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 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