摘要:

570 AnaZyst, September, 1974, Vol. 99, $9. 570-576 The Determination of Residues of Volatile Fumigants in Grain REPORT BY THE PANEL ON FUMIGANT RESIDUES IN GRAIN OF THE COMMITTEE FOR ANALYTICAL METHODS FOR RESIDUES OF PESTICIDES AND VETERINARY PRODUCTS I N FOODSTUFFS OF THE MINISTRY OF AGRICULTURE, FISHERIES AND FOOD THE Panel was formed in 1973 by the Committee for Analytical Methods for Residues of Pesticides and Veterinary Products in Foodstuffs to study gas - liquid chromatographic methods for the determination of fumigant residues in grain. Members were invited from United Kingdom government and trade laboratories closely involved with work on fumigant residues in grain, and from the Association of Public Analysts. Official laboratories in The Netherlands also participated as they had carried out significant work on the analysis of fumigants in cargoes in international trade.The members of the Panel are listed in Appen- dix 111. The terms of reference of the Panel were- “To establish by collaborative study a gas - liquid chromatographic method for the determination of residues of volatile fumigants in grains, bearing in mind its possible application to other fumigated foodstuffs.” The residues referred to are those of intact volatile fumigants. Although methods involving steam distillation and also cold ball-mill extraction of the fumigant were considered by the Panel, the method chosen for collaborative study was that reported by Heuser and Scudamore,l in which the intact grains are extracted with aqueous acetone at room temperature, without grinding, for gas - liquid chromatographic detemina- tion.(These authors also reported the use of aqueous acetonitrile for extraction, but this method was not investigated by the Panel.) PREPARATION OF SAMPLES The normal spiking procedures used in collaborative studies are unsatisfactory for investi- gations with volatile fumigant residues. In practice, fumigants pass into grains in an ageing process and the fumigant, while still largely intact, cannot be recovered so readily after some weeks as it can be soon after addition. The Panel, therefore, decided to use for each trial aged and aired samples of grain drawn from a single batch of fumigated material and distributed to members for analysis on a given day. In this way, as nearly as is possible, the same sample of grain in the same condition was analysed on the same day in all laboratories.The absolute value of the incurred residue could not be ascertained but a comparison of the analytical data gave a measure of the reproducibility of the method. The levels of fumigant residues incurred in this work were similar to those found in normal commercial fumigation practice and are of the order of the limits under consideration by the Joint Meeting of the FA0 Working Party of Experts on Pesticide Residues and the WHO Expert Committee on Pesticide Residues.2 Details of the preparation of these samples are given in Appendix 11. TREATMENT OF RESULTS The means and standard deviations of results given in this work were calculated for individual analyses, irrespective of the originating laboratories, and without distinguishing results from laboratories that reported fewer analyses.Eight laboratories took part in all three collaborative exercises and the number of results from each laboratory for any particular substrate and conditions varied from one to four, but generally results were obtained in duplicate or triplicate. Means and standard deviations were therefore weighted towards those laboratories which carried out most analyses. If means from laboratories had been taken for statistical analysis, the conclusions would have been weighted in favour of results from those analysts who carried out fewer analyses. The simple approach used is an approximation and emphasises the importance of the analytical method rather than the expertise of the analyst.0 SAC; Crown Copyright Reserved.PANEL ON FUMIGANT RESIDUES I N GRAIN 57 1 This approach is fitting in a collaborative study, although it must be noted that analytical experience is important in the use of any method. It was also the case that, in these studies, not all laboratories investigated all conditions and a complete statistical analysis was not possible. FIRST COLLABORATIVE STUDY In the first collaborative study, wheat and maize fumigated with carbon tetrachloride were investigated by the method described in Appendix I. Batches of 300 g were sent to the collaborating laboratories and 20-g samples taken for analysis, using 60 ml of mixed solvent. Extraction periods of 2 and 7 days were compared. For wheat, the means of results for 2 and 7 days’ extraction were 15 and 17 mg kg-l, with standard deviations for individual analyses of &5 and &l mg kg-l, respectively.For .maize, the means of results for 2 and 7 days’ extraction were both 43 mg kg-l, the 2-day extraction period giving a standard deviation of &9 mg kg-l and the 7-day extraction period a standard deviation of &lo mg kg-l. Un- treated grain generally showed a negligible apparent fumigant content although, in one laboratory, values as high as 1 rng kg-l were reported. Some laboratories used the recom- mended polypropylene glycol column for gas - liquid chromatography, whereas others used a Porapak Q column. It was concluded from this study that there was apparently little differ- ence between 2- and 7-day extraction periods for wheat or maize.However, the standard deviations reported with maize were rather large and there were one or two very low figures for the fumigant extracted in 7 days, which depressed the mean, and it was felt that the situation should be re-examined by using 50-g samples of maize. This study represented the first occasion on which some members had used the method and it was felt that useful results could be obtained by broadly repeating the work. Additionally, in order to provide evidence about the reproducibility of gas - liquid chromatographic conditions, it was agreed in subsequent work, to exchange final extracts between collaborators. SECOND COLLABORATIVE STUDY In the second collaborative study, wheat and maize were again fumigated with carbon tetrachloride.On this occasion, two 300-g samples from the same fumigation of each cereal were distributed to each laboratory for analysis. For the extractions, 20g of wheat and 50 g of maize were used. The results obtained using a polypropylene glycol column for gas - liquid chromatography (see Appendix I) are given in Tables I and 11. These tables also include results from one laboratory using a Porapak Q column. Only the polypropylene glycol figures have been included in the statistical analyses. Samples distributed to laboratories 1 and 2 were held up in transit, possibly at a high ambient temperature, and were analysed 1 day later than the other samples so that the results may be slightly low. TABLE I DETERMINATION OF CARBON TETRACHLORIDE IN FUMIGATED WHEAT gas - liquid chromatographic column and were not included in the statistical calculations Results are given in mg kg-l.The results in parentheses were obtained using a Porapak Q Sample A Laboratory 1 2 3 4 5 6 7 8 2-day extraction 7, 7, 8 6 , 8, 7, 6 8, 9 8, 9, 9 12, 11 11 9, 8 (8, 8) 8, 10 7-day extraction a, 9. 7 6 , 6, 7 , 6 10, 10 10, 11, 10 9, 10 9, 11 11, 11 (9, 9) 10, 1 1 Sample B c A \ 2-day extraction 7-day extraction 7, 6, 8 8, 7. 9 7, 6, 7, 6 8, 7, 7, 7 9, 9 10, 11 11, 10, 9 10, 11, 10 10, 10 10, 10 10 11, 11 8, 9 (8, 9) 11, 11 (10, 10) 9, 8 11, 10 Mean . . . . . . 8.5 9.2 8.4 9.5 Standard deviation . . k l . 7 k 1.7 & 1.6 k 1.7 Exchange mean* . . 7.8 8.9 7.7 8.7 Exchange s.d.* . . . . 1.3 & 1.7 & 1.3 1.8 Obtained from interlaboratory exchange of final solutions.572 PANEL ON FUMIGANT RESIDUES IN GRAIN: THE DETERMINATION [Analyst, VOl.99 The final solutions were examined by gas - liquid chromatography both in the laboratory in which the extraction was made and also in another exchange laboratory. Laboratory 1 exchanged with laboratory 2, laboratory 3 with laboratory 4, and so on, for the investigation of the variability of the gas - liquid chromatographic determinations. TABLE I1 DETERMINATION OF CARBON TETRACHLORIDE IN FUMIGATED MAIZE gas - liquid chromatographic column and were not included in the statistical calculations Results are given in mg kg-l. The results in parentheses were obtained using a Porapak Q Laboratory 1 2 3 4 5 6 7 8 Sample A 2-day extraction 7-day extraction r 16, 18, 14 14, 13, 14, 15 20, 19 21,21 18, 19, 18 24, 23 23, 23 24 21, 24 20, 19 (18, 18) 17, 17 21, 19 A -l 17, 16, 17 17, 14, 19, 14 23, 23, 22 21, 27 (20, 23) Mean .. . . . . 18.1 Standard deviation . . 53.3 20.2 & 3.4 Sample B , I. 2-day extraction 7-day extraction 17, 16, 16 16, 15, 13, 16 19, 20 24, 22 17, 20, 17 22, 22 22,24 22 23,23 16, 16, 13 18, 18, 18, 14 22, 25, 22 21, 21 (19, 19) 19, 17 22,21 24, 23 (21, 21). 18.2 2.7 Exchange mean* . . 16.3 20.3 18.0 Exchange s.d.* . . . . k3.1 2 4-0 k3.1 * Obtained from interlaboratory exchange of final solutions. 20-5 3.6 19.3 * 3.3 A summary of the results obtained is given in Tables I and 11. Agreement of results between laboratories for any one sample and set of conditions was good. Results were closer than in the first collaborative study and the increased reproducibility of the analyses of maize was particularly noticeable.Also, there was clearly no sampling error from the fumigated bulk in that there was no significant difference at 95 per cent. confidence limits between samples A and B, taken from the same original batch. In this study, a 7-day extraction consistently gave higher residues than a 2-day extraction (by about 10 per cent.), although under any particular set of conditions in the study the difference was not always significant at 95 per cent. confidence limits. However, a 7-day extraction period, while probably giving as nearly as is. possible a figure for 100 per cent. extraction, is not realistic in actual situations in which fumigant residues have to be determined.To a good approximation, therefore, a 2-day extraction period is adequate. For an indication of absolute recovery levels to be expected in these periods, reference can be made to the calculations of Heuser and S c ~ d a m o r e , ~ ~ ~ ~ based on amounts of fumigants used. Comparison of results from exchange of solutions for gas - liquid chromatography gave no significant evidence to suggest that chromatographic operating conditions were especially critical beyond the factors given in the method. Results from exchange analyses were slightly lower, but in some instances there are few values for a particular set of conditions and the differences are not significant. One laboratory carried out analyses of individual grains of maize in one of the treated samples.Twelve grains were taken at random and the carbon tetrachloride content was found to vary from 2 to 97 mg kg-l. Using a number of approximations, it can be deduced empirically that for a 20-g sample of maize the content of every third sample would be expected to be outside For a 50-g sample of maize, one sample in eighteen would be expected to be outside *lo per cent. of the mean. This work corroborated the finding from the collaborative. studies that 50 g is the minimum sample to be taken for the analysis of maize. It was agreed that the method in Appendix I can be recommended for determining carbon tetrachloride in wheat and maize. 8 per cent. of the mean. THIRD COLLABORATIVE STUDY In the third collaborative study, wheat and maize fumigated with chloroform, trichloro- ethylene and 1,2-dibromoethane were examined by the method in Appendix I using 50-g samples of wheat and maize for analysis and both 2 and 7 days for extraction.The resultsSeptember, 19741 OF VOLATILE FUMIGANT RESIDUES IN GRAIN 5 73 TABLE I11 DETERMINATION OF CHLOROFORM, TRICHLOROETHYLENE AND 1,Z-DIBROMOETHANE IN FUMIGATED WHEAT Results are given in mg kg-l. The results in parentheses were obtained using an Apiezon L gas - liquid chromatographic column and were not included in the statistical calculations Chloroform Trichloroethylene 1 ,Z-Dibromoethane r - v ---7 7 2-day 7-day 2-day 7-day 2-day 7-day Laboratory extraction extraction extraction extraction extraction extraction 1 36,36, 37 35, 36,39 33,33,34 34,34,38 50, 50,47 49, 49, 52 2 30,30 22,23 29,29 31,33 40,40 43.44 3 33,35 39,39 36,37 38,37 49,50 51,52 4 31,32,32 36,33,33 25,25,25 29,25,25 48,47,49 48, 43, 43 5 35,37,34 37,38,37 34,36,33 35,36,35 52,51, 51 48, 48, 49 6 41,42 37,38 41,42 7 36,36 (37,37) 35,36 (38,40) 33,34 (34,34) 34,35 (35,37) 47,48 (48, 50) 52, 48 (47, 47) 8 41,43 37,36 42,42 39,38 59,62 51,50 (37, 40,34) (36,34) (35,38, 33) (35,38,36) (54, 56,47) (48, 52, 50) Mean .35 35 33 34 49 48 Standard deviation 5 4 +5 +5 +4 +5 - + 4 mean* . . 32 36 33 33 46 47 s.d.* . . +_7 *7 +5 *6 +7 + 5 Exchange Exchange * Obtained from interlaboratory exchange of final solutions. obtained in members' own laboratories are given in Tables I11 and IV. These tables contain results obtained using mostly polypropylene glycol but also sometimes Apiezon L columns for gas - liquid chromatography.Only the polypropylene glycol figures have been included in the statistical analyses. As before, the final solutions were also examined by gas - liquid chromatography in other laboratories and a summary of the exchange results obtained is also given in Tables 111 and IV. TABLE IV DETERMINATION OF CHLOROFORM, TRICHLOROETHYLENE AND ~,Z-DIBROMOETHANE IN FUMIGATED MAIZE Results are given in mg kg-l. The results in parentheses were obtained using an Apiezon L gas - liquid chromatographic column and were not included in the statistical calculations Chloroform Trichloroethylene 1,2 Dibromoethane r - 7 7-d- '1 r--- A 7 %day 7-day %day 7-day 2-day 7-day Laboratory extraction extraction extraction extraction extraction extraction 1 83,88,85 80,82,85 114, 122, 121 117, 126, 122 115, 116, 113 119, 129, 123 2 63,63 45,47 86,89 91,99 92,95 96,102 3 78,85 97,111,111 129,134 98,104, 116 129,129 4 69,63, 66 72,72,70 84,84,80 89,88,79 111,111, 111 109,112, 104 5 78,78,70 80,81,76 115, 110, 101 118,114,109 148,117, 113 124,113,111 (78, 78, 71) (74, 76,69) (124, 112, 101) (121, 119, 106) (144, 117, 113) (120,114,109) (85,gO) (71,741 (117, 122) (106, 108) (127, 127) (106, 103) 6 71,81 110,95 120,119 7 86,92 68,76 115, 118 98,111 135,131 104.105 8 86,88 74,77 118, 127 113, 115 135, 143 118, 126 Mean .. . .77 74 106 108 117 115 Standard deviation 5 11 * 11 + 15 15 16 * 10 Exchange Exchange mean* . . 75 76 107 102 115 114 s.d.* . . A14 2 14 A 13 & 19 & 20 & 11 * Obtained from interlaboratory exchange of final solutions.574 [Analyst, Vol.99 These figures are in good agreement with the means and standard deviations of the original analyses and again it can be concluded that the description of the gas - liquid chromatographic conditions given in Appendix I are adequate. There was generally no apparent or statistical difference (at the 95 per cent. confidence level) between the 2- and 7-day periods of extraction for any of the three fumigants examined in the third collaborative study. The results for maize in the third collaborative study are relatively more variable than those for wheat but, considering the inherent difficulties of sample preparation, sampling and distribution in this work, they were considered to be adequate.It was agreed that the method in Appendix I can also be recommended for determining chloroform, trichloroethylene and 1 ,%dibromoethane residues in wheat and maize. PANEL ON FUMIGANT RESIDUES IN GRAIN: THE DETERMINATION A 2-day extraction period is, therefore, sufficient. CONCLUSION The method of Heuser and Scudamorel has been established by collaborative study to be suitable for the determination of carbon tetrachloride, chloroform, trichloroethylene and 1,2-dibromornethane residues in wheat and maize. It was not possible to distribute grain samples for the collaborative analysis of unreacted bromomethane residues because of the high volatility of the fumigant. A normal electron- capture detector is unsuitable for the accurate determination of 1,2-dichloroethane at the usual levels and special apparatus is necessary, making the subject unsuitable for immediate collaborative study.Nevertheless, as a result of the work reported here and taking into account the evidence provided by Heuser and S c u d a m ~ r e l ~ ~ ~ ~ of similar results obtained with other fumigants and commodities, the Panel was encouraged to accept the validity of the method for wider use. Appendix I RECOMMENDED METHOD FOR DETERMINING RESIDUES OF VOLATILE FUMIGANTS I N GRAINS APPARATUS- Conical $&-Capacity 250 ml, with 24/29 ground-glass sockets and glass stoppers. Graduated cylinders-Capacity 25 ml and 10 ml, stoppered. Microsyringe-Capacity 1 pl. Gas - liquid chromatografihic column-A 4m x 2-2mm i.d. stainless-steel column packed with 15 per cent.m/m polypropylene glycol (LB 550X, Ucon fluid) on 60 to SO-mesh Chromo- sorb W was found suitable by all collaborating workers. Limited experience also showed a 2m x 2.2mm i.d. stainless-steel column packed with 15 per cent. Apiezon L on Chromosorb P and a 50 to 80-mesh Porapak Q column to be satisfactory. Gas chromatograph-An isothermal model fitted with a tritium (preferred) or nickel-63 source heated electron-capture detector and a glass-lined heated injection block is required. The use of a 100 to 200-mCi tritium source for ionisation in the detector (e.g., Perkin-Elmer) makes possible its use in the p-ionisation mode if pure argon is used as the carrier gas. This modification increases the potential of the method for multi-residue purposes, as shown by Heuser and Scudamore.The much lower energy source provided by commercial nickel-63 detectors precludes this extension of scope. Recorder-A 1-mV potentiometric recorder with a response time of 1 s (maximum) and a chart speed of 0.5 cm min-l is suitable. REAGENTS- A cetorte-Check for interfering impurity peaks by gas - liquid chromatography before use. Sodium chloride-Analytical-reagent grade. Calcium chloride-Anhydrous. Pure fumigants. De-ionised water. Nitrogen-Oxygen-free; carrier gas for gas - liquid chromatography.September, 19741 OF VOLATILE FUMIGANT RESIDUES I N GRAIN 575 METHOD If it is necessary to store the sample overnight, cool the container to below 5 "C. Refore analysis, thoroughly mix the sample in the sealed container. EXTRACTION- Quickly weigh a 50-g sample and immediately immerse it in 150 ml of a 5 + 1 V/V mixture of acetone and water in a 250-ml conical flask and insert the stopper (ungreased). Allow the flask to stand for 48 hours in the dark at room temperature (20 to 25 "C) with swirl- ing after 24 hours.Pour 10ml of the supernatant liquid into a 25-ml graduated cylinder, add 2 g of sodium chloride, insert the stopper, shake the cylinder vigorously for 2 minutes and allow it to stand until two layers separate. Pour 5 ml of the clear upper layer into a 10-ml graduated cylinder, add 1 g of anhydrous calcium chloride, insert the stopper, shake the cylinder for 2 minutes and then allow it to stand for 30 minutes with occasional shaking. Withdraw 0.5-pl aliquots from the upper layer into a 1-p1 syringe for gas -liquid chromatography.Dilute the extracts ten or one hundred fold or more with dry acetone, if necessary. Rinse the syringe very thoroughly between injections of different samples or concentrations. Inject all solutions into the gas chromatograph in triplicate. G A S - LIQUID CHROMATOGRAPHY- Use a nitrogen carrier gas pressure of 175 kN m-2 (25 lb in-2) at an oven temperature of 95 "C with the polypropylene glycol column for carbon tetrachloride. The latter then has a retention time of about 6 minutes. Use an oven temperature of 120 "C for chloroform, trichloroethylene and 1,2-dibromoethane; under these conditions their retention times are about 3, 4 and 8 minutes, respectively. Construct a calibration graph daily from peak heights obtained by injecting into the gas chromatograph 0-5-pl aliquots of a suitable range of solutions of fumigant in acetone.Avoid overloading the detect or. The limits of detection of the gas - liquid chromatography in the different laboratories collaborating in this work varied from 0.0002 to 0.04 mg kg-l for carbon tetrachloride, from 0.005 to 0.2 mg kg-l for chloroform and trichloroethylene and from 0.005 to 0.8 mg kg-l for 1,2-dibromoet hane in 50-g samples of grain. Peak heights should prove satisfactory for quantitative measurements. CALCULATION- Relate the amounts of fumigant indicated in the aliquots of dried sample extracts to a total volume of anhydrous solvent of 125 ml and hence to the content of total mass of sample. Appendix I1 PREPARATION OF SAMPLES FOR COLLABORATIVE TESTING Wheat or maize was sieved, spread evenly on porcelain-enamelled trays and placed in a room maintained at constant temperature and humidity (25 "C and 70 per cent.). After 7 days' conditioning, the trays were transferred to a 1700-litre steel fumigation chamber and a calculated amount of liquid fumigant was vaporised in. A mixture was used for the third test.Gas samples were drawn, after 1 hour and shortly before the end of treatment, into partially evacuated gas sampling flasks containing toluene, for the determination of vapour concentration. After a 72-hour fumigation, air was drawn through the chamber for 0.5 hour before opening it and removing the trays. These trays were then kept in a constant-temperature room for 1 month in order to air them.Samples of wheat or maize were taken at intervals and analysed to monitor the residue levels present. A period of 24 hours before the distribution of samples to Panel members, each commodity was thoroughly mixed and subdivided into eight or sixteen portions using a Boerner divider. Each portion was placed in a screw-topped glass bottle and sealed with a layer of aluminium foil. Samples were then kept at 10 "C until distribution.576 PANEL ON FUMIGANT RESIDUES I N GRAIN Appendix I11 MEMBERSHIP OF THE PANEL The membership of the Panel was: Mr. R. H. Thompson (Chairman) (Pest Infestation Control Laboratory), Mr. F. B. Fishwick (Pest Infestation Control Laboratory), Mr. M. Green (Lancashire County Laboratory), Dr. W. B. F. Grevenstuk (Rijks Instituut Voor de Volks- gezondheid, The Netherlands), Mr. A. H. Harris (Tropical Stored Products Centre, Tropical Products Institute), Mr. H. V. Hart (Flour Milling and Baking Research Association), Mr. S. G. Heuser (Pest Infestation Control Laboratory), Dr. R. A. Hoodless (Laboratory of the Government Chemist), Mr. K. A. Scudamore (Pest Infestation Control Laboratory), Dr. A. J. Speek (Central Institute for Nutrition and Food Research, TNO, The Netherlands) and Dr. N. A. Smart (Secretary) (Plant Pathology Laboratory). REFERENCES 1. 2. 3. 4. 5. Heuser, S. G., and Scudamore, I<. A., J . Sci. Fd Agric., 1969, 20, 566. “Evaluations of Some Pesticide Residues in Food, 1971,” WHO Pesticide Residue Series, No. 1, Heuser, S. G., and Scudamore, K. A., Chem. G. Ind., 1968, 1154. Scudamore, K. A., and Heuser, S. G., Pestic. Sci., 1973, 4, 1. Heuser, S. G., and Scudamore, K. A., Analyst, 1968, 93, 252. WHO, Geneva, 1972, pp. 263 and 272. Received Mavclr 26th, 1974 Accepted May 8th, 1974 COMMITTEE FOR ANALYTICAL METHODS FOR RESIDUES OF PESTICIDES AND VETERINARY PRODUCTS IN FOODSTUFFS (DR. N. A. SMART, SECRETARY), MINISTRY OF AGRICULTURE, FISHERIES AND FOOD, PLANT PATHOLOGYLABORATORY, HATCHING GREEN, HARPENDEN, HERTFORDSHI RE.

