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ANALYST, OCTOBER 1993, VOL. 118 1247 Determination of Nanogram Amounts of Iodine in Foods by Radiochemical Neutron Activation Analysis Raghunadha R. Rao and Amares Chatt" Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Three different neutron activation analysis methods have been developed for the determination of ppb levels of iodine in food samples. The methods are based on the separation of iodine using ( i ) toluene extraction followed by bismuth sulfide coprecipitation; ( i i ) bismuth sulfide coprecipitation followed by radiochemical purification with palladium iodide; and (iii) radiochemical isolation by bismuth sulfide coprecipitation. The accuracy of these methods was evaluated by analysing replicate samples of reference materials.The measured values of iodine in A-I1 Milk Powder, H-4 Animal Muscle and H-9 Mixed Human Diet from the International Atomic Energy Agency (IAEA) are statistically indistinguishable from the IAEA recommended values, and those for the Standard Reference Materials 1571 Orchard Leaves and 1577 Bovine Liver from the National Institute of Standards and Technology (NIST) are in good agreement with the NIST information values. The precision, in terms of relative standard deviation, is 5% at 50-100 ppb and 10% at 5-20 ppb levels of iodine. The absolute detection limits of these methods vary between 0.5 and 10 ng of iodine. All three methods were used t o measure the iodine content of several food samples. The method involving bismuth sulfide coprecipitation followed by radiochemical purification provides the best detection limit and highest precision.Keywords: Iodine determination; neutron activation analysis; toluene extraction; bismuth sulfide coprecipitation; radiochemical purification There is a considerable interest in determining the iodine content of human foods and diets. Iodine is an essential nutrient; its lower limit of safe daily dietary intake for adults has been suggested in many countries. For example, the Canadian Recommended Nutrient Intake (RNI) is 160 pg d-l,' the US Recommended Dietary Allowance (RDA) is 150 pg d-l,? and values between 150 and 200 pg d-l are common in European3 and other countries. Deficiency of iodine leads to goitre disease and excessive intake may contribute to thyroiditis.3-5 Iodine intakes of adults in different regions of the USA6 and Canada7 have been reported to exceed the respective RDA and RNT values by more than six times.Interlaboratory comparison studies for iodine determination employing several analytical techniques revealed that the measured values varied between 0.05 and 2.6 ppm for the International Atomic Energy Agency (IAEA) certified reference material (CRM) A-1 1 Milk Powder,8 and by a factor of 3 at about the 0.2 ppm level for the Community Bureau of Reference (BCR) CRM Skim Milk Powders.3 One of the reasons for this large spread of results could be the lack of reliable methods for the determination of very low levels of iodine in biological materials. Activation analysis, in general, has a high sensitivity for iodine.Non-destructive photon activation analysis9 and epithermal instrumental neutron activation analysis (EINAA)l('--lY methods have been used to determine iodine at higher levels (>SO0 ppb) in a variety of samples. However, for measuring trace amounts of iodine, it is necessary to employ either a preconcentration NAA (PNAA) or a radiochemical NAA (RNAA) proeedure to separate iodine from elements such as Br, CI, Na and Mn, which induce high activities in the samples on neutron irradiation. A radiochemical separation method involving alkali fusion in the presence of iodine carrier and solvent extraction has been described for analysing plant materials.") The determination of iodine in biological mater- ials has been accomplished by ignition of an irradiated sample and gas phase separation of 12x1 on hydrated manganese dioxide." Iodine has also been determined in biological and ' To whom correspondence should be addressed. diet samples by combustion o f the activated samples in an oxygen atmosphere and separation of 1281 using oxidation- reduction and extraction-stripping cycles.2'-23 Pre- and post- irradiation separation procedures involving combustion of the sample in an oxygen stream and collection of the liberated iodine on charcoal has been applied to the determination of lz9I content and 12W1271 ratios in soils.'S--'8 A method involving microwave acid digestion and PNAA for measuring low levels of iodine in biological and diet samples has recently been reported by the present authors.2"3(J In an attempt to improve the detection limits and possibly the precision of measurement of our PNAA method29 developed earlier, further studies have been carried out and are reported here. Three additional methods have been developed for more effective separation of iodine from major interfering elements in a given matrix using (i) toluene extraction followed by bismuth sulfide coprecipitation; (ii) bismuth sulfide coprecipitation followed by radiochemical purification with palladium iodide; and (iii) post-irradiation isolation of iodine by bismuth sulfide coprecipitation. The details of these methods together with the evaluation of various analytical parameters and application to biological and diet samples are described in this paper.Experimental Reagents The following reagents were used: ultrapure ammonium iodide (SPEX); analytical-reagent grade bismuth nitrate, thioacetamide and toluene (Mallinckrodt Chemicals); Puriss grade hydrazine sulfate and sodium nitrite (Fluka); palladium chloride (BDH); and ultrapure nitric acid and ammonia solution (Scastar).Stock solutions of bismuth nitrate (40 mg ml-l BP+ in 2% nitric acid), and thioacetamide (8.5 mg mlP1 S2-) and 10% sodium nitrite in de-ionized, distilled water (DDW) were used. A 0.5 mg ml-l iodide stock standard solution was prepared by dissolving the requircd amount of ammonium iodide in 5% v/v ammonia solution. This stock solution was diluted with DDW to prepare a working standard of 501248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity.Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution.The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb.The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I.The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry.Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel.The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid.The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min.The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine.Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity.Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution.The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed.The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added.Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution.The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801247