ISSN:0003-2654    

DOI:10.1039/AN9749900570

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Analyst, September, 1974, Vol. 99, Pp. 577-579 Modified Methylene Blue Method for the Micro-determination of Hydrogen Sulphide BY N. A. MATHESON (Rowett Research Institute, Bucksburn, Abcrdem, AB2 9SB) 577 A method is described whereby hydrogen sulphide can be determined simply and rapidly in the 0.04 to 0.4 pmol range. QUANTITATIVE chemical methods for determining hydrogen sulphide on a micro-scale tend to be inaccurate, laborious or require special apparatus. Some of the problems have been sum- marised by Koren and Gierlinger.1 One of the best known methods is that involving the use of NN-dimethyl-t-phenylenediarnine, which forms methylene blue with hydrogen sulphide; it gives variable results as normally used but, after investigation, it proved possible to develop suitable apparatus and conditions for a simple and accurate technique.Other workers have tried to use the methylene blue method quantitatively, notably Almy,l Sands, Grafius, Wainwright and Wilson,3 Fogo and Popowskid and Kuratomi, Ohno and A k a b ~ r i , ~ but each group used a different version of the technique, which indicates that the earlier methods were not entirely satisfactory. The method for the determination of hydrogen sulphide used by the last authors gave erratic results in this laboratory (see also Goas). EXPERIMENTAL REAGENTS- NN-Dimethyl-p-phenylenediamivte reagent-NN-Dimethyl-+-phenylenediamine sulphateS was obtained from BDH Chemicals Ltd. and 75 mg were dissolved in 100 ml of 4 N sulphuric acid. Iron reagent-This reagent was a solution of 1-2g of iron(II1) sulphate in 100ml of 0.015 M sulphuric acid.Zinc acetate reagent-Each 100-ml volume of this reagent contained 0.5 g of zinc acetate dihydrate and 5 mg of Brij (polyoxyethylene lauryl ether) detergent. APPARATUS- One unit of the apparatus is shown in Fig. 1. The complete apparatus consists of a battery of five or six of these units and a reservoir to help to distribute nitrogen uniformly to all tubes. PROCEDURE- The hydrogen sulphide sample solution is prepared by diluting 0.2 ml of a fresh, approxi- mately saturated solution to 50 ml with 0.01 N sodium hydroxide solution. A more concen- trated solution is prepared by another dilution (ten times more concentrated but prepared at the same time) and its concentration determined by iodimetric titration.The sample tubes (F) contain 2 ml of 2 N hydrochloric acid and sufficient 0.01 N sodium hydroxide solution to give a total volume of 4 ml after the hydrogen sulphide solutions have been blown in. The receiver tubes (G) contain 10 ml of zinc acetate reagent. The apparatus is assembled and nitrogen is passed through the sample and receiver tubes for 10 minutes a t the flow-rate of 20 mlmin-l. Various amounts of the solution of hydrogen sulphide in 0.01 N sodium hydroxide (containing 0-04 to 0.4 pmol of hydrogen sulphide, determined by iodimetric titra- tion of the more concentrated solution7) are placed in the bulbs of the inlet tubes by means of a pipette attached to a flexible plastic tube. The nitrogen lines are reconnected and the samples are blown in, and gassing with nitrogen is continued for 90 minutes at the flow-rate of 20 ml min-l.The delivery lines are then lifted out of the receiver tubes while nitrogen is passing and any drops of liquid on the ends of the delivery tubes are transferred into the receiver tubes. These receiver tubes are brought to 18 "C, then 4 ml of NN-dimethyl-$- phenylenediamine reagent and, 5 minutes later, 1 ml of iron reagent are added, both at 18 "C. After a further 5 minutes, the extinctions of the solutions in each tube are read at 630 nm. Q SAC and the author.578 MATHESON: MODIFIED METHYLENE BLUE METHOD FOR THE Nitrogen [AnaLvst, Vol. 99 b--lOcm- Fig. 1. Unit for the en- trainment and trapping of hydrogen sulphide. The gas inlet tube (A) and the gas outlet tube (€3) are made from capillary tubing, which forms gas-tight seals with screw joints C and D.There is a bulb of about 2-ml capacity on the gas inlet tube. The multiple adaptor has two screw threads (C and D, size 13) and one ground-glass joint (E, 19/26). Also shown are the sample (F) and receiver (G) tubes RESULTS AND DISCUSSIOPU' A graph of hydrogen sulphide present, determined titrimetrically, against extinction is shown in Fig. 2. Each point represents one experiment, except for that with no hydrogen sulphide present, which is the mean of four experiments. The points are represented by the straight line y = 1 . 2 4 ~ + 0.046, but several points do not fall close to this line (residual standard deviation = h0.014). At first, the volume of hydrogen sulphide solution was plotted against the colorimetric reading but the scatter was large.It was found that the concentration of apparently saturated hydrogen sulphide solutions varied, but when the titrimetrically determined concentrations of hydrogen sulphide were used instead, the graph in Fig. 2 resulted. There was still some variation but it was much less; its cause has so far not been found. Several of the choices of apparatus or reagents made may appear to be arbitrary, but were found to be necessary for good recoveries. The apparatus was made from readily obtained parts, except that the capillaries were further shaped in the laboratory. The bulbs on the inlet capillaries are convenient for holding the alkaline hydrogen sulphide solution until it enters the sample tube, mixes with the hydro- chloric acid and releases the hydrogen sulphide gas.Sintered-glass diffusers were not employed in the apparatus as hydrogen sulphide was found not to be stable in contact with them. Fine capillaries are a satisfactory alternative. Brij detergent was incorporated in the trapping solution because at the level used it permits slight foaming to occur but not severe frothing. Without the detergent, hydrogen sulphide escapes from the recovery solution before it can be absorbed. The flow-rate of nitrogen was set at 20 ml min-1, which permitted complete removal of hydrogen sulphide from the sample tube in less than 90 minutes and its complete absorption in the receiving tube. In order to[September, 1974 MICRO-DETERMINATION OF HYDROGEN SULPHIDE 579 prevent oxidation, the apparatus was flushed with nitrogen so as to remove air before the hydrogen sulphide was added.Surprisingly, the rate of expulsion of hydrogen sulphide was slightly faster with hydrochloric acid than with sulphuric acid. 1 I I I I I I 0.1 0.2 0.3 0 Hydrogen sulphide presendpg Fig. 2 . Extinction of the solution after reaction with NN- dimethyl-p-phenylenediamine reagent plotted against the amount of hydrogen sulphide present, found by titration of a more concentrated solution. Blank values are not deducted but are plotted. The line shown is the best straight line, fitted to the experimental points by calculation The concentrations of zinc acetate, NN-dimethyl-9-phenylenediamine, iron and sulphuric acid in the final reaction mixture were fixed at the lowest levels that gave maximum reaction.The temperature of 18 “C was chosen because less colour was produced from a given amount of hydrogen sulphide at lower or higher temperatures. The NN-dimethyl-p-phenylenediamine reagent was 4 N with respect to sulphuric acid because more concentrated acid gives heating effects when mixed with the sample and these effects interfere with the colour yield. The method worked well on the hydrogen sulphide solutions examined in that the graph of the titrimetric versus colorimetric results was rectilinear. The method should therefore be suitable for use with any reactions in which hydrogen sulphide gas can be freed quantitatively from solution. REFERENCES 1. 2. 3. 4. 5 . 6. 7. Koren, H., and Gierlinger, W., Mikrochi,m. Acta, 1953, 220. Almy, L. H., J . Amer. Chem. Soc., 1925, 47, 1381. Sands, A. E., Grafius, M. A., Wainwright, H. W., and Wilson, M. W., Rep. Invest. U.S. Bur. Mines, Fogo, J. K., and Popowski, M., Analyt. Chem., 1949, 21, 732. Kuratomi, I<., Ohno, K., and Akabori, S., J . Biochem., Tokyo, 1957, 44, 183. Goa, J., Acta Chem. Scand., 1961, 15, 853. Vogel, A. I., “Textbook of Quantitative Inorganic Analysis,” Third Edition, Longmans, Green and Received April 22nH, 1974 Accepted May lst, 1974 R.I. 4547, 1949. Co. Ltd., London, 1961, p. 370.

ISSN:0003-2654    

DOI:10.1039/AN9749900577

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

580 AnaLyst, September, 1974, Vol. 99, pp. 580-587 The Determination of Nitrogen- 15 in Plant Material With an Emission Spectrometer BY C . P. LLOYD-JONES, G. A. HUDD AND D. G. HILL-COTTINGHAM ( Lrfiivc~sity of Rvisfol, Department of Agriculture and Horticulture, Research Station, Long ,4 shton, Bristol, BS18 9A F ) Instrumental and procedural modifications for the use of the Statron, Model NOI-5, emission spectrometer in the determination of nitrogen- 15 are reported. An interpolative method is recommended for estimating the background of the nitrogen-14 - nitrogen-15 peak. This method gives a linear relationship between the apparent percentage of nitrogen- 15 and the actual percentage of nitrogen-15 over the lower, most commonly used range. Assays repeated a t intervals over 2 months had standard errors of 0.75 per cent.The preparation of nitrogen gas samples for the emission spectrometer has been treated as a separate operation from the determination of total plant nitrogen. After a standard Kjeldahl digestion of the plant material, the ammonium ion is precipitated with a Nessler reagent. The supernatant is rejected and the precipitate treated with dilute hydrochloric acid to give an ammonium chloride solution, from which an aliquot is taken for a Dumas combustion in a discharge tube. Two operators can prepare and measure up to fifty samples per day. The method has been used to analyse a range of plant tissues from an apple nutrition experiment, during which replicate digestions and analyses gave values for nitrogen-15 enrichment that differed by an average of only 2.5 per cent.THE use of nitrogen-15 as a tracer in agricultural research has, until comparatively recently been limited by the need to use a mass spectrometer to determine the isotope. Emission spectroscopy, an alternative method for isotopic analysis, was first applied to the determina- tion of nitrogen by Broida and Chapman. More recently, the method has been developed and applied to agricultural studies by other workers. 2?3 Commercial instruments designed for the determination of nitrogen-15 are now available and are considerably cheaper than mass spectrometers and less demanding of their operators. One of these emission spectrometers, a modified Statron, Model NOI-5, has been found to give reliable, reproducible and accurate values for the percentage of nitrogen-15 in plant material.The principle of this technique is that when nitrogen gas contained in a discharge tube is excited by a high-frequency oscillator, the wavelengths of the light emitted depend on the isotopic composition. Thus in the 2-0 band, the nitrogen molecules 14N1*N emit light at wavelength 297.7 nm, 14N15N at 298-3 nm and 15N15N at 298.9 nm. In the Model NO14 spectrometer, the light produced in the nitrogen sample by a 27-MHz oscillator is analysed by a monochromator containing a rock-salt prism and fitted with an automatic scanning device that traverses once a minute the wavelengths corresponding to the nitrogen molecules of relative molecular masses 28, 29 and 30. The monochromator output is detected by a photo- multiplier and, after further amplification, the signal is fed to a chart recorder, which traces the variation of light output with wavelength and enrichments are calculated from measure- ments of the peaks.For the measurement of nitrogen-15, a Dumas technique4 is used in order to generate the nitrogen gas directly in the discharge tubes. For the analysis of plant material, it is necessary to take a substantial amount of sample so a5 to avoid sampling errors.6 The nitrogen con- stituents of plants are most commonly brought into solution by a Kjeldahl digestion procedure and the nitrogen-15 determinations are carried out on aliquots of these digests. However, the use of sulphuric acid makes it impossible to use these aliquots directly for Dumas combustions, and separation of the ammonia by distillation can result in memory effects and give rise to cross-contamination. * p 7 We have overcome this difficulty by precipitation of the ammonium ion with Nessler reagent.The precipitate is treated with dilute hydrochloric acid to give a Q SAC and the authors.LLOYD- JONES, HUDD AND HILL-COTTINGHAM 581 solution of ammonium chloride, aliquots of which are dried and used for Dumas combustions in discharge tubes. Our method for the preparation of samples for nitrogen-15 measurement is independent of the determination of the total chemical nitrogen, which is made on another aliquot of the Kjeldahl digest by a modern automatic technique. The procedure is sufficiently versatile to deal with materials that have a very wide range of nitrogen contents and nitrogen- 15 enrichments, and has a capacity of about fifty samples per day. EXPERIMENTAL APPARATUS- Zeiss Scientific Instruments Ltd.Emission spectrometer-Statron, Model NOI-5, nitrogen-15 analyser supplied by Carl Recorder-Ten-inch chart recorder, Type 28000, from Bryans Southern Instruments. Furnace-Tube furnace, Type MF3, from Severn Science Ltd. Controller-Model 101, indicating type, from Eurotherm. Vacuum equipment-Rotary diffusion pump (1-inch) combined unit. Pirani vacuum gauge, Model 9A. M.F. tester, Model T1, from Edwards Vacuum Components Ltd. REAGENTS- iodide in 100 ml of water. Nessler reagent-A solution of 1.35 g of red mercury(I1) iodide plus 1.2 g of potassium Sodium hydroxide solution, 10 N. Hydrochloric acid-1 N in water containing 10 per cent.V/V of ethanol. Digestion acid-A solution of 1 g of selenium in 200 ml of concentrated sulphuric acid. Dumas reagents-Calcium oxide (0.5 to 1.0 mm) and copper oxide (stick form). SAMPLE PREPARATION- Digestion and precipitation of ammonium ion-Weight about 150 mg of finely milled, dry plant material into digestion tubes, add 2 ml of digestion acid and digest for 4 hours. Allow the mixture to cool, dilute it to 20 ml with water, then take an aliquot from this diluted solu- tion for the determination of the total nitrogen by any standard method. After this stage, exact quantitative procedures are not required, as it is the ratio of nitrogen-14 to nitrogen-15 that is to be determined. Transfer a further aliquot, containing about 200 to 500 pg of nitrogen, of the diluted digest into a centrifuge tube.This operation is best carried out by pouring the solution into a calibrated tube; the use of a pipette is undesirable and could be a source of contamination. Add 10 N sodium hydroxide solution so as to make the final concentration approximately 0.4 N with respect to hydroxide. Stopper the tube, mix the contents well and leave at room temperature in the dark for 4 to 6 hours. Centrifuge the mixture at 2000 r.p.m. for 10 to 15 minutes, decant the supernatant liquor, invert the tube and allow it to drain for 5 minutes. Add about 1 ml of ethanolic hydrochloric acid to the precipitate, the volume being chosen to give a concentration of about 500 pg ml-l of nitrogen. Mix the acidic solution thoroughly and then transfer it into a tapered centrifuge tube; stopper the tube and leave it in a refrigerator overnight.Centrifuge the mixture for 10 to 15 minutes at 2000 r.p.m. and remove any solid that remains in the surface film of the solution. Fill a short length of glass tubing (0.d. 3 mm; i.d. 2-5 mm) with the solution by capillary action. The length of tube should be such that the solution it holds will contain about 10 pg of nitrogen. Wipe the outside of the tube and evaporate the solution to dryness in a warm atmosphere. The tube, containing a deposit of about 10 pg of nitrogen as ammonium chloride, is then ready for the combustion procedure. Add to this tube 4 ml of Nessler reagent and mix well. COMB USTI ON- Place 2 to 3 g of calcium oxide in a silica tube, evacuate the tube and, when the pressure is be low 0.13 Nm-2, heat the calcium oxide gently with a blow-torch until the initial vigorous reaction subsides, then strongly to red heat.Allow the tube to cool under vacuum. Pyrex glass sample tubes (Fig. 1) were made locally and incorporate cheap unground sockets that can be sealed with wax to the ground cone joints of the vacuum manifold. This manifold accommodates up to six sample tubes, and by use of a ground-glass joint they can be rotated from the horizontal The calcium oxide is reactivated before use in the Dumas combustion.582 LLOYD-JONES et al. : THE DETERMINATION OF NITROGEN-15 [Analyst, VOl. 99 to the vertical position as required. Place 15 to 20 mg of the freshly activated calcium oxide into section A of a sample tube, then add 15 to 20 mg of stick copper oxide together with the tube containing the deposit of ammonium chloride in section D. Keeping the tube horizontal so as not to disturb the contents, attach it to the vacuum manifold by the joint at D and seal with wax.With a vacuum below 0.013 Nm-2 applied to the tube, heat the end of the tube that contains the calcium oxide so as to ensure it is fully activated and then gradually heat the whole tube as far as the constriction C until the flame becomes bright orange. Apply the high-frequency tester to the hot tube for about 10 s to assist in the desorption of gases from the walls of the tube. When the tube has cooled, move it from the horizontal to the vertical position by rotation of the vacuum manifold so that the copper oxide and capillary tube drop out of section D.Allow the detached discharge tube to cool, then tip the calcium oxide into section B with the copper oxide and sample capillary. Produce the nitrogen gas by a Dumas combustion in a furnace at 575 “C for 5 to 6 hours. Fit the discharge tube into the emission spectrometer for measure- ment. Tubes prepared by this technique can be re-measured at intervals over many months if required. The unground sockets, after removal from the manifold, can be re-incorporated into new sample tubes. Seal the tube at constriction C, and pull it away from section D. A B C D 50 mm Fig. 1. Sample tube as attached to manifold. For key to lettering, see text NO14 EMISSION SPECTROMETER- Preliminary investigations showed that the instrument as received required modification, in that there were frequent failures of the main fuse owing to inrush current.The signal to noise ratio was unsatisfactory, and the photomultiplier output was non-linear and varied unpredictably with H.T. voltage. All of the work reported here was carried out on the instrument after modification, as described below. Modi$cations-A surge-limiting resistor of 100 I2 (10 W) was fitted between the trans- former and bridge rectifier of the power supply to the H.F. oscillator. The output of the amplifier was increased from a nominal 100 mV to a nominal 1000 mV by use of an alternative tapping on the output resistor chain. The photomultiplier was changed to an EM1 9824 B with a dynode chain of 220-kQ resistors and a 150-V Zenor diode (IN 5276 B Motorola) between the cathode and first dynode.The last dynode was decoupled to ground with a 0.94-pF capacitor, and the next three dynodes were decoupled t o it with capacitors of 0.47, 0.22 and 0.1 pF, respectively. The magnetic shield for the photomultiplier was suitably insulated and run at cathode potential. The wavelength at which the micro-switch changes the attenuation of the input to the amplifier (referred to hereafter as “gain”) was altered by cutting a new operating cam (see below). Instrument operation-The analyser was operated with a slit width of 0-04 mm and height 3 mm, as recommended. As the amplifier output limits at about 800 mV, the amplifier gain was adjusted so as to bring the maximum output at the peaks to not more than 700 mV.The photomultiplier was run at a constant 1150 V H.T. Once a discharge tube had been aligned correctly and the appropriate gain settings found, any minor adjustments required to bring the peak heights to near full-scale deflection on the chart were made by using the recorder gain control. From each tube, four consecutive scans were recorded for measurement and the mean ratios used for the calculation of enrichments. The variation of light output with wavelength from a sample of 4-15 per cent. nitrogen-15 is shown in Fig. 2. For ease of measurement, the peaks are brought to comparable size by the use of different gain settings, pre-set by the operator to the appropriate values. Thus, scan B shows a trace suitable for measurement where peaks 30 and 29 were recorded at gain x 3000 and peak 28 at Trace A was recorded at constant gain ( x 300) throughout the scan.September, 19741 IN PLANT MATERIAL WITH AN EMISSION SPECTROMETER 583 x300.The ratio of the gains used for recording the peaks is required for calculation of the abundance. With our instrument, no corrections were required to the calibration of the gain controls. The change-over is accomplished by the micro-switch on the scanning device. RESULTS CALCULATION AND CALIBRATION- If the relative heights of the peaks for nitrogen-28, -29 and -30 molecules are known, then the percentage of nitrogen-15 can be calculated by using standard equations. At low abundance, the heights of peaks 28 and 29 are used to give the ratio R = height,,/height,, for insertion in the expression nitrogen-15 (per cent.) = 100/(2K + 1).At higher percent- ages of nitrogen-15, when the peaks 29 and 30 are more conveniently used, the corresponding expressions are R’ = height,,/height,, and nitrogen-15 (per cent.) = 100/(+R’ + 1). The instrument requires calibration with known standards because of the limited resolu- tion. From the measured heights of the peaks, an apparent percentage of nitrogen-15 is calculated by the standard equations and this value is then corrected to the true value from a calibration graph prepared by using samples of known abundance. A B C c X300 ” 4 + - ~ 3000 +-x 300 -;-x 3000--+ Fig. 2. Variation of light output with wavelength from a sample containing 4.16 per cent. of nitrogen-16, indicating the use of two different amplifier settings and alternative methods of measuring peak 29.Scan A is recorded a t a constant gain ( x 300). In scan B, peak 28 is recorded at x 300 and peaks 29 and 30 at x 3000. Scan C: as in scan B, but using modified cam for interpolative method In order to obtain the height of peak 29, the manufacturers recommend measuring from the top of the peak to the dip between peaks 29 and 30 (Fig. 2, scan B). We have found that when using this method over the range 0.366 to 8.0 per cent. of nitrogen-15, the results give a calibration graph with a slight but definite curvature that cannot be represented by any simple relationship (Table I). An alternative method8 for the measurement of the height of peak 29 is to interpolate between the dip 28 to 29 and the dip 29 to 30 (Fig.2, scan C). When using this method with our instrument, the apparent percentage of nitrogen-15 is linearly related to the actual values and can be corrected by the simple expression: Actual nitrogen-15, per cent. = 1.083 x (apparent nitrogen-15, per cent.) - 0,022 If the results obtained for the interpolative method in Table I are corrected by this expression,584 LLOYD-JONES et id. : THE DETERMINATION OF NITROGEN-15 [AIZdySt, VOl. 99 there is virtually negligible error: Actual nitrogen-15, per cent. . . 0.366 0.718 1-36 2.11 4.15 6-11 8.0 Corrected value, per cent. . . 0-366 0.714 1.35 2.10 4.16 6.10 8-07 We prefer to use the interpolative method and apply a simple arithmetic correction to the apparent values rather than read each individual correction from a constructed graph.In the instrument as supplied, the change of gain between peaks 28 and 29 takes place as in Fig. 2, scan B. For the interpolative method, it is essential that the dip 28 to 29 is scanned at the same gain as the dip 29 to 30, hence the necessity to cut a new cam to alter the time at which this change takes place (Fig. 2, scan C). For the manufacturers’ recommended method, a chart speed of about 0.5 mm s-1 is adequate, but the interpolative method requires a chart speed of about 2 mm s-l for peak 29 and associated dips. A device has been developed to provide this higher chart speed automatically for peak 29, reverting to 0-5 mm s-l for peak 2g9. TABLE I CALIBRATION OF APPARENT PERCENTAGE OF NITROGEN-15 Comparison of results obtained using alternative methods of measuring background for peak 29 Actual nitrogen-15, per cent.0.366 0.718 1-36 2.1 1 4.15 6.1 1 8.0 Apparent nitrogen-15, per cent. 7 A \ Calculated from dip 29 to 30 as base Calculated from interpolated base I A 1 r -l Replicate tubes Mean Replicate tubes Mean 0-4980, 0.5 150, 0.4962, 0.4798, 0.495 0.357, 0.350, 0.361, 0.362, 0.358 0.9022, 0.9028, 0.8832 0.896 0.688, 0.679, 0,672 0.680 1.639, 1.614, 1.576 1.61 1.274, 1.266, 1.274 1.27 2.455, 2.366, 2.378 2.40 1.995, 1.956, 1.942 1.96 4.528, 4.584, 4.552 4.55 3.818, 3.901, 3.864 3.86 6.613, 6.565, 6.537 6-57 5.660, 5.707, 5.506 5.66 8.603, 8.671, 8.576 8.62 7*441,7*504,7*476 7.47 0-4970, 0.4824, 0.4968 0.348, 0.370, 0.358 There is no advantage in continuing to use the interpolative method beyond 8 per cent.of nitrogen-15, and therefore above this value we use the dip 29 to 30. Over the range 8 to 25 per cent. of nitrogen-15 the ratio of the heights of peaks 28 and 29 gives apparent abundance values that are linearly related to the actual values by the equation Actual nitrogen-15, per cent. = 1.112 x (apparent nitrogen-15, per cent.) - 1-73 Above 25 per cent. of nitrogen-15, the values are calculated from the ratio of the heights of peaks 29 and 30 and a calibration graph is required. When using nitrogen-15 in nutrition experiments, most samples fall into the lowest range, and therefore in practice the interpolative method described above is that most commonly employed. In investigations with plants, it is more meaningful to subtract the naturally occurring nitrogen-15 (0.366 per cent.) from the total measured on a sample and thus to express the results as atom per cent. excess.INSTRUMENT REPRODUCIBILITY- The results in Table I1 obtained from four consecutive scans of three discharge tubes prepared from plant material with different nitrogen-15 enrichments demonstrate the excellent reproducibility. Four replicate scans were therefore judged to be sufficient, and this number was adopted as standard. TABLE I1 REPRODUCIBILITY OF REPEAT SCANS OF DISCHARGE TUBES Nitrogen-15, per cent. Standard Standard error < A \ error, as a percentage Sample Four consecutive scans Mean per cent. of the mean A 0.6951, 0.7138, 0.6982, 0.7060 0-703 0.008 1.1 B 3.718, 3.762, 3.740, 3.718 3.73 0.02 0.6 C 6.841, 6.891, 6.868, 6-903 6.88 0.03 0.5September, 19741 IN PLANT MATERIAL WITH AN EMISSION SPECTROMETER 585 Two discharge tubes were assayed on sixteen different occasions over a period of 2 months, and the results given in Table I11 demonstrate the long-term stability of the instrument.TABLE 111 REPEAT ASSAYS FOR PERCENTAGE OF NITROGEN-15 CARRIED OUT ON 16 DIFFERENT OCCASIONS OVER 2 MONTHS USING TWO DISCHARGE TUBES Tube D was prepared by Dumas combustion; tube E was supplied by manufacturers Nitrogen-15, per cent. Standard error A r-- - Standard as a Repeat assays error, percentage Sample (each the mean of 4 scans) Mean percent. of the mean D 0.3612, 0.3627, 0.3617, 0.3672 0,3617 0.0027 0-75 0.3604, 0.3608, 0.3623, 0.3645 0.3621, 0.3623, 0.3645, 0.3552 0-3597, 0.3595, 0.3637, 0.3588 1.519, 1.517, 1.539, 1.535 1.534, 1.529, 1.542, 1.515 1.545, 1.515, 1-522, 1.531 E 1.510, 1.510, 1.522, 1.517 1 -525 0.0114 0.75 COMPARISON OF PRECIPITATION WITH DISTILLATION- A comparison was made of the results obtained using either Nessler precipitation or distillation and evaporation for the removal of ammonia in preparation for a Dumas com- bustion.Two Kjeldahl digests of plant tissues were enriched by the addition of known amounts of nitrogen-15 as ammonium chloride; three replicates from each were precipitated and three were steam-distilled in a Markham apparatus into dilute hydrochloric acid and this distillate was evaporated in a Buchler Evapomix. From each of the six samples of ammonium chloride thus obtained, two replicate discharge tubes were prepared and measured on the spectrometer.The results, given in Table IV, show that the over-all mean values obtained by the precipitation procedure are virtually identical with the calculated enrichments. With only one exception the individual results obtained by distillation are slightly lower than the calculated values, although the over-all means do not differ significantly. TABLE IV COMPARISON OF RESULTS OBTAINED WHEN NH,t IS SEPARATED FROM PLANT DIGESTS BY EITHER NESSLER PRECIPITATION OR DISTILLATION Nitrogen-15 found, per cent. , 7 ~~ Nessler precipitation Distillation A A I \ f \ Nitrogen- 15 Mean from Mean from in digest, Replicate each Over-all Standard Replicate each Over-all Standard per cent. tubes precipitation mean deviation tubes distillation mean deviation 0.457 0-461,0.454 0.450 0.456 0.007 0*444,0*447 0.446 0.446 0.005 (1-4 per 0.455,0.439 0.447 (1 -2 per 0*466,0.453 0.460 0*447,0*457 0.452 cent.) 0.446, 0.443 0.444 cent.) 0.880 0.863, 0.883 0-873 0.880 0.009 0.871,0.865 0.868 0.871 0.012 (1.0 per 0.888, 0.875 0.882 (1 -4 per 0.879, 0.881 0-880 0.888, 0.884 0.886 cent.) 0.851, 0.875 0.863 cent.) APPLICATION TO PLANT NUTRITION EXPERIMENT- About 300 samples of plant tissues obtained from a typical nutrition experiment with apple trees were digested and analysed for nitrogen-15 at intervals over a period of 2 to 3 months.As a check on the reproducibility of the whole procedure, several samples were re-analysed at a later date and some results, given in Table V, were selected to cover a range of type of tissue, of different total nitrogen concentration and of enrichment.These results show that the procedure was equally successful with all of these plant tissues, although there was about a 30-fold range in their total nitrogen concentrations. For these twelve samples, the maximum difference between repeat analyses was just over 6 per cent.; the average586 LLOYD-JONES e.t d. : THE DETERMINATION OF NITROGEN-15 [Analyst, VOl. 99 difference was 2-5 per cent., which is an acceptable value in the context of plant nutrition experimentation. DISCUSSION The Statron NOI-5 emission spectrometer, as modified, has been found to be reliable, reproducible and stable over long periods. The degree of accuracy obtained (standard error 0.5 to 1.4 per cent.) is eminently suitable for nitrogen-15 analysis in plant nutrition experi- mentation.The equivalent error of a mass spectrometer has been given as 0.1 to 0.6 per cent.,b a value that does not justify the difference in cost and the extra complexity of the technique for this type of work. In addition, it should be noted that the emission spectrometer requires only 1Opg of nitrogen for a determination of the enrichment, whereas a mass spectrometer may need up to 1 mg of nitrogen.1° Hence, with the emission spectrometer, there is the opportunity to determine the enrichment of very small amounts of nitrogen, such as eluates from chromatographic columns, provided that the samples are homogeneous. TABLE V VALUES OBTAINED FROM DUPLICATE DIGESTION AND NITROGEN-15 ANALYSES OF SAMPLES FROM A TYPICAL EXPERIMENT WITH APPLE TREES TREATED WITH LABELLED FERTILISER Code No.Tissue 157 Leaves 4 243 37 Stem bark 17 44 169 Stem wood 106 27 1 23 Roots 21 24 1 Nitrogen-15, atom per cent. excess Total nitrogen, per cent. of A \ dry mass 1st digestion 2nd digestion 3-80 0.628 0.655 1.68 6.70 6.97 1.39 2.62 2-68 1-77 0.235 0.24 1 1.66 0.783 0.763 1.39 3.01 2.89 1.02 0.283 0.279 0.45 3.2 1 3-15 0.13 0.593 0.556 2-05 3.04 3-0 1 1-56 1.93 1 -92 0.387 0.860 0.808 In order to interpret nutritional experiments, it is necessary to determine the total plant nitrogen chemically as well as nitrogen-15 enrichments. Solutions that have been analysed by standard automatic colorimetric methods are unsuitable for subsequent isotopic analysis, and the older technique for the determination of nitrogen involving distillation of ammonia can incur isotopic contamination.In order to take advantage of the automatic colorimetric procedure and to exploit fully the capacity of the emission spectrometer, we have developed the technique described above in which chemical analysis and isotopic analysis have been treated as separate operations. In addition, the emission spectrometer, like the mass spectro- meter, measures the ratio of nitrogen-14 to nitrogen-15 and, provided that the sample is representative, there is no need for the preparative procedure to be fully quantitative. Hence non-quantitative manipulations can be used in order to shorten the procedure by elimination of rinsing and washing precipitates, and the possibilities for cross-contamination of samples during dispensing are reduced.By using the simple precipitation procedure with a Nessler reagent, about 80 per cent. of the nitrogen in the digest was recovered in the ammonium chloride solution used for the Dumas combustion. There is no evidence that isotopic fractionation occurs during this procedure, as shown by the results in Table IV. With one operator dealing with sample preparation and one operating the instrument, about fifty samples per day can be analysed. With this instrument, the nitrogen-15 technique now comes within the capacity of most analytical laboratories. The authors thank Mrs. J. S. Adam and Mrs. D. R. Cooper for their assistance during these investigations, and also Mr. B. Perkins for construction of the vacuum manifold and the production of sample tubes.587 September, 19741 IN PLANT MATERIAL WITH AN EMISSION SPECTROMETER REFERENCES 1 . 2. 3. 4. 5. 6. 7. 8. 9. 10. Broida, H. P., and Chapman, M. W., Analyt. Chem., 1958, 30, 2049. Leicknam, J . P., Middelboe, V., and Proksch, G., Analytica Chim. Acta, 1968, 40, 487. Goleb, J. A., and Middelboe, V., Ibid., 1968, 43, 229. Faust, H., Isotopenpraxis, 1967, 3, 100. Fiedler, R., and Proksch, G., Plant Soil, 1972, 36, 371. Bremner, J. M., Cheng, H. H., and Edwards, A. P., “The Use of Isotopes in Soil Organic Matter Martin, A. E., and Ross, P. J., Trans. 9th Int. Congr. Soil Sci., 1968, 3, 521. Ferraris, M. M., and Proksch, G., Analytica Chim. Acta, 1972, 59, 177. Skerrett, E. J., and Lloyd-Jones, C. P., in preparation. Jansson, S. L., “The Use of Isotopes in Soil Organic Matter Studies,” Pergamon Press, Oxford, Received February 21st, 1974 Accepted May 21st, 1974 Studies,” Pergamon Press, Oxford, 1966, p. 429. 1966, p. 415.