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I.The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry.Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel.The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb.The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation.The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'.To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity.Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution.The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed.The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution.The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801253

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801261

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801265

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I.The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry.Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel.The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb.The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation.The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'.To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity.Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution.The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed.The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution.The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801269

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801277

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801281

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801285

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801289

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder.H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 .I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml.This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%).The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure.The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven.The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min.The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I.The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction.The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution.The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate.The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA.The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples.To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples.This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials.The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet.Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples.The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine. The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW.The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate.The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel.Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel.The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min.Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer.The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I.The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry.Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples. The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I .They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo. The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'.The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction. The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min.Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW. The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel.The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine. Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid.Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb.The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation.The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'.To this solution, 0.5 ml aliquots of1248 ANALYST, OCTOBER 1993, VO12. 118 ng ml-l iodide. The iodide comparator standards were prepared by transferring microlitre portions of the iodide standard solution onto pre-cleaned Gelman Sciences GA-6S membrane filters (0.45 pm pore size, 47 mm diameter) placed in pre-cleaned 1.2 ml polyethylene irradiation vials. The comparator standards and samples were prepared to have identical geometry. Reference Materials A number of CRMs and Standard Reference Materials (SRMs) obtained from the IAEA and the National Institute of Standards and Technology (NIST) , respectively, were analysed to evaluate the accuracy of the methods at the low levels of iodine that might be present in some biological and diet samples.The RMs used were: NIST SRM 1571 Orchard Leaves and SRM 1577 Bovine Liver, and IAEA A-11 Milk Powder. H-4 Animal Muscle and H-9 Mixed Human Diet. Irradiation and Counting All samples and comparator standards were irradiated in the Dalhousie University SLOWPOKE-2 research reactor at a maximum integral neutron flux of 1 x 1012 n cm-2 s-1 or a maximum epi-cadmium neutron flux of 1 x lo'* n cm-2 s - I . They were counted on a 25 ml active volume Aptec hyperpure Ge detector connected to a Link high count rate pulse processor and a Nuclear Data ND-66 analyser. The detector had a resolution of 2.08 keV (full-width at half-maximum) at the 1332 keV photopeak of 6OCo.The 443 kcV y-ray of 12*l was free from interference and was therefore used for assaying iodine. Microwave Acid Digestion of Samples Microwave acid digestion bombs (Parr Instrument Co.) were used for the dissolution of samples. The bomb contained a chemically inert Teflon sample cup of 45 ml capacity. Details of the development of the sample digestion procedure are given