ISSN:0003-2654    

DOI:10.1039/AN9749900580

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

588 Analyst, September, 1974, Vol. 99, #$. 588-594 Atomic-absorption Determination of Some Common Trace Elements in Aluminium Oxide and Other Aluminium Compounds Using a Co-precipitation Separation Technique BY P. N. W. YOUNG* (New Zealand Aluminium Smelters Limited, Invercargill, New Zealand) Atomic-absorption spectroscopy has been used to determine micro- gram amounts of calcium, iron, manganese, silicon, titanium, vanadium and zinc in aluminium oxide and other aluminium-rich materials after co- precipitation of these elements on zirconium hydroxide from an alkaline solu- tion of the sample. The proposed method serves both to separate the trace elements from aluminium and also to concentrate the elements, thus improving both accuracy and sensitivity. The method is shown to be as accurate and precise as colorimetric procedures, very much quicker and less tedious.The trace constituents are determined in the range 5 to 400 p g in the final 25 ml of sample solution on a 2-g sample of aluminium oxide, and in the range 10 to 2000 pg on a 0-5-g sample of other materials (25-nil final volume). Interference effects are reduced to a minimum by separation of the elements from aluminium and the provision of closely matched standard solutions. THE determination of microgram amounts of various elements in aluminium oxide and other aluminium-rich materials presents problems that are not easily solved by the use of conven- tional analytical techniques. Current methods of analysis for these trace impurities, mainly colorimetric, give reliable results but are time consuming and require lengthy separation procedures together with closely controlled experimental conditions.Although the more sensitive elements such as sodium and calcium can be determined by direct atomic-absorption spectro- scopy, inter-element effects are encountered from the matrix element and either releasing agent^^,^ or closely matched standards must be used to counteract these effects. Refractory elements such as vanadium, titanium and silicon present at or below the 100 p.p.m. level are especially difficult to determine by direct instrumental methods. Separation and concentra- tion procedures for the determination of certain trace elements in aluminium compounds have been r e p ~ r t e d , ~ but generally are time consuming and frequently require the use of more than one separating reagent or technique for the determination of several trace impurities.In the present paper a method is reported for the determination of calcium, iron, mang- anese, silicon, titanium, vanadium and zinc in aluminium oxide and combinations of these elements in other aluminium-rich materials by their separation and concentration with a single co-precipitant in combination with sodium hydroxide solution. The proposed proce- dure allows for the determination of the seven elements by direct atomic-absorption spectro- scopy on a single solution without further dilution, which is free from matrix interferences. The elements calcium, iron, titanium, silicon and vanadium are determined by the use of a, nitrous oxide - acetylene flame while manganese and zinc are determined with the air - acetylene flame.Features that make the technique of co-precipitation as a separation technique especi- ally attractive are as follows. Determination by direct instrumental methods is difficult. * Present address : Waste Water Treatment Laboratory, Invercargill City Council, P.O. Box 7, Invercargill, Xew Zealand. 0 SAC and the author.YOUNG 589 By use of a suitable co-precipitating element or precipitating reagent, or both, it is possible to precipitate trace elements quantitatively. I t is possible by choice of a suitable selective precipitant to isolate individual trace elements. It is also possible by the use of a non-selective reagent to determine several trace elements in a given sample on a single separation. By isolation of the trace elements from matrix elements and other potential interference effects accuracy and sensitivity will be improved.Several papers have appeared on the application of co-precipitation to the determination of trace amounts of impurities in various materials but none has, as yet, dealt specifically with the separation of several impurities in aluminium oxide and similar materials by the use of a single co-precipitating reagent. Marshall and West reported on the atomic-absorption determination of trace amounts of calcium and magnesium and iron and nickel in aluminium salts by co-precipitation with iron(II1) hydroxide and hydrated manganese(1V) oxide, respectively, Burke6 has reported on the determination of trace amounts of antimony, bismuth, tin and lead in nickel and copper-base systems with the aid of a co-precipitation separation procedure.Luke' has published a comprehensive introductory paper to the technique of co-precipitation as a concentration and separation method for the determination of trace impurities, DEVELOPMENT OF METHOD Analysis of aluminium oxide for trace impurities in order to comply with specification requirements calls for the determination with reasonable accuracy of calcium, iron, manganese, phosphorus, sodium, silicon, titanium vanadium and zinc. Other materials presented to the laboratory such as cryolite, aluminium fluoride and miscellaneous samples with a high aluminium content required the determination of calcium, iron, silicon and phosphorus.Of the foregoing elements, sodium (present at about the 0.5 per cent. level) can easily and conveniently be determined by direct atomic-absorption spectro- scopy ; phosphorus, however, cannot readily be determined by conventional atomic-absorption spectroscopy as its strong resonance lines lie below 190 nm and therefore cannot be utilised by normal laboratory instrumentation. These two elements were disregarded and investigations were then carried out on the feasibility of separating the remaining elements with the aid of a single co-precipitant or precipitating reagent, or both. Consideration of the properties and known reactions of the various elements prompted investigation into the possibility of precipitating their oxides or hydroxides in alkaline solution with the aid of a suitable carrier.Sodium diethyldithi~carbamate,~ in combination with ammonia solution, has been shown to be a valuable non-selective reagent for the co-precipita- tion of a large number of elements. This reagent was, however, found to be unsuccessful under the existing conditions because of the simultaneous precipitation of insoluble aluminium hydroxide. Use of sodium hydroxide, instead of ammonia, in combination with sodium diet hyldithiocarbamate, although effective in keeping aluminium in solution as soluble sodium alumiriate (NaAlO,), was not successful. Attention was then focused on those elements other than iron, manganese and titanium which form hydroxides that are insoluble in solutions made alkaline with sodium hydroxide as possible co-precipitants.Iron and manganese have previously been used as co-precipitants5 and of the other reagents investigated zirconium as the hydroxide Zr(OH), was found to be extremely effective in precipitating the required elements. Quantitative co-precipitation of the elements occurred when 8 mg of zirconium (maximum level of trace impurity of 0.4 mg) was added to a solution of the sample made alkaline with sodium hydroxide; in practice, 10 mg of zirconium is used [lo ml of zirconium solution (see under Reagents)j. Close control of pH does not appear to be necessary provided the pH is higher than 9. At or above this pH there is no tendency for aluminium to co-precipitate. Other possible co-precipitants investigated included thorium and cerium in combination with sodium hydroxide, a mixture of sodium dietliyldithiocarbamate, copper(I1) and sodium hydroxide and precipitation with hydrogen sulphide, iising bismuth as a carrier, in acidic solution.Although some success was achieved with these reagents, results obtained with zirconium and sodium hydroxide were by far the most successful.590 YOUPU’G : ATOMIC-ABSORPTION DETERMINATION OF TRACE ELEMENTS [Analyst, VOl. 99 CHOICE OF CALIBRATION RANGE- Calibration graphs were prepared by precipitating and determining known amounts of the trace elements in the manner to be used in the actual analysis. It is essential for blank and calibration solutions to contain the same amount of sodium hydroxide in the preparative stage as in the sample solution. [Even analytical-reagent grade sodium hydroxide pellets contain substantial amounts of impurities such as silicon and iron (up to 0-005 per cent.).] For a 2-g sample of aluminium oxide based on a final volume of 25 ml, the calibration range for each element is set out in Table I below.TABLE I CALIBRATION RANGE FOR ELEMENTS Element Calcium Iron Manganese Silicon Titanium Vanadium Zinc Concentration rangelpg per 25 ml 100-400 50-300 0-50 50-400 0- 100 0-100 0-100 Calibration range/pg ml-l 0-20 0-20 0-5 0-20 0-20 0-20 0-5 Only one set of standards containing all of the above elements in appropriate proportions For a 0-5-g sample of cryolite, aluminium fluoride or similar material based on a is necessary. final solution volume of 25 ml, the relevant calibration values are set out in Table 11.TABLE I1 CALIBRATION RANGE FOR CALCIUM, IRON AND SILICON Concentration Calibration Element range/pg per 25 ml range/pg ml-1 Calcium 10-200 0-10 Iron 10-1 000 0-50 Silicon 50-2000 0-1 00 Although direct atomic-absorption spectroscopy may suffice to determine some of the elements in Table 11, when present at a moderately high concentration (up to 0.4 per cent. total), it is preferable to separate and concentrate the analytes because of the increased sensitivity, accuracy and freedom from matrix effect. The increased time factor involved in co-precipitation, etc., at the most amounts to 1 hour. Because of the higher concentration of iron and silicon present 15mg of zirconium are added as co-precipitant. INTERFERENCE EFFECTS- Interferences resulting from inter-element effects mainly associated with the matrix element aluminium are almost entirely removed by the separation procedure.Other possible inter-element effects are reduced to a minimum by matching of standard and sample solutions and because of the dilution effect of solutions containing low concentrations of the analytes. PROPOSED METHODS APPARATUS- A Varian Techtron, ModelAA5, atomic-absorption spectrophotometer equipped with both air - acetylene and nitrous oxide - acetylene burner heads was used. Varian Techtron hollow- cathode lamps were used as source lamps and a Model D1-30 digital indicator unit in the absorbance mode was used to facilitate data read-out. INSTRUMENT OPERATING PARAMETERS- Calcium, iron, titanium, silicon and vanadium are determined by using a nitrous oxide - In all instances the burner height was shown not to be a critical factor but Best sensitivities were obtained with a Zinc and manganese were determined by using a Instru- acetylene flame.was maintained at 10 mm for all of the elements. “red feather’’ height of about 10mm. fuel-lean air - acetylene flame and the burner height was again maintained at 10 mm. -n-+ ,-n++:nmn T T 7 n - n n e f n l l n x x T m -September, 19741 IN ALUMINIUM COMPOUNDS WITH SEPARATION BY CO-PRECIPITATION 591 Ca Fe Mn Si Ti V Zn Wavelength/nm 422.7 248.3 279.5 251.6 364.3 318.5 213.9 Spectral band width/nm 0.32 0.32 0.16 0.20 0.16 0.16 0.50 Acetylene flow-rate/l min-l 4.5 4.5 1.5 5.0 4.5 4.5 1.5 8.5 Lamp current/mA 6 8 5 15 10 20 6 The acetylene flow-rates in the above table should be used only as a general guide.For silicon, the flow must be carefully adjusted to give a flame with the maximum “red feather” height without luminescence. REAGENTS- demineralised water was used for all dilutions. - Nitrous oxide flow-rate/l min-1 7.7 7.7 7.7 7.7 7.7 - Air flow-rate/l min-l - - - - - 8.5 Analytical-reagent grade acids and reagents were used unless otherwise stated, and Hydrochloric acid (sp. gv. 1.18). Boric acid. Lithium carbonate. Sodium hydroxide, pellets-Baker, analysed reagent, low in silica. Zirconyl nitrate-Hopkins and Williams laboratory reagent, recrystallised. Silicon stock solution, 1000 pg mZ-l-Fuse 1.0700 g of freshly ignited silica with about 10 g of sodium carbonate in a platinum crucible, extract the melt with dilute hydrochloric acid (1 + 10) and make the volume up to 500 ml.Stock solutions of elements-Prepare 1000 pg ml-1 solutions of calcium, iron, manganese, titanium, vanadium and zinc from analytical-reagent grade salts of the metals (the titanium stock solution should be prepared in 6 M hydrochloric acid in order to prevent hydrolysis). Dilute the solutions further for use as calibration standards. Zirconium working solution, approximately 1 mg ml-l-Dissolve 0.50 g of recrystallised zirconyl nitrate in 10 ml of hydrochloric acid (1 + 1). Dilute the solution to 100 ml. Sodium hydroxide solution, 20 per cent. m/V-Dissolve 200.0 g of sodium hydroxide pellets in water and dilute the solution to 1 litre. Either of the two sample dissolution procedures is used. The method of choice is depen- dent upon the availability of reagents, glass tubes, etc.Method I1 is normally used only for aluminium oxide samples because of sample dissolution problems that sometimes occur in the fusion method. METHOD I- Accurately weigh approximately 2 g of aluminium oxide or 0-5 g of cryolite, aluminium fluoride or other material into a 70-ml capacity platinum dish, together with 2.8 g of lithium carbonate and 6.2 g of boric acid. Mix thoroughly and heat the materials in an electrical muffle furnace for 1 hour at 1000 “C. Extract the resulting molten bead by warming it on a hot-plate with a mixture of 10ml of hydrochloric acid (sp. gr. 1-18> and 20ml of water, transferring the solution and washings into a 600-ml beaker. Then submit either an aliquot or, more usually, the whole solution to the following co-precipitation procedure.If an aliquot is required, make the solution up to 200 ml, transfer the required volume of solution into a 600-ml beaker and dilute it to approximately 200 to 300 ml. Add 10.0 ml (or 15-0 ml for materials other than aluminium oxide) of the zirconium working solution followed by an excess of sodium hydroxide solution (adding a similar amount of sodium hydroxide solution to the blank and standard solutions). Stir the precipitate of zirconium hydroxide well, then warm it on a hot-plate for about 15 minutes before filtering it off on a No. 540 Whatman filter-paper. Wash the precipitate on the filter with water and transfer the paper and funnel into a clean dry 25-ml calibrated flask.Dissolve the precipitate on the filter by the slow addition of 7 ml of hot hydrochloric acid (1 + 1) followed by two washings with 2 to 3ml of hot water. Cool and dilute the contents of the flask to the mark with water. Run a blank using identical amounts of reagents with each sample solution. Sample and blank solutions are then ready for direct aspiration and determination of the analyte elements. Preparation of calibration solutions-Into each of four 600-ml beakers, transfer 10.0 ml (or 15.0 ml) of zirconium working solution, approximately 200 ml of water, suitable volumes of analyte working stock solutions and a known volume of sodium hydroxide solution. Proceed as specified for sample solutions in Method I after precipitation of zirconium hydroxide. Store in a plastic container.592 YOUNG : ATOMIC-ABSORPTION DETERMISATION OF TRACE ELEMENTS C.4 nalyst, Vol.99 METHOD II- Introduce 1.000 g of the sample into a clean dry glass “Vycor” tube that is sealed at one end (the glass tube should have approximate dimensions as follows: wall thickness, 1.2 mm; internal diameter, 10.6 mm; and length, 450 rnm). Add 7.3 ml of hydrochloric acid (sp. gr. 1-18> and seal the open end of the tube. Shake the contents so as to ensure thorough mixing. Place the tube in a steel guard-tube and heat in an electric oven at 250 “C for about 16 hours. Allow the tube to cool and carefully open one end by cutting and breaking it. Filter the solution through a Whatman No. 540 filter-paper into a 100-ml calibrated flask and dilute to the mark.Again submit either an aliquot or the whole solution to the co-precipitation procedure as described in Method I. DETERMINATION OF ELEMENTS- Set up the instrument for each element under the specified conditions. Aspirate blank and calibration solutions followed by the sample solutions. Plot absorbance versus concen- tration for each element and obtain the concentration of each element in the sample from the graphs. RESULTS Table I11 shows the results obtained for four analysed spectrographic standards, together with accepted values and our values obtained by alternative (mainly colorimetric) methods of analysis. TABLE I11 COMPARISON OF RESULTS ON ANALYSED SPECTROGRAPHIC STANDARDS Element content, p.p.m. f A -l Spectrographic standard Low- Accepted AA4S - co-precipitation Colorimetric Working- Accepted AAS - co-precipitation Colorimetric Accepted AAS - co-precipitation Colorimetric Accepted AAS - co-precipitation Colorimetric Medium- High- Ca 332 320 310 672 681 665 590 600 582 871 890 895 ITe 42 45 38 53 60 62 53 60 65 500 485 510 Mn 4 6 6 13 10 10 9 11 15 27 20 22 Ti 21 25 22 36 39 40 27 20 28 63 75 72 Si 191 200 198 163 165 168 247 225 220 47 1 485 477 V Zn 36 33 40 - - - 117 70* 120 98” 110 - 262 - 230t 275 2957 284 * 100 p.p.m.of vanadium added. t 300 p.p.m. of vanadium added. AAS denotes atomic-absorption spectroscopy. Table IV shows the results of analysis of a cryolite, an aluminium fluoride and a miscel- laneous sample. The results obtained with the proposed method show good agreement with accepted values and results obtained with other methods of analysis.The precision of a series of analyses conducted for calcium, iron and titanium by the proposed method is contrasted with the values for precision obtained by using International Standards Organisa- tion approved colorimetric methods for determining these elements. Results show good agreement on standard deviations for the two methods of analysis. Vanadium was the only element tested that showed a relatively large deviation from the expected values but as it is almost always present in aluminium oxide a t concentrations below 0.005 per cent. the discrepancy is not considered to be serious. The results given in Table V show the precision of the proposed method.September] 19741 I N ALU1\IIKIUM COMP0UNI)S WITH SEPARATION BY CO-PRECIPITATION 593 TABLE IV COMPARISON OF RESULTS ON MISCELLANEOUS MATERIALS Sample Method Iron, p.p.ni.Silicon, 13.p.m. Calcium, p.p.m. Cryolite C12 AAS - co-precipitation 74 1220 - Colorinictric 70 1235 - Aluminium fluoride AAS - co-precipitation 24 1012 I Colorimetric 31 1031 - I h s t 1‘4 v/s AAS - co-precipitation 1700 766 83 Colorimetric 1721 781 78 AA4S denotes atomic-absorption spectroscopy. Table IV shows the method to be acceptably accurate even for determination of element concentrations at about the 0.1 per cent. level. Even with the comparatively high values, the proposed method was quicker and at least as accurate as conventional methods. In addition] the increased sensitivity and freedom from matrix effects makes this the preferred method for the determination of the elements in question.Inter-element effects, ii any, due to the relatively large excess of silicon are eliminated by using closely matching sample and standard solutions with respect to the analyte elements. TABLE V (SPECTROGRAPHIC WORKING STANDARD) COMPARISON OF PRECISION VALUES OBTAINED I N THE ANALYSIS OF ALUMINIUM OXIDE Element determined A r ’r Method Calcium Iron Titanium AAS - co-precipitation- No. of analyses Mean element content, p.p.in Standard deviation Colorimetric, ISO- No. of analyses Mean element content, p.p.m. Standard deviation 12 10 10 678 60 40 8-3 2.2 3.8 12 10 10 658 62 41 8.0 2.8 3.3 AAS denotes atomic-absorption spectroscopy. IS0 denotes International Standards Organisation DISCUSSION It is probable that the elements are co-precipitated as their hydroxides or oxides, with zirconium hydroxide acting as a carrier for the trace precipitants.Zinc and vanadium if present as hydroxides would be expected to dissolve in excess of sodium hydroxide solution as the soluble sodium zincate [Na, (ZnO,); and sodium vanadate (Na,VO,), but dissolution of vanadium does not occur or perhaps only to a limited extent. It would appear, therefore, that some other factor, such as adsorption on to the carrier precipitant, takes place, thus effectively “shielding’] the trace precipitant from attack and subsequent dissolution by the solvent. The mechanism of co-precipitation appears to be very complicated* and no attempt has been made to study it in detail in the present work. Experimental results obtained in the present work and other published data show that, up to a given value, the co-precipitation of a particular element is more effective the higher the ratio of co-precipitant to trace element present.Thus quantitative recovery of calcium, iron and silicon (major impurities in the study) in aluminium oxide was not achieved until more than 8 mg of zirconium were present in the solution (based on a 2-g sample). The method permits the determination of microgram amounts of calcium, iron, mang- anese, silicon, titanium] vanadium and zinc, iree from matrix effects normally associated with the presence of large amounts of aluminium. It also simplifies the task of determining in- dividual elements as it is not necessary completely to isolate the element to be determined.Sensitivities for the required elements is adequate at the levels generally encountered in the specified materials. Lower concentrations could be determined either by increasing the amount of sample or by concentrating the final volume to 10 ml or less, or both. Flameless atomic-absorption spectrophotometric techniques such as the carbon-rod atomiser, with its594 YOUNG improved sensitivities for most elements and need of a minimum aliquot or sample solution would be ideally suited to this type of analysis. The present work has been limited to the determination of the required elements in aluminium compounds. It is hoped to extend the method to the determination of trace impurities in other materials either by using the same precipitating reagents or by introducing other precipitants.Co-precipitation as a technique for separation and concentration of trace elements has much to recommend it, and it is perhaps surprising that the method has not found wider application in the trace analysis field compared with such techniques as chelate formation and solvent extraction. Co-precipitation is an inherently simple and rapid separation technipe and has been shown in this and other reported W O X - ~ ~ , ~ to possess a sound quantitative basis for trace-metal analysis. The proposed method has shown zirconium in alkaline solution to be of great value as a non-specific wide-range precipitant €or the simultaneous determination of several trace impurities in aluminium compounds. The method is very much quicker than existing pro- cedures (mainly colorimetric), it is as accurate and requires less operator expertise for attain- ment of results of comparable accuracy and precision. The author thanks the management of New Zealand Aluminium Smelters for permission to publish this paper. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8 . IS0 Recommended Procedures Nos. R805, R900, R1232, 111618, R2070. 132071 and R3390. Elwell, W. T., and Gidley, J. A. F., “Atomic hbsorption Spectrophotometry,” Pergamon Prcss, David, D. J., Analyst, 1955, 83, 655. Danchik, R. S., Analyt. Chem., 1971, 43, 109R. Marshall, G. B., and West, T. S., Talaxta, 1967, 14, 823. Burke, K. E., Analyt. Chem., 1970, 42, 1536. Luke, C. L., Analytica Chiin. Acta, 1968, 41, 237. Wahl, A. C., “Radioactivity Applied to Chemistry,” J. Wiley and Sons, New York, 1951, p. 104. Received Februaqy 18th, 1971 Accepted April 2nd, 1974 Oxford and London, 1961.

ISSN:0003-2654    

DOI:10.1039/AN9749900588

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Analyst, September, 1974. Vol. 99, p p . 595-601 595 Atomic-absorption Studies on the Determination of Antimony, Arsenic, Bismuth, Germanium, Lead, Selenium, Tellurium and Tin by Utilising the Generation of Covalent Hydsides BY K. C. THOMPSON AND D. R. THOMERSON (Shandon Southern Instruments Ltd., Frimley Road, Camberley, Surrey, G U16 5ET) A method for the determination of arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium by means of hydride generation is described. The hydrides are generated by adding the acidified sample to dilute (1 per cent. m/ V ) sodium borohydride solution. The liberated hydrides are passed directly into a 17 cm long silica tube mounted in an air - acetylene flame. The advantages of the proposed system are its simplicity, high Sensitivity, high speed of analysis and the fact that background correction facilities are not required.The generation of plumbane for analytical purposes does not appear to have been reported previously. THE hydride generation technique, with subsequent atomic-absorption spectrophometry in a suitable flame (usually an argon - hydrogen diffusion flame) , is now a well known method for the determination of arsenic. The original methods for determining arsenic made use of a zinc metal reduction1 -5 with some form of collection of the arsine prior to actual measurement. The method has been extended to ~elenium~-~ and antimony.' Pollock and West, 8 ~ ) further extended the technique to the determination of bismuth, antimony and tellurium by using a magnesium - titanium(II1) chloride reduction, and to germanium by using a reduction with sodium borohydride.Sodium borohydride has been shown to be a promising reducing agent lo -12 and has been used for the determination of arsenic, antimony, bismuth, germanium, selenium, tin and tellurium12 by the method of hydride generation. There are certain disadvantages when using the methods described : appreciable collection times are normally required for generation of the hydride; a collection vessel (or balloon) is usually required to store the hydride and liberated hydrogen; the sensitivity is rather limited because of the large dilution factor involved when the liberated hydrides are passed into an argon - hydrogen diffusion flame ; and background (non-specific) absorption is usually observed below 200 nm when the hydrides and hydrogen are passed from the collection vessel to the flame.L4n improvement in the speed of arsenic determinations by using a zinc column has been reported by Lichte and Sk0gerb0e.l~ The samples were injected on to the column and the arsine was evolved rapidly while the sample percolated through the column ; no collection vessel was used. The replacement of the flame with an electrically heated, 15 cm long, 2.5 cm i.d. silica tube14 was found to double the arsenic sensitivity compared with that obtained by use of a flame. This paper reports the use of a 17 cm long, 0.8 cm i.d. silica tube mounted in an air - acety- lene flame in order to effect atomisation of the generated hydrides. The advantages of this technique are that no collection vessel is required, that background flame absorption for arsenic, selenium and tellurium determinations is effectively eliminated, thus obviating the requirement of an automatic background corrector, that the narrow silica tube gives a relatively large increase in sensitivity compared with direct sample injection into an argon - hydrogen diffusion flame, and that very little modification to the atomic-absorption spectro- photometer, other than the mounting of two Terry clips on the grid of a wide-path air - acety- lene burner, is required. A cell for the rapid generation of hydride (without associated collection vessels) has been developed that utilises a reduction with sodium borohydride. The reduction products are Q SAC and the authors.596 THOMPSON AND THOMERSON : ATOMIC-ABSORPTION DETERMINATION OF [Analyst, Vol.99 continuously swept into the heated silica tube by using a constant flow of carrier gas. One sample can be run every 40 s while using this form of generation. The borohydride method has been found to be applicable to the determination of arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium. The generation of plumbane (lead hydride) by using an aqueous solution of sodium borohydride does not appear to have been reported before. Some results have also been obtained for arsenic by use of a zinc column similar to that described by Lichte and Skogerboe.13 One sample could be run every 30 s and the column could be used for fifty to seventy samples before it was necessary to replace the zinc granules. EXPERIMENTAL Results were obtained by using a Shandon Southern Instruments A3400 atomic-absorp- tion spectrophotometer, Shandon Southern Instruments hollow-cathode lamps and a Kipp and Zoiien BD8 recorder used in the 10-mV range. The wavelengths used in this study are reported in Table I.TABLE I Element AS AS Bi Ge Pb Sb Se Sn Te DETECTION LIMITS AND CHARACTERISTIC CONCENTRATIONS Sample volume = 1 ml Characteristic concentration Volume of 1 per cent. Concentration Nitrogen (1 per cent. Wavelength/ m/ V NaBH,/ of HC1/ flow-rate/ absorption)/ nm ml M 1 min-1 pg ml-1 193-7 2 1.5 1.2 0.000 52 193.7 Zinc column 1.5 1.0 0.001 223.1 2 1.5 1.2 0*000 43 265.1 2 1.5 1-2 1.0 233.3 1 0.2 1.2 0.08 2 17.6 2 1.5 1.2 0.000 61 196.1 2 1.5 3.0 0.0021 224.6 2 0.5 1.2 0*000 44 214.3 2 1.5 3.0 0.0020 Detection limit pg ml-l 0.0008 0.00 15 0*0002 0.5 0.1 0.0005 0.001s 0~000.5 0.0015 (24 / Stock solutions containing 1000 pg ml -l of arsenic(III), arsenic(V), bismuth(III), germanium(IV), lead, antimony(III), selenium(IV), selenium(VI), tin(II), tellurium(1V) and tellurium(V1) were prepared. The results for the higher oxidation state of some of these elements (i.e., arsenic, selenium and tellurium) are discussed later.High-purity analytic21 grade (Aristar) hydrochloric acid and Alfa Inorganics sodium borohydride 10/32-inch pellets* (98 per cent. pure) were used. The tube was mounted above the burner grid of a wide-path air - acetylene burner. The generated hydride, contained in the stream of nitrogen, was introduced through the side-arm (A) in the middle of the silica tube.Provision was made for cooling the outside of this side-arm via annulus(B), and a cooling air flow-rate of 5 1 min --I. was used. An auxiliary nitrogen stream was injected into the two transverse tubes (CC’) that were positioned 1 cm from the ends of the silica atomising tube (Fig. 1). This transverse stream of nitrogen prevents the liberated hydrogen from igniting at one or both ends of the tube. When ignition does occur, in the absence of the transverse nitrogen stream, the resulting flame absorbs radiation weakly below 200nm and causes a small background absorption. If ignition occurs when the auxiliary gas stream is flowing, the resulting flame does not ignite on the optical axis but on the ends of tubes D and D‘ (Fig.1). Instead of these trans- verse tubes, silica windows could be positioned over the ends of the tube, leaving a Z to 2-mm gap. This device would eliminate molecular absorption as the resulting flame ignition would take place away from the optical axis but this system, when used, caused a slight loss in sensitivity. The main disadvantage was that transmission at wavelengths below 200 nni rapidly decreased with use if the windows were not frequently removed and repolished. The measurement procedure was as follows. A 1 to 2-ml amount of 1 to 2 per cent. m/V sodium borohydride solution was injected into the generator cell (see Fig. 2). The sample, containing a suitable concentration of hydrochloric acid (see below), was contained in a l-ml MLA pipette (Shandon Southern Instruments Ltd.), which formed an air-tight seal with the side-arm of the cell.When a stable base-line was obtained, the sample was injected into * Obtainable from Ralph N. Emanuel Ltd., 264 Water Road, Wembley, Middlesex. The silica atomising tube is depicted in Fig. 1.September, 19741 SOME ELEMENTS UTILISING GENERATION OF COVALENT HYDRIDES 597 B A t N2 t hydride Fig. 1. Silica atomising tube. 1 cm from ends of atomising tube. A, side-arm; B, annulus; and C , C’, D, D’, transverse tubes, Half actual size the cell and the signal monitored. The cell was then removed, emptied and the procedure repeated. Arsine could also be generated from the simple zinc column (see Fig. 3). The column contained 30-mesh, arsenic-free zinc powder (obtained from Fisher Scientific).The sample, 0.5 to 1 ml in 1.5 M hydrochloric acid, was injected through the septum cap on to the top of the column and the resulting arsine was carried in the nitrogen stream out of the bottom of the column into the silica tube. This system gave satisfactory results only for arsenic and was less sensitive than the sodium borohydride reduction method (see Table I). OPTIMISATION OF OPERATING CONDITIONS- Hydride generator design-The design of the sodium borohydride cell is shown in Fig. 2. The-gas was not bubbled through the solution but simply passed over the surface of the liquid in the cell. The volume of the cell was sufficient to ensure that actual sample carry-over due to rapid evolution of hydrogen when the acidified sample was added to the cell did not occur to any appreciable extent.This property was demonstrated with cadmium, which should not It was possible to obtain a result every 40 s. +- N2in N2 in Zinc 30 Fig. 2. Hydride generator cell (containing Fig. 3. Zinc column (for generation of NaBH,). One third actual size arsine). One third actual size698 THOMPSON AND THOMERSON ATOMIC-ABSORPTION DETERMINATION OF [AIZalyst, VOl. 99 form a volatile hydride although any carry-over will be readily atomised; the high sensitivity for cadmium should make detection of any carry-over very easy. A 1 pg ml-l solution of cadmium in 1.5 M hydrochloric acid gave a negligible signal with 1 ml of a 2 per cent. m/V solution of sodium borohydride. If the sodium borohydride concentration was increased to 4 per cent.m/V a small signal was observed. The cadmium signal increased rapidly with further increases in the bordhydride and hydrochloric acid concentrations and it gave a sharper peak than the hydride peaks. Severe memory effects were also observed with cadmium, but not with the hydrides. When the generator cell was replaced by a much smaller cell, and the PVC and silicone rubber tubing connecting the cell outlet to the silica tube was shortened from 60 to 40 cm, the cad- mium response, unlike the hydride response, increased greatly (100 times). It is extremely unlikely that the lead (plumbane) peaks observed were due to carry-over, for a number of reasons : firstly, the sensitivity for cadmium at 228.5 nm is much greater than that for lead at 283.3 nm; secondly, the lead response greatly decreased with increasing acid concentration ; and thirdly, the response to lead did not change appreciably (unlike the cadmium response) when the cell shown in Fig.2 was replaced by the much smaller cell. The nitrogen inlet was at the top of the column and the exit at the bottom. Evolution of hydrogen did not occur to the same extent as with the borohydride generation system. Acid concentration-The response to arsenic, bismuth, germanium, antimony, selenium and tellurium when using sodium borohydride was not very dependent on the hydrochloric acid concentration (1 to 4 M). For tellurium, a slight increase was observed with increasing acid concentration from 1 to 4 M. With tin, the response decreased markedly if the acid concentration exceeded 0-7 M.These results are similar to those obtained by Fernandez.12 With lead, both the acid and the borohydride concentrations were critical. The optimum concentration of hydrochloric acid was 0.2 M when a suitable excess of sodium borohydride was used. With the zinc column the arsenic solutions were prepared in 1.5 M hydrochloric acid.13 Sodium borohydride solutiofi concentration-For all of the elements studied except lead, the sodium borohydride concentration was not very critical; 2 ml of 1 per cent. wz/V sodium boro- hydride solution for a 1-ml volume of sample was found to be satisfactory. When using certain samples reg., determining arsenic in a 10 000 pg ml-l solution of iron (11) or in the diluted solution resulting from the fusion with sodium peroxide of a tungsten alloy], it was found that the borohydride concentration should be increased to 2 per cent.m/V in order to ensure adequate reduction of arsenic, bismuth, germanium, antimony, selenium, tin and tellurium. Other solution components can consume sodium borohydride and consequently minimise hydride formation. This increased borohydride concentration gave a slight reduction in sensitivity, probably due to the greater volume of hydrogen that is evolved resulting in dilution of the hydrides. The decrease in sensitivity was more marked for selenium than for the other elements. In the determination of lead the sodium borohydride concentration was critical. For a 1-ml solution of sample in 0.2 M hydrochloric acid, the optimum volume of 1 per cent.mjV sodium borohydricle solution was 1 ml. If the borohydride concentration was increased the response slowly declined, and if the acid concentration was increased the response rapidly decreased to zero. It would appear that for generation of the unstable lead hydride the final solution in the cell must contain an excess of borohydride. The relatively poor sensitivity obtained for lead indicated a poor efficiency of conversion to the hydride (less than 5 per cent.). The use of an ice-bath in the preparation of all samples and for the immersion of the generator cell did not markedly increase the lead response. Similarly, if the sodium borohydride solution was mixed with a neutral lead solution and allowed to stand in the generator cell for 1 to 10 minutes and then 1 ml of 0-2 M hydrochloric acid was added to the cell, no increase in response was observed.Carvier gas-Nitrogen and argon gave similar responses ; nitrogen was used because it was cheaper. The optimum nitrogen flow-rate was 1.2 1 min -l for all of the elements except selenium and tellurium when a flow-rate of 3.0 1 min -l was used. With lower gas flow-rates the precision of determination of selenium and tellurium decreased and the selenium signal took appreciably longer t o return to the base-line. The total auxiliary transverse nitrogen flow-rate was 3 lmin-l. With the zinc column the optimum gas flow-rate was 1 lmin-l, The zinc column is shown in Fig. 3. The acid concentrations used in this study are listed in Table I.September, 19741 SOME ELEMENTS UTILISING GENERATION OF COVALENT HYDRIDES 599 although the rate was temporarily increased to 3 1 min-1 just before the addition of a new sample.This increase ensured a stable base-line with no memory effects. FZame-An air - propane flame was tried initially, but it gave much lower results for arsenic than the air - acetylene flame, probably because of incomplete breakdown of the hydride when the silica tube was surrounded by the relatively cool air - propane flame. All of the results given were obtained by using a stoicheiometric air - acetylene flame, supported on a wide-path air - acetylenc burner. Damj?kg-In order to attain the optimum signal to noise ratio the A3400 spectrophoto- meter was operated at damping position 1 (time constant = 0.5 s).The pen recorder response was 0-8 s (full-scale deflection). RESULTS DETECTION LIMITS AND CHARACTERISTIC CONCENTRATIONS- listed in Table I. shown in Fig. 4. Detection limits (20) and characteristic concentrations (1 per cent. absorption) are A typical arsenic trace is These correspond to a l-ml addition of sample. 1 ml 0.01 pg mi-’ arsenic (in 1.5 M HCI) I Time - Fig. 4. Typical arsenic trace (2 ml of 1 per cent. NaBH, solution in the generator cell) With germanium small blank signals were observed, which were thought to be caused by the deposition of involatile germanium metal from the decomposition of the hydride on the inside of the silica tube. The hydrogen and the small amount of hydrochloric acid liberated when a subsequent blank was added to the generator cell probably caused volatilisation of some of this metal film.The only other significant blank was observed with arsenic, which was traced to arsenic in the sodium borohydride. By using 2 ml of a 1 per cent. m/V solution of the sodium boro- hydride reagent from Alfa Inorganic the blank corresponded to 0-0025 pg of arsenic, and when sodium borohydride from BDH Chemicals Ltd. was used the corresponding figure was 0.005 pg. The background signal at 193.7 nm when using a deuterium hollow-cathode lamp corresponded to less than 0.0008 pg of arsenic for both reagents. The relative blank could be reduced by using less borohydride. Various methods for removing this blank were tried; passing helium through a 1 per cent. m/V borohydride solution has been reported to remove arsenic as the hydride.1° On passing nitrogen through 1 per cent. m/V sodium borohyride solution for 1 hour no significant reduction in the arsenic blank was observed.However, if 2 pg of arsenic(II1) were added to 20 ml of 1 per cent. m/V sodium boro- hydride solution, this additional arsenic “blank” could be removed by bubbling nitrogan through the solution for 10 minutes ; nevertheless, the original reagent blank signal remained.600 THOMPSON AND THOPV.:ERSON : ATOMIC-AESQRPTIQN DETERMINATION OF [Analyst, Vol. 99 With arsenic(V) a longer bubblicg time (30 minutes) was required to remove the added arsenic. Increasing the hydrochloric acid concentration from 1.5 M to 3 M merely increased the blank by 25 per ccnt. and did not affect the sensitivity to arsenic.Hence, it would appear that the arsenic blank originates mainly from the sodium borohydride rather than the hydrochloric acid and that the arsenic present in the borohydride is not in the I11 or V oxidation states. Recrystallisation of the sodium borohydride would probably decrease the reagent blank and would give an improved arsenic detection limit (see Fig. 4). CALIBRATION GRAPHS- The graph obtained for tin (not shown) was very similar to that for bismuth. The linearity of the arsenic, selenium and tellurium graphs could be improved by using an electrodeless microwave15~ l6 or radioirequency-poweredl2 lamp. Some calibration graphs are shown in Fig. 5. w 0 Fig. 5 (a) and ( b ) . Calibration graphs (conditions as described in Table I) PRECISION STUDIES- It was important that any air in the generator cell should be flushed through the system prior to injection of the sample.The results of some precision studies are summarised in Table 11. TABLE 11 PRECISION STUDIES WITH SODIUM BOROHYDRIDE Sample volume = 1 ml Concentration/ Number of Relative standard Element pg ml-l measurements deviation, per cent. As 0.01 10 Bi 0.1 10 Sb 0.02 10 Se 0.1 10 Sn 0.02 10 Te 0.1 10 OXIDATION STATE OF THE ELEMENT WITH ARSENIC, SELENIUM AND TELLURIUM- All previous results were obtained by using arsenic(III), selenium(1V) and tellurium(1V) solutions. With 1 nil of 1 per cent. nz/V sodium borohydride solution the signal from 0.1 pg of arsenic(V) was 37 per cent. of that from 0.1 pug of arsenic(II1). On increasing the sodiumSeptember, 19741 SOME ELEMENTS UTILISING GENERATION OF COVALENT HYDRIDES 601 borohydride concentration to 4 per cent.mlV, the arsenic(V) signal was found to be 90 per cent. of that obtained for arsenic(II1). Solutions of selenium(V1) and tellurium(V1) gave a negligible response compared with equivalent amounts of selenium(1V) and tellurium(IV), even when 1 ml of 4 per cent. m/V sodium borohydride solution was used. The selenium(V1) and tellurium(V1) solutions and the corresponding selenium(1V) and tellurium(1V) solutions were simmered with an equal volume of aqua regia (3 + 1 V/V hydrochloric acid - nitric acid) for 15 minutes, allowed to cool and then diluted to twice the volume of sample originally taken. It was then possible to add 1 ml of the resulting solution directly to the generator cell containing 2 ml of 2 per cent.m/V sodium borohydride solution. Under these conditions tellurium(V1) was quantitatively reduced by the hydrochloric acid to the tellurium(1V) state, but less than 50 per cent. of the selenium(V1) was reduced. For solid samples, dissolution in aqua regia followed by a suitable dilution (1 + 1 or greater) with distilled water is recommended. This technique would ensure that the resulting solution contained arsenic(V), selenium(1V) and tellurium(1V). The only other element in this study that commonly occurs in two oxidation states is tin. It was assumed that all dilute tin solutions (concentration of less than 0.1 pug ml -l) would be in the tin(1V) oxidation state. With the zinc column the signal from 0.1 pg of arsenic(V) was 55 per cent.of that from arsenic(II1). The prior addition of tin(I1) chloride and potassium iodide to the sample is normally recommended for the reduction of arsenic(V) when using zinc in order to generate arsine . CONCLUSIONS The hydride generation technique using sodium borohydride, coupled with a flame-heated, narrow-bore, silica atomising tube, constitutes a very sensitive method for the determination of arsenic, bismuth, antimony, selenium, tin and tellurium. The method can also be used to determine germanium and lead. (This is thought to be the first report of the generation of plumbane for analytical purposes.) The proposed method has several advantages over the use of zinc granules for the generation of hydride, followed by atomisation of the liberated hydride in an argon - hydrogen flame, viz., negligible background absorption for arsenic and selenium, increased sensitivity, applicability to a larger range of elements and increased speed of analysis. The authors thank Mr. R. G. Godden for many helpful suggestions and the Directors of Shandon Southern Instruments Ltd. for permission to publish this paper. 1 . 2. 3. 4. 5. 6. 7 . 8. 9. 10. 11. 12. 13. 14. 14. 16. REFERENCES Holak, W., A7zalyt. Chem., 1969, 41, 1712. Madsen, R. E., Atom. Absorption Newsl., 1971, 10, 57. Dalton, E. F., and Malanoski, H. J., Ibid., 1971, 10, 92. Manning, D. C., Ibid., 1971, 10, 123. Fernandez, F. J., and Manning, D. C., Ibid., 1971, 10, 86. Yamamoto, Y . , Kumamaru, T., Hayashi, Y . , and Kanke, M., Analyt. Lett., 1972, 5, 717. Yaniamoto, Y., Kumamaru, T., Hayashi, Y., and Tsujino, R., Ibid., 1972, 5, 410. Pollock, E. N., and West, S. J., -4tom. Absorption ArewsE., 1972, 11, 104. Sullivan, E. A., “Sodium Borohydride : Handling, Uses, Properties, Analytical Procedures,” Braman, R. S., Justen, L. L., and Forebank, C. C., Analyt. Chem., 1972, 44, 2196. Fernandez, F. J., Atom. Absovption Newsl., 1973, 12, 93. Lichte, F. E., and Skogerboe, R. K., Analyt. Chem., 1972, 44, 1480. Chu, R. C., Barron, G. P., and Baumgarner, P. A. W., Ibid., 1972, 44, 1476. Dagnall, R. M., Thompson, K. C., and West, T. S., Talanta, 1968, 15, 677. , , Ibid., 1973, 12, 6. -- Booklet No. 01915, Ventron Corporation, Beverley, Mass., U.S.A. , , Ibid., 1967, 14, 557. --- Received March 12th, 1974 Accepted March 26th, 1974