clsewhere.29 Briefly, 200-250 mg of a sample were accurately weighed into a pre-cleaned Teflon sample cup, 5 ml of ultrapure concentrated nitric acid were added, and the mixture was heated for 35 s at a power of 675 W in a microwave oven. The digestion step was repeated after a 20 rnin cooling period, if necessary. This procedure yielded sufficiently complete decomposition of the sample with respect to iodine.The contents of the cup were poured into a pre-cleaned 250 ml beaker containing 1 g of hydrazine sulfate. The cup and its lid were rinsed successively with 3 x 5 ml aliquots of each of a 5% hydrazine sulfate solution and DDW. The washings were added to the sample solution. The resulting solution was diluted to 100 mi, maintaining a final acidity of 0.2 rnol 1-l. Preconcentration of Iodine by Bismuth Sulfide Coprecipitation Details of the development of this method have recently been published.'" Bricfly, it involved the sequential dropwise addition of 1 ml of each of bismuth nitrate and thioacetamide stock solutions to a 100 ml digested sample solution at an acidity of 0.2 rnol I-'. The dark brown precipitate formed was allowed to settle for 20 rnin at room temperature and then filtered through a pre-cleaned Gelman membrane filter under vacuum suction.The precipitate was washed three times with 5 ml aliquots of a 0.2 mol I-' nitric acid solution containing 0 . I% hydrazine sulfate. The filter containing the precipitate was folded, placed in a 1.2 ml polyethylene vial and heat- sealed. The vial was irradiated in an epi-cadmium neutron flux of 1 .0 x 1010 n cm-2 s- for 30 or 60 min and counted for either 30 or 60 rnin after a decay period of I min. Preconcentration of Iodine by Toluene Extraction The digested sample solution was poured into a 250 ml beaker. The sample cup and lid of the microwave digestion bomb were thoroughly rinsed with 2 x 5 ml portions of a 5% hydrazine sulfate solution and subsequently with 2 x 5 ml portions of DDW.The washings were added to the sample solution and the final acidity of the resulting solution was adjusted to between 1 and 2 rnol I-', maintaining a total volume of 30 ml. This solution was transferred into a 125 ml Pyrex separating funnel. Then, 10 ml of toluene and 5 ml of a 10% NaN02 solution were added. Iodine was extracted into the organic phase by shaking on a wrist-action mechanical shaker for 10 min. The organic phase was transferred into another separat- ing funnel. The extraction procedure was repeated three times with 10 ml of toluene and 2 ml of a 10% NaN02 solution. The extraction recovery of iodine was checked by irradiating 1 mi aliquots of the organic phasc after each extraction and calculating the yield of iodine.Preliminary experiments with spiked standard iodine and with 1251 tracer revealed that three extractions were sufficient for the complete recovery of iodine (>99%). The organic phases were combined in a separating funnel. The iodine was reduced to iodide and back-extracted into the aqueous phase by equilibrating with 2 x 10 ml portions of a 5% hydrazine sulfate solution. The aqueous phases containing iodide were combined and diluted to 100 ml, maintaining a final acidity of 0.2 mol I-' with rcspect to nitric acid. Samples containing relatively large amounts of chlorine and bromine were first treated by this toluene cxtraction method to isolate iodine. Then, the bismuth sulfide coprecipitation procedure described above was used to concentrate the iodide further and collect it in a precipitate which is suitable for NAA. The iodine content of the precipitate was measured by irradiating in an epi-cadmium neutron flux of 1 x 1OlO n cm-? s-I for 60 rnin and counting for 60 rnin after a decay period of 1 min. Radiochemical Separation of Iodine by Bismuth Sulfide Coprecipitation About 250 mg of a sample were accurately weighed into a pre- cleaned polyethylene vial and irradiated for 60 min in an integral neutron flux of 1 X 1012 n cm-? s-I. The vial was opened and the sample was transferred into a Teflon sample cup of the microwave digestion bomb. The vial was rinsed three times with 1 ml portions of ultrapure concentrated nitric acid. The irradiated sample was spiked with 100 pI of 1251 tracer solution and digested according to the procedure described under Microwave Acid Digestion of Samples. To the digested sample solution, 2 ml of bismuth nitrate stock solution, 1 ml of ammonium iodide (1 mg ml-I I - ) and 2 ml of thioacetamide stock solution were added sequentially while stirring the solution with a magnetic stirrer. The resulting dark brown precipitate was allowed to settle for 20 min and then filtered through a Gelman membrane filter under vacuum suction. The entire procedure was completed within 50 min from the end of the irradiation. The filter was folded, placed in a new 1.2 ml polyethylene vial and counted for 30 min. The recovery of iodine was checked by measuring the activity of the 1251 tracer. Radiochemical Purification by Palladium Iodide Precipitation Thc sample was digested according to the procedure described under Microwave Acid Digestion o f Samples. This solution was taken through the bismuth sulfide coprecipitation proce- dure. The precipitate was collected and irradiated for 60 rnin in an integral neutron flux of 1 X 1012 n cm-2 s-I. The irradiated precipitate was washed off the filter into a beaker with 5 ml of concentrated nitric acid. The filter was rinsed thoroughly with 4 mol I-' nitric acid and subsequently with 10 ml of a 5% hydrazine sulfate solution. The sample solution was diluted to SO ml with DDW, maintaining a final acidity between 2 and 4 rnol I-'. To this solution, 0.5 ml aliquots of

ISSN:0003-2654    

DOI:10.1039/AN9931801293

出版商:RSC    

年代:1993

数据来源: RSC