ISSN:0003-2654    

DOI:10.1039/AN9749900595

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

602 Analyst, September, 1974, Vol. 99, p p . 602-607 Electrolytic Extraction Combined With Flame Atomic-Absorption for the Determination of Metal Ions in Aqueous Solution BY J. B. DAWSON, I). J. ELLIS AND T. F. HARTLEY (Department of Medical Physics, General Infirmary, Leeds, LSl 3EX) MRS. hl. E. A. EVANS AND K. W. METCALF (Johnson Matthey & Co. Ltd., Group Research Laboratory, Wembley, Middlesex) Metals were deposited from aqueous solutions onto an iridium wire either by electrolysis a t pH 2 or by auto-deposition a t pH 9. The amount deposited was determined by atomisation with an air - hydrogen flame into a long-tube (10 cm) atomic-absorption spectrophotometer. The sensitivities obtained by electrolysis were comparable with those obtained by conventional sample aspiration for magnesium, lead and zinc and an order of magnitude greater for cadmium, copper and mercury.The presence of other ions in the sample solution generally reduced the sensitivity of the method. ONE of the first uses of electro-deposition in combination with atomic absorption was reported by Brandenberger and Bader,l who deposited metallic mercury on a copper wire from which it was vaporised into an absorption cell by electrical heating of the wire. Newton, Chauvin and Davis2 described a procedure in which metal ions were concentrated by adsorption on a tungsten-wire loop and Fairless and Bard3 applied electro-deposition techniques to carbon-rod flameless atomic absorption. We have previously reported the results of preliminary experi- ments in which flame atomic absorption was combined with electro-deposition for the deter- mination of zinc,4 copper5 and other elements.6 The advantages of electrolytic extraction combined with atomic-absorption spectroscopy are: concentration of the analyte; separation of the analyte from interfering substances; efficient atomisation; and measurement of the ionic concentration of the analyte.A dis- advantage of the technique is the complexity of the electro-deposition process. The purpose of this study was an investigation of the application of electrolysis to the examination of trace metals in biological materials. The elements are usually bound to organic molecules as complexes from which, in some instances, they can be released by changing the pH of the solution. Measurement of the ionic concentration of the metal can be used in order to study the dynamics of the release mechanism.In order to prevent disturbance of the equilibrium between free and bound metals, it is necessary that only a small fraction (about 1 per cent.) of the metal ions should be removed. Hence, in contrast to most procedures that involve electro-deposition, in which every effort is made to extract all of the metal, we required incomplete but reproducible extraction. EXPERIMENTAL APPARATUS- The combined electrolysis - atomic-absorption equipment is shown in Fig. 1. The sample cup, a 3-ml plastic vial, was placed in a holder mounted on the shaft of a 1 rev s-1 synchronous motor and the complete assembly was raised so as to immerse the electrodes to a depth of 5 mm in the sample.The electrodes were mounted on the ends of two brass arms attached to the shaft of a synchronous motor. This motor drove the arms against a fixed stop, which ensured reproducible insertion of the electrodes into the flame. Single, straight iridium wire (0.5 mm diameter and 2-5 cm long) was used for the electrodes as it could withstand the atomising temperature (about 2300 K) of the air - hydrogen flame. The atom reservoir was an electrically heated metal tube 12 cm long and of 9 mm internal diameter, fabricated from nimonic 75 sheet about 0.2 mm thick (International Nickel Ltd.). 0 SAC and the authors.DAWSON, ELLIS, HARTLEY, EVANS AND METCALF 603 A current of 100 to 160 A at 2 V a.c. was sufficient to prevent condensation of atomic vapour within the tube.The long axis of the tube was mounted on the optical axis of a simple atomic-absorption spectrophotometer, which consisted of a hollow-cathode discharge lamp and an Optica CF4 monochromator fitted with an EM1 9558 QC photomultiplier operating at 1400 V with its output connected to one input of a fast-response two-channel potentiometric recorder (full-scale deflection in 0-5 s). The other input was connected across the resistor R (Fig. 1) to record the current passed during electrolysis. Electrically heated /.<optical axis nirnonic tube \&* Power connection Reed switch f l Pulse Power generator amplifier - - Fig. 1. Apparatus for combined electro-deposition and atomic-absorption analysis For the cadmium studies, a similar electrolysis - atomisation system was fitted directly on the optical rail of a Varian Techtron, Model AA5, atomic-absorption spectrophotometer, in place of the conventional burner assembly; the atom reservoir tube was made of a rhodium - platinum alloy (Johnson Matthey & Co.Ltd.). PROCEDURE- The response of the atomic-absorption instrumentation was assessed by placing a small volume (4 p1) of a dilute solution of the analyte on the iridium-wire cathode, where it was dried and subsequently inserted into the flame for atomisation. Volatilisation of the sample was complete in 1 s. The sensitivities obtained (Table I, row 1) are expressed in terms of peak absorbance and, except for magnesium, are greater than those obtained using conventional sample aspiration techniques (Table I, row 2a).The detection limit was calculated as the concentration that will generate a signal equal to 22/2 times the standard deviation of the background signal. Using solutions of similar ionic composition to that of blood plasma,5 the optimum potential for the deposition of each metal was determined. The voltage response graphs were sigmoid for all of the metals studied and the optimum potential used for subsequent work corresponded to the start of the appropriate plateau. This condition was selected as it gave maximum discrimination against other metals with minimum sensitivity to small changes in the applied potential. Approximately 0.2 p1 of solution adhered to the cathode on its removal from the solution in the sample cup. This carry-over could be removed by rinsing the electrode with distilled water, but much of the electro-deposited material was also removed.The problem of solu- bility of electro-deposited material has also been described by Rogers.' As the error intro- duced by carry-over (1 to 2 per cent.) was less than that due to dissolution of the deposit604 [Analyst, 1701. 99 (20 to 25 per cent.), the cathode was not rinsed. In the measurement of copper in bl-od plasma, adhesion of the sample to the electrode generated a signal equivalent to less than 0.01 mg 1-l. DAWSON et al.: ELECTROLYTIC EXTRACTION AND FLAME AAS TABLE I ATOMIC-ABSORPTION SENSITIVITIES OBTAINED USING DIFFERING TECHNIQUES OF SAMPLE PRESENTATION Metal and wavelength Technique I 1 Cd, CU, Hg, Mg, Pb, Zn, 228-8 nm 3244 nm 253.7 nm 258-2 nm 283.3 nm 213.9 nm (1) 4-pl sample vaporised from Ir wire/ absorbance per mg 1- 1 .. . . 0.72 0.132 0.004 0.172 0.072 0-48 (2) Sample aspirated into a 10-cm flame- (a) Sensitivity/absorbance per mg 1-1 . . .. . . 0.13 0.043 0.00065 0.45 0.0074 0.20 (b) Detection limit/mg 1-1 . . 0.004 0.005 0-5 0*0003 0.03 0.002 (3) Auto-deposition from sample onto Ir wire (40 s)/absorbance per m u 1-1- (a) PH9 . . . . . . 0.19 0.018 0.0021 0.0165 0.0028 0-39 (b) PH2 . . . . . . 0.0056 <0*001 <0*0002 <0*002 <0*00003 0-0013 It was found that, in addition to the adhesion of solution to the electrode, metals were deposited on the electrodes without an applied potential (Table I, row 3a). Auto-electrolysis has been used for the deposition of thallium on zinc,a lead on ~admium,~ mercury on copper1 and cadmium and lead on a tungsten - rhenium alloy wire.2 In the last instance, an ion- exchange mechanism was postulated in order to account for the deposition.Our investiga- ions of the auto-deposition of cadmium, copper, lead, magnesium, mercury and zinc on the iridium-wire electrodes showed it to be a time- and pH-dependent process,G unrelated to the position of the metal in the electrochemical series. These observations could be attributed to the precipitation of hydroxides on the electrodes, a conclusion that is consistent with the findings of SmithlOtll on the pH dependence of the stability of dilute solutions. As little auto-deposition of metals on the iridium wire occurred from solutions at low pH (Table I, row 3b), most subsequent electrolysis experiments were carried out at about pH 2. Polarisation of the electrodes, particularly the anode, was found to be a recurrent problem, which could not be satisfactorily solved by the conventional use of platinum-black elec- trodes or the addition of hydroquinone to the electrolysis solution.The effect of polarisa- tion was minimised by using recurrent pulse electrolysis with flame cleaning of the electrodes between each measurement. Pulse electrolysis was effected by applying the depositing voltage to the electrodes €or a short period followed by open circuiting the electrodes by the reed switch so as to allow accumulated polarisation to disperse. When this procedure was used, the peak current was usually between 0.1 and 0-5mA. The effect of variations in deposition and relaxation times on the absorption signal was studied for each metal and the results for copper, shown in Fig.2, are typical. A pulse duration of 0.2 s and a relaxation period of 2.0 s were found to be suitable for most metals. The sensitivity of an analysis was improved by increasing the number of depositing pulses. Thus, as the concentration of the analyte decreased, the analytical signal could be maintained approximately constant by increasing the number of depositing pulses. This rule did not apply, however, at very low concentrations; e.g., as the concentration of copper was reduced by a factor of lo3 from 1 mg 1-1 to 1 pg l-l, it was found that the efficiency of its deposition was halved.6 Below this concentration, the time required to obtain useful absorbance signals became impracticably long (about 20 minutes).September, 19741 FOR THE DETERMINATION OF METAL IONS I N SOLUTION 605 0 1 -0 2.0 3.0 Time/s Fig.2. Effect on copper deposition of variations in deposition pulse duration with a constant relaxation time of 5 s (A) and in relaxation period with a constant deposition time of 0.5 s (B) . Experimental conditions : solution contains 0.2 p.p.m. of copper + elec- trolytes; depositing potential, 1.5 V; and number of pulses, 5 RESULTS AND DISCUSSION Operating conditions and the results obtained in the determination of metals are sum- marised in Table 11. A typical concentration was 0-1 mg 1-1 and the amount of metal removed was of the order of 10-9g, ie., approximately 1 per cent.of the sample. Except for magnesium and zinc, the analytical sensitivity for metals using the electrolysis method was greater than that obtained by the conventional aspiration technique (Table 11, row 2a) and only zinc was less sensitive by electrolysis than by auto-deposition (Table 11, row 2c). The contribution of the electro-deposition process to the over-all analytical sensitivities of lead and zinc was considerably less than that for other elements (Table 11, row 3b). The coefficients of variation of measurements (Table 11, row 3c) are similar to those commonly obtained with transient-response atomic-absorption systems. l2 The ionic composition of the sample solution was a major factor in determining the amount of a metal electro-deposited. Solutions that contained ions in concentrations similar to those found in serum (“serum equivalent” solutions5) generally gave detection limits higher than those for pure solutions (Table 11, rows 4a and 4b).Cadmium was determined in a 3 per cent. m/V solution of acetic acid in water and in a 0.5 per cent. mjm solution of citric acid in 1 + 9 m/m ethanol - water at approximately pH 2-5. Calibration graphs for these cadmium solutions were linear up to 20 pg 1-1 and the most reproducible results were obtained by rcmoving the electrodes from the solution on the decaying edge of the electrolysis pulse, allowing the electrodes to dry for 30 s before inserting them into the flame and leaving the electrodes to cool in air for 20 s after flame cleaning and before immersion in the next sample.In order to obtain electrolytic deposition of mercury, it was necessary to acidify inorganic solutions to pII 2 with nitric acid before the metal could be extracted. Attempts to determine mercury in a “serum equivalent” solution were unsuccessful. Magnesium was readily auto- deposited from inorganic solutions at pH 8 to 9, but below pH 4 neither auto-deposition nor electro-deposition occurred. No auto-deposition of copper occurred from fresh untreated serum and electro-deposition from serum was effected only below pH 6. Acidification of serum to approximately pH 1-5 with hydrochloric acid appeared to liberate all bound copper. A steadily increasing signal due to the release of bound zinc from ten-fold diluted serum followed its progressive acidifica- tion to 0-1 moll-1 with hydrochloric acid.In inorganic solutions of pH greater than 9, there was some loss of zinc due to precipitation. Extended electrolysis (600 pulses) of whole blood diluted ten-fold with distilled water and acidified to pH 2 with nitric acid to give a solution containing about 20 pg 1-1 of lead gave a small signal equivalent to 1 ng of lead on606 DAWSON et aZ. : ELECTROLYTIC EXTRACTION AND FLAME AAS [,4dyst, Vol. 99 the electrode. This technique, however, does not appear to have any significant advantage over the more conventional Delves’ cup method. 13 Attempts to measure iron, cobalt, nickel and chromium were unsuccessful. No absorp- tion signals were detected following the attempted electro-deposition of these metals. Known amounts of salts of the metals were placed on the iridium wire and signals of 1 per cent.absorption or less were generated by 20 ng of iron, 16 ng of cobalt, 2 ng of nickel and 10 ng of chromium. Antimony, arsenic and selenium generated multiple absorption peaks following their electro-deposition from solutions in hydrochloric acid of concentration 0.001 moll-1: all gave poor sensitivity. The multiple peaks of arsenic and antimony may have been due to the formation of arsine and stibine in the air - hydrogen flame. The largest selenium peak had a sensitivity of 0.0027 absorbance unit per milligram per litre and the addition of ascorbic acid to the sample enhanced the response to give a sensitivity of 0.022 absorbance unit per milligram per litre. TABLE I1 ATOMIC-ABSORPTION SENSITIVITIES OBTAINED USING ELECTROLYTIC EXTRACTION OF THE METAL Metal r A 1 Pb Zn Hg :: 4.0 3-0 P.d.between electrodeslv . . 15.0 1.5 4-0 Cd c u Current pulse duration/s(repetition rate, 0.5 Hz) .. . . . . 0.6 0.2 0.2 0.2 0.2 0.2 pH of solution . . . . . . 2-5 2.5 3.0 9.0 2-0 2.0 (1) Sensitivitylabsorbance per nig 1-1 (2) Relative sensitivities for solutions per current pulse . . . . . . 0.84 0,023 0.0015 0.014 0.00088 0.0098 containing 1 mgl-1- (a) Electrolysis (20 pulses) . . 129.2 10.7 46.2 0-62 2.38 0.98 Continuous aspiration Vaporisation from wire (1-1) Electrolysis (20 pulses) . . 23-3 3-48 7.5 1 -63 0.24 0.4 1 (c) Electrolysis (20 pulses) 88-4 25.6 14.3 16.4 6.3 0.5 Auto-deposition (40 s) a t pH 9 (3) Precision- (a) Concentrationlrng 1-1 . , 0.01 0.1 10 0.1 1.2 0.5 per cent.. , .. * . 4 5 8 9 12.5 9 (b) Standard deviationlmg 1-1 0.0004 0.005 0.8 0.009 0.15 0.045 (c) Coefficient of variation, (4) Detection limits (20 pulses)/mg 1-1- (a) Pure solutions . . . . 0.0001 0-005 0.5 0.017 0-35 0.05 lyte solutions . . . . 0.000 16 0.025 150.0 0-5 0-75 0.03 (b) “Serum equivalent” electro- CONCLUSION I t has been demonstrated that for cadmium, copper, mercury and zinc, electrolytic extraction is a feasible means of quantitative analysis, even though only a small fraction (less than 1 per cent.) of the metal is deposited on the electrode. The detection limits obtained, however, are not better than those which can be achieved by using conventional atomic- absorption procedures except for cadmium, for which a forty-fold improvement is obtained.In solutions of mixed inorganic ions, detection limits are worse. The method is most success- ful in simple solutions in which the metal to be determined is the major cation. The ease with which auto-deposition occurred with most metals justifies further study of this process as a means for extracting metals from solution. We would expect auto-deposition or electro-deposition combined with atomic absorption to be of greatest value iii special situ- ations such as the determination of the ionic components of certain metals in biological systemsSeptember, 19741 I‘ 4OR THE DETERMINATION OF METAL IONS IN SOLUTION 607 or where electro-separation can be used to deposit preferentially the metal of interest in the presence of a large amount of matrix material.The limiting factor in the use of these tech- niques is the complexity of the deposition processes and further studies of these processes are necessary. We thank the Medical Research Council for its support of the studies on the electrolysis of biological samples and the Directors of Johnsoii Matthey & Co. Ltd. for permission to publish some of the information contained in this paper. 1. 2. 3. 3 . 5. (i . 7. 8. 9. 10. 1 1 . 12. 13. REFERENCES Brandenberger, H., and Bader, H., Atom. Absorfition Newsl., 1967, 6, 101. Newton, hf. P., Chauvin, J. V., and Davis, D. G., Analyt. Lett., 1973, 6, 89. Fiarless, C., and Bard, A. J., Analyt. Lett., 1972, 5, 433. Walker, B. E., Dawson, J. B., and Ellis, D. J., “International Atomic Absorption Spectroscopy Ellis, D. J., Hartley, T. F., and Dawson, J. B., “Third International Congress on Atomic Absorption Hartley, T. F., and Ellis, D. J., Proc. SOC. Analyt. Chem., 1972, 9, 281. Rogers, L. B., Analyt. Chem., 1950, 22, 1386. Truhaut, R., and Soulet, M., Rep. 15dme Congr. G.A.M.S., 1952, 201. Seith, W., and Gremm, W., Mikrochim. Acta, 1956, 339. Smith, A. E., Analyst, 1973, 98, 65. -, Ibid., 1973, 98, 209. Anderson, W. N., Broughton, I”. M. G., Dawson, J. B., and Fisher, G. W., Clinica Chim. A d a , 1974, Delves, H. T., A m l y s t , 1970, 95, 431. Conference, Sheffield, 1969,” Hilger, London, 1969, Abstract G2. and Atomic Fluorescence Spectroscopy, Paris, 1971,” Hilger, London, 1972, p. 641. 50, 129. Received March 7th, 1974 Accepted May Gth, 1974

ISSN:0003-2654    

DOI:10.1039/AN9749900602

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

608 Analyst, September, 1974, Vol. 99, pp. 608-611 A Potentiometric Titration Method for the Rapid Determination of Salt in Meat Products BY M. KAPEL AND J. C. FRY (Procter Department of Food and Leather Science, Lceds University, Leads, LS2 9 J T ) h method is described for the rapid determination of salt in meat pro- ducts by titration of the macerated sample with silver nitrate. The acidic and oxidising conditions specified permit the use of an ion-selective electrode with a silver sulphide membrane in the potentiometric detection of the end-point. The method is slightly more precise and very much more rapid than existing routine and reference methods. MOST existing methods for the determination of salt in meat products depend upon either an aqueous extraction or an ashing pre-treatment with the subsequent titrimetric deterrninatioii of chloride, typically by the Volhard or Mohr m e t h ~ d .l - ~ Alternatively, the meat can be digested with nitric acid in the presence of silver nitrate.* Such pre-treatments are slow, and the methods based on their use involve several further time-consuming operations. In addi- tion, ashing must be carried out with great care and in the presence of sodium carbonate in order to minimise the risk of loss of hydrogen chloride, while extraction procedures are not applicable to certain meat products. This latter fault arises because it is essential, in the Volhard titration, that chloride be precipitated as silver chloride and that the precipitate should be either filtered off or effectively removed from the system by a surface coating of a substance such as nitrobenzene.The colloidal precipitates of silver chloride that are produced by some meat products (e.g., luncheon meat) cannot be removed in this way and accurate titration is therefore impossible. The following method overcomes the above difficulties and is generally applicable to meat products, being accurate and faster than existing methods. It depends upon the direct titration of chloride in a sample macerate under conditions that permit the use of an ion-selective electrode to detect the end-point. In this context certain amino-acids and their residues in proteins seriously interfere with solid-state electrodes that include silver salt membranes. In particular, histidine and cysteine are known to form silver complexes,5~6 which have been shown to interfere with the solubility product equilibria and hence the potentials of such electrodes.The mode of action of these amino-acids will be reported elsewhere, but the conditions required for the elimination of such interference are embodied in the present method. These conditions are, firstly, a low pH to ensure that protonation of the imidazole group in histidine takes place, and, secondly, the presence of an oxidising agent to convert cysteine into cystine. Advantage has been taken of the necessity to provide a low pH in order to effect control over the ionic strength of the macerated sample by using a strong acid at a high concentration. The ionic strength and, consequently, activity coefficients and liquid-junction potentials, are thus maintained virtually constant.A mercury - mercury(I) sulphate reference electrode was employed in preference to the more widely used calomel instrument, as the latter would cause difficulties with the diffusion of chloride into the sample solution as well as the precipitation of silver chloride at the liquid junction. EXPERIMENTAL APPARATUS- All readings were made by means of an ion-selective electrode with a silver sulpliide membrane (Philips 1s 650-S) and a mercury - mercury(1) sulphate reference electrode, with saturated potassium sulphate solution as the outflowing electrolyte (Activion Glass Ltd.) . These electrodes were connected to a pH meter (Corning - EEL Digital 110) used in the millivolts mode. 0 SAC and the authors.An MSE homogeniser was used in order to macerate samples.KAPEL AND FRY 609 REAGENTS- All of the inorganic reagents used were of analytical-reagent grade. Background sohtion-This was a 0.01 M solution of potassium dichromate in 1 M nitric Titrant-A 0.171 M solution of silver nitrate in 1 M nitric acid. Emulsijier-Teepol-L detergent. SAMPLE PREPARATION- In accordance with British Standard methods,l bulk meat samples, normally greater than 200 g, were rendered homogeneous by at least two treatments with a meat mincer that had plate holes less than 4 mm in diameter. They were then thoroughly mixed and stored in air-tight containers. PREPARATION OF MACERATED SAMPLES- which were added about 50 ml of background solution and about 0.2 ml of Teepol-I1. meat was then macerated on the homogeniser for not less than 15 s.DETERMINATION OF CHLORIDE- The macerated sample was carefully rinsed into a 250-ml beaker with approximately 50 ml of background solution. While the liquid was being stirred by means of a magnetic rotor, the electrodes were introduced into the sample, which was then titrated to a potential of +30 mV with the silver nitrate solution titrant (Note 1). The salt content was obtained by substitution of the values into the following equation- acid. About 10 g of the prepared meat sample were accurately weighed into a vortex flask, to The Volume of titrant used (ml) Mass of sample taken (g) Sodium chloride, per cent. mlm = After each determination, the electrodes were rinsed in distilled water and the silver sulphide membrane was dried on a tissue by use of very firm pressure and a polishing motion (Note 2).COMPARISON WITH EXISTING METHODS- The above procedure was compared with two reference methods for a range of meat products. The reference techniques were : firstly, a modification of the British Standard method for the “Determination of Chloride in Meat and Meat Products,”l in which the British Standard method was followed, except that deproteinated extracts were first filtered and then made up to volume, the reverse order being specified in the standard; and secondly, preliminary ashing for 3 hours at 450 “C under an ample layer of sodium carbonate, this process being followed by dissolution in a slight excess of nitric acid, filtration and a Volhard titration, as in the British Standard method.From every bulk sample six sub-samples were taken and analysed for chloride by all three methods. RESULTS In Table I NA indicates that analyses by the modified British Standard method were attempted but that colloidal silver chloride precipitates were produced, so that no results were TABLE I SALT CONTENTS OF VARIOUS MEAT PRODUCTS KESULTING FROM THE USE OF DIFFERENT ANALYTICAL METHODS Bulk sample Canned sausage Chopped ham with pork Canned ham Canned meat balls Luncheon meat Corned beef Salami Salt, mean per cent. m/ni on “as received” basis 7- A \ Modified BS Ashing Present method method method 1.36 1-37 1-36 NA 1.43 1.45 3.38 3.38 3.41 NA 0.99 1.00 NA 1-95 1.93 2.02 1.95 1.97 6-34 6.03 5.92 Best estimate of standard deviation 7-- \ Modified RS Ashing Present method method method 0.009 22 0.0198 0.007 00 NA 0.0156 0.006 24 0.02 15 0.0227 0-0124 NA 0-006 06 0.004 89 NA 0-008 11 0.0137 0.007 14 0.0211 0.009 06 0.115 0-0905 0-0537610 KAPEL AND FRY: A POTENTIOMETRIC TITRATION METHOD [Analyst, vO1.99 obtained. minations. error relative to the mean result indicated under “Present method” in Table I, are shown. ,4s already explained, each of the values in the table is based upon six deter- In Table I1 the expected limits of error for the proposed method, recorded as percentage TABLE I1 . ERRORS IN PROPOSEI) METHOD Relative error limits, per cent. Bulk sample Canned sausage Chopped ham with pork Canned ham Canned meat balls Luncheon meat Corned beef Salami for 95 per cent. population (1.9ti x s.d.) $I 1.01 3: 0.843 A0.713 f 0.958 f 1.39 *0.901 f 1.78 for 99 per cent.population (2-58 x s.d.) f 1.33 f l . 1 1 *Om938 5 1-26 f 1.83 f 1-19 4 2-34 Correlation coefficients, calculated from the mean results in Table I, were : modified British Standard method with proposed method, r = 0.999 37 ; ashing method with proposed method, The results indicate a close correlation between the proposed method and the two refer- ence procedures. In a majority of the experiments the proposed technique proved to be more precise than the reference methods and, for most materials, it can be expected to give results to within about &l per cent. relative error. = 0.999 86. RAPIDITY OF THE METHODS- The British Standard method took about 34 hours per single sub-sample, excluding the bulk sample preparation time; the ashing procedure required rather more time, while the proposed method furnished single results in 10 to 12 minutes with a routine throughput of about eight samples per hour.Although virtually the whole duration of the proposed method comprises operator time, the actual working time involved is at most only one third of that required by either of the reference methods. The proposed method can also be carried out with an auto-titrator, a device that would both reduce the total operator time and increase the throughput. NOTES- The exact end-point potential depends upon the electrodes used and is best determined by replicate titrations of standard sodium chloride solution with standard silver nitrate solution under conditions identical with those under which subsequent determinations are to be made. The quoted value of f 3 0 mV was obtained by means of this method and can be expected to be correct only for the electrodes specified.The proposed method is unsuitable for use with silver-metal electrodes, which are attacked by the background solution. The silver sulphide membrane of the ion-selective electrode used is also dissolved slowly, and the indicated drying procedure is accordingly necessary in order to prevent the formation of a porous surface layer, with consequent deterioration in the speed of response of the electrode. 1. 2. C o N c L u s I o N The proposed method yields results which show excellent correlation with those obtained by use of two reference methods over a range of typical samples.The results obtained with the proposed method are usually of greater precision than those furnished by the reference methods. The proposed method also makes available a large saving in operator time, being rapid in execution. We gratefully acknowledge the generous financial assistance for this work provided by J. Sainsbury Ltd., to whom one of us (J.C.F.) is also indebted for a maintenance grant. Our thanks are also due to Professor A. G. Ward for his interest and encouragement.September, 19741 FOR THE RAPID DETERMINATION OF SALT IN MEAT PRODUCTS 61 1 REFERENCES 1. “Methods of Test for Meat and Meat Products, Determination of Chloride Content,” British 2. “Official Methods of Analysis of the Association of Official Analytical Chemists,” Eleventh Edition, 3. Pearson, D., “The Chemical Analysis of Foods,” Sixth Edition, J. and A. Churchill Ltd., London, 4. “Official Methods of Analysis of the Association of Official Analytical Chemists,” Eleventh Edition, 5 . Greenstein, J. P., and Winitz, M., “Chemistry of the Amino Acids,” Volume 3, John Wiley and 6. -,- , op. cit., p. 1973. Standard 4401 : Part 6 : 1970. The Association of Official Analytical Chemists, Washington, D.C., 1970, p. 398. 1970, p. 378. The Association of Official Analytical Chemists, Washington, D.C., 1970, p. 392. Sons, New York, 1961, p. 1885. Received February 25th, 1974 Accepted March 2292d, 1974

ISSN:0003-2654    

DOI:10.1039/AN9749900608

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

612 Analyst, September, 1974, Vol. 99, 9. 612 The Use of Filter-papers in the Determination of Nitrogen in Foods By D. PEARSON (National College of Food Technology, University of Reading, St. George’s A v e n w , Weybvidge, Surrey) Ix their paper, 0 hAlmhain and 6 Danachairl imply that it is inadvisable to use filter-papers for weighing out samples preparatory to the determination of nitrogen in foods as recom- mended by Pearson.2 The inclusion of an amount of ammonia-nitrogen of the order of 300 pg in a filter-paper of 12-5 cm diameter would appear to increase the volume of 0.1 N (0.05 M) sulphuric acid required for the titration in the Kjeldahl procedure by approximately 0.17 ml. Assuming, therefore, that the method is applied to a 2-g sample, the presence of the filter-paper would cause an increase in the calculated percentage of nitrogen of approximately 0.012 per cent.This difference is unlikely to he of significance in the analysis of most foods. For instance, the over-all effect in the examination of meat products would be an indication that a further 0.3 per cent. of meat was present. Taking into account the procedures used in our laboratories, the effect on the results obtained by the incorporation of a filter-paper (not an ashless or hard grade) in the digestion flask is considerably less than is indicated above. Apart from the fact that smaller (9 cm) papers are used, a similar filter-paper is included in the blank determination. This practice is referred to in the conclusions of d hAlmhain and d Danachairl and was recommended by Pearson3 in another text-book that was designed mainly to meet the requirements of less experienced workers. By using filter-papers from the centre of the box, variations in the nitrogen content between different papers should be extremely small. In the author’s opinion, filter-papers are most useful for the efficient transfer of moist foods into the digestion flask, but if the amount of nitrogen being determined is extremely small the sample should be weighed out in a perforated glass cup. REFERENCES 1. 2. 3. 6 hAlmhain, L., and 6 Danachair, D. , Analyst, 1974, 99, 211. Pearson, D., “The Chemical Analysis of Foods,” Sixth Edition, Churchill, London, 1970, p. I). “Laboratory Techniques in Food Analysis,” Batterworths, London, 1973, p. 53. Received May 6th, 1974 Accepted June 4th, 1974 0 SAC and the author.

ISSN:0003-2654    

DOI:10.1039/AN9749900612

出版商:RSC    

年代:1974

数据来源: RSC

摘要:

Analyst, September, 19741 613 Book Reviews QUANTITATIVE THIN LAYER CHROMATOGRAPHY. Edited by JOSEPH C. ToucHsTom. Pp. xvi + 330. New York, London, Sydney and Toronto : Wiley-Interscience. 1973. Price k7-50. In a book with such a general title, one might expect to find up-to-date information on all aspects of quantitative thin-layer chromatography, including fundamental aspects, some critical assessments of the various techniques that have been reported and are in present use as well as critical reviews of the applications of the techniques in biochemical, inorganic, pharmaceutical and organic problems. The book deals mainly with biochemical aspects, the chapters dealing with the fundamental aspects of the techniques are not sufficiently critical to be of much practical use.The sole paper dealing with the use of quan- titative thin-layer chromatography in air pollution does not contain references to work after 1968! As the contributions are from several workers, the editing in such a book must be fairly ruth- less in order to avoid much overlap and the impression is that we are presented with semi-review articles intermingled with some research papers that would probably not be accepted, in their present form, by most international journals dealing with chromatography. The editing has not been ruthless, the standard of the chapters varies considerably and in some it is low even with respect to grammar. Although there is a fair amount of good work in the book, for those workers already in the field it offers little, and for those who wish to start in the field it is too superficial in its treatment of the fundamental aspects to be of much use.The contents of the book do not justify the title. L. S . BARK THERMAL ANALYSIS. By T. DANIELS. Pp. 272. London: Kogan Page Ltd. 1973. Price k6. Dr. Daniels is to be Congratulated on producing a volume that can be recommended without reservation to everyone interested in thermal analysis, be he novice or expert. It is indeed refreshing these days to be able to make such a statement, especially on a work relating to a subject that has recently generated such a volume of literature as has thermal analysis. This book very neatly closes a gap in the literature, as there was previously no single treatise covering in such an easily digested manner all the important thermoanalytical techniques, their instrumentation and their main applications. As a measure of coverage, thermogravimetry, differential thermal analysis, differential scanning calorimetry, dilatometry, mechanical, electric and magnetic methods in thermal analysis, evolved gas detection, evolved gas analysis and corn- binations of various techniques are all considered.Major emphasis is laid on instrumentation and in such a way that aspects relevant to the production of reliable results are well brought out. Criticism might perhaps be levelled a t the small amount of space devoted to applications, yet these are well chosen to illustrate the type of information that each method yields and are probably adequate to satisfy the author’s criterion “. . .that each reader can critically assess the approach most suited to his particular problem.” The novice to thermal analysis will therefore find this book a mine of information and the expert in one technique, while he might perhaps cavil at the coverage given to his particular hobby-horse and observe omissions, will undoubtedly garner facts about other techniques that were previously unknown to him. Having read this book, one is left not only with an awareness of the wide variety of techniques that come under the heading of “thermal analysis” but also with some impression of the enormous range of materials and problems thev can be applied to with success. The book is well written and easy to read, although the pedant might object to the occasional split infinitive, and the author is to be congratulated on giving due attention to standards for nomenclature and methods of reporting results enunciated by the International Confederation for Thermal Analysis.The references are well selected for illustrative purposes but do not always represent the first recorded observation, e.g., the system described in the first few lines of p. 21 was used much earlier by both Czech and American workers. Commendably, there are few misprints and those that do occur, such as that in equation 3.5 on p. 62, should cause no difficulty. Although the expert might perhaps question some observations, the only mis-statement noted was on p. 114 : IITA peak areas in (degI2 are particularly sensitive to heating-rate variation and the correct dimensions here should be deg s.All the figures are reproduced in a particularly clear and lucid form and full indexes are provided. Overall, this book is excellent value for money and should be on the bookshelves not only of those already interested in the subject but also of those who wish to know “what thermal analysis is all about.” R. C. MACKENZIE614 BOOK REVIEWS [Analyst, Vol. 99 CHEMICAL ANALYSIS OF ORGANOMETALLIC COMPOUNDS. ELEMENTS OF GROUPS 1-111. By T. R. CROMPTON. A n International Series of Monographs, No. 4. Pp. x + 258. London and New York: Academic Press. 1073. Price i 5 . 8 0 ; $16.25. From the generic heading of this series, The Analysis of Organic Materials, and the titles of the monographs that have already appeared, or are likely to appear in the series, it is evident that an extensive coverage is contemplated.For good measure, “Chemical Analysis of Organometallic Compounds” adds to this coverage by its incursion into the field of inorganic analysis. The author is not new to the analysis of organometallic compounds, and this publication can be regarded as a logical sequence to his earlier book, “Analysis of Organoaluminium and Organozinc Compounds” (Pergamon Press Ltd.) . The latter, together with three proposed volumes in this series on “Chemical Analysis of Organometallic Compounds,” are intended to cover the most important compounds. This volume (Volume 1) deals with elements in the conventional Groups I to 111, with major chapters devoted to lithium, magnesium, mercury and boron and short chapters on sodium, potassium, copper, beryllium, calcium and thallium, for example.Each (element) chapter contains a review of the most important aspects of the particular organometallic compound. This is followed by critical comments on the relative importance and weaknesses of published methods for determining the element and its functional groups. Here is a publication that is destined to be well received by the busy analyst (and others) who requires reliable information quickly without undue recourse to the original literature. There is one small complaint: like most analysts, I prefer the word “determination” to the presumed equal alternative used extensively and almost invariably throughout the book. Volume 1. W. T. ELWELL NEW DEVELOPMENTS IN GAS CHROMATOGRAPHY. Edited by HOWARD PURNELL.Advances in New York, London, This book contains a collection of seven reviews on different aspects of gas chromatography. Each review is written by an acknowledged expert and all are backed by a comprehensive list of references. The first chapter, on the “Application of Gas Chromatography to Forensic Science,” is an interesting illustration of the use of the technique for practical purposes. Methods for the deter- mination of alcohol in blood and urine and the detection and identification of drugs are well described. The effort required to place a gas-chromatographic system under computer control is considerable and David Leathard has produced a useful guide to what is involved; this is mainly directed to those whose knowledge of computers is minimal.The second half of this chapter is devoted to the problems of curve fitting and the resolution of overlapping peaks. While this is an important aspect the use of a computer should not be seen to be an excuse for the promotion of inferior chromatographic separation. Phase changes in gas-chromatographic stationary phases have usually been regarded as a nuisance factor. However, discontinuous retention time behaviour over temperature ranges showing such phase changes can be recognised as falling into one of six categories, characteristic of the different log Vg versus 1/T relationships. P. F. McCrea shows how the phenomenon can be put to many different applications, which include the choice of specialised stationary phases for enhanced separation factors and the characterisation of solid - solid, solid - liquid, liquid - liquid, etc.transitions. A more specialised application of this phenomenon, for the study of the physical properties of polymers, e.g., crystallinity and surface properties, is dealt with in a separate chapter by J . E. Guillet. The use of gas chromatography for the production of pure compounds on an industrial production scale is well described in technical and economic detail by John Condor and a favourable comparison is made with contemporary distillation techniques. Studies in complex formation between the gas-phase component and the stationary phase are reviewed by C. A. Wellington and the theme is continued in another review by S. H. Langer and J. E. Patton, where chemical interaction between the gas-phase components and the stationary phase enables this technique to be used as a chemical reactor to study reaction kinetics.As well as citing actual applications, e.g., isomerisation, a fairly comprehensive mathematical treatment is given of the theory behind the application. Undoubtedly this book will be of interest to students of gas chromatography who wish to keep abreast of recent developments, and to research workers seeking basic information and further references in these particular aspects of the technique. Analytical Chemistvy and Instrumentation, Volume 11. Sydney and Toronto: John Wiley & Sons. 1973. Price L l O . Pp. viii + 408. W. R. MCLEANSeptember, 19741 BOOK REVIEWS 615 GUIDE TO MODERN METHODS OF INSTRUMENTAL ANALYSIS. Pp.xiv + 495. New York, London, Sydney and Toronto: Wiley-Interscience. 1972. Price k7.75. The first impression made by this book is of the very wide yet detailed knowledge required by the modern analytical chemist. The range of techniques of which he has to be knowledgeable is covered (with some glaring exceptions) in the text a t a level sufficient to give a reasonable apprecia- tion of their theory and practice, and to help the reader to make a critical selection of the best approach to a particular problem. The twelve chapters deal in turn with Gas Chromatography (J. F. Johnson), High Resolution Liquid Chromatography (T. H. Gouw and R. E. Jentoft), Thin Layer and Paper Chromatography (D. J. Shapiro and V. W. Rodwell), Gel Permeation Chromatography (M. J. R. Cantow and J .F. Johnson), Visible and Ultraviolet Spectroscopy (C. R. Hare), Infrared and Raman Spectroscopy (3. B. Colthup), NMR Spectroscopy (D. L. Rabenstein), ESR (C. P. Poole, Jr.), Gas Chromato- graphy - Mass Spectrometry (R. A. Flath), Mass Spectrometry (D. H. Smith), Electroanalytical Methods (D. D. Gilbert) and Differential Thermal and Thermogravimetric Analysis (E. M. Barrall, The book will be most useful for analytical chemists dealing with organic compounds. It contains relatively little inorganic material, and the absence of accounts of flame spectroscopy and of X-ray fluorescence in this respect is regrettable. Also, the treatment of all electrochemical techniques in 40 pages, although comprehensive, is necessarily lacking in detail. The description of applications of the techniques is deliberately left brief, with references made to other sources of information.These criticisms apart, this book provides a very acceptable treatment of modern instrumental analytical techniques. It is well written and presented, with few typographical errors, and achieves a standard higher than that of many of its predecessors. Edited by T. €3. Gouw. 111). A. TOWNSHENU METHODS OF BIOCHEMICAL ANALYSIS. Volume 21. Edited by DAVID GLICK. Pp. x + 572. New York, London, Sydney and Toronto: John Wiley & Sons. D. Zakim and D. A. Vessey of San Francisco discuss techniques for the characterisation of UUP-glucuronyltransferase and other tightly bound microsomal enzymes. They describe the preparation and sub-fractionation of microsomes and review the choice of aglycones for assaying the transferase enzyme.An absorption peak a t 400 nm for p-nitrophenol is lost on formation of the glucuronide while, as an alternative, o-aminophenol-glucuronic acid can be diazotised selectively in the presence of unreacted o-aminophenol (by careful control of pH) and the diazotised glucuronide can then be complexed with N- (-naphthyl) ethylenediamine dihydrochloride (Amax. 555 nm ; c, 2.9 x lo4 cm2 mol-l). Glucose-6-phosphatase is multifunctional and differential assay pro- cedures are given. The preparation and purification of phospholipases is described together with the effects of these enzymes on UDP-glucuronyltransferase and glucose-6-phosphatase. 0. E. Olson, I. S. Palmer and E. J. Whitehead of Brookings, South Dakota, deal with the determination of selenium in biological materials.The element occurs naturally a t widely different concentrations and methods are needed for determining less than 1 p.p.m. and up to a few per cent. The organic moiety can be destroyed and the element oxidised to Se4+ or Se6+ and then determined. Neutron-activation analysis has become popular as a non-destructive method. The details of procedures for destructive analysis are described and different methods of determining selenium are assessed critically. Selenious acid reacts with o-diamines ; thus 3,3’-diaminobenzidine (DAB) forms yellow monopiazselenol (Amax. 420 nm), which can be extracted with toluene. DAN (2,3- diaminonaphthalene) gives a complex that can be extracted with cyclohexane and is very strongly fluorescent (606-nm filter) ; a method based on this is fully recorded and found to be particularly suitable for samples containing less than 2 p.p.m.of selenium. A second method separates selenium from interfering materials by arsenic coprecipitation. A gravimetric method is described for determining large amounts of selenium. C. Horvath of Yale University offers a long article on high-performance ion-exchange chroma- tography with narrow-bore columns. He devotes particular attention to the rapid analysis of nucleic acid constituents a t the sub-nanomole level. Thin ion-exchange resin shells on the surface of glass beads act as a skin (pellicular ion-exchange resin). A similar product (Zipax, made by Du Pont) consists of a porous crust of resin on a spherical silica core; it makes efficient columns for the analysis of nucleic acid constituents.Pellionex resins (Reeve-Angel) are deposited on glass micro- beads and their properties make them suitable for the analysis of minute samples. Theoretical aspects of the procedures are considered and there is a useful table of molar absorptivities of RNA 1973. Price k11.50.616 BOOK REVIEWS [Analyst, Vol. 99 constituents a t 260 nm. The actual instrument used for high-pressure liquid chromatography requires “the successful solution of complex engineering problems” and although home-made systems have been successful, the author prefers to describe in detail the Varian LCS 1000 model. Many problems open to attack by high-performance liquid chromatography are truly important and new highly sophisticated analytical techniques permit great gains in separation efficiency.These advances, however, demand much improved methods for preparing small samples as well as more efficient and rapid identification of unknown components. J. Okuda and I. Miwa of Nagoya, Japan, consider newer developments in enzymic determina- tion of D-glucose and its anomers. (In water, glucose equilibrates: a, 36.5; p, 63.5; aldehyde form, 0.003 per cent.). The enzymes concerned include (2) @-D-glucose oxidase, (ii) hexokinsse and (iii) acyl phosphate : ~-glucose-6-phosphotransferase. Methods based on (i) require the use of the oxygen electrode or of colorimetry or fluorimetry ; each approach is described and evaluated. Methods based on (iz) may involve spectrophotometry, colorimetry, fluorimetry or radioisotopes.The enzyme (iii) is highly specific but the authors note that i t is not commercially available. K. G. Oldham of Amershsm surveys radiometric methods of enzyme assay in a long article covering general principles, techniques, classes of enzymes, different radiometric assay procedures and optimum conditions. The author observes that the number of labelled enzyme substrates now marketed is large and increasing steadily while counting equipment is satisfactory and widely avail- able. The run-on cost of radiometric assays has fallen and the time taken for each radiometric assay has been reduced. Conventional methods and radiometric methods are compared in an instructive and critical manner.There is increasing advantage to be gained by the use of radio- isotopes in clinical biochemistry and in pharmacology as well as in general biochemistry. ‘The article covers the ground very competently and 484 references are cited. P. J. Elving, J . E. O’Reilly and C. 0. Schniakel of Ann Arbor, Michigan, deal with polarography and voltammetry of nucleosides and nucleotides and their parent bascs as an analytical and investi- gative tool. The authors state that “one reason for the relatively small use made of polarographic techniques by chemists interested in biological compounds is their lack of sufficient theoretical back- ground in electrochemistry.” A substantial section (about 10 000 words) is devoted to a general discussion, which is followed by a somewhat longer section on pyridine (nicotinamide) derivatives.The treatment of pyrimidine and purine derivatives is systematic and thorough and includes a wide range of compounds including aza-, thio- and chloro-derivatives in addition to all the classical sub- stances. Finally, there is a shorter and instructive discussion of riboflavin and flavin nucleotides. J. R. Majer of Birmingham (England) and A. *4. Boulton of Saskatoon deal with the integrated ion-current (IIC) technique of quantitative mass-spectrometric analysis with particular emphasis on chemical and biological applications. It is difficult, despite the use of powerful physicochemical methods, to identify with certainty many unknown substances present in minute amounts in com- plex biological extracts. Precise mass analysis now allows positive identification a t the 2 p.p.m. level even when there is gross contamination, while the integrated ion-current method permits quantification in the to range, even in the presence of contaminant ions. The substances (or derivatives) must be volatile in high vacuum a t temperatures between -50 and +500 “C. Methods are described and the applications discussed include analysis of trace metals (via chelates), pollutants, detergents, surface decontaminants and isomers. The section on polln- tants includes an interesting treatment of the detection and analysis of polycyclic hydrocarbons. The authors point out that the IIC technique is the method of choice in biochemical applications, especially in problems of neurochemistry and neuropharmacology. They add however that, “the only snag would seem to be the expense of the installation (approximately $100000 to $150000) and the need for skilled and knowledgeable personnel. ” There can be no doubt that more and more difficult problems are being tackled successfully but the “snag” referred to by Majer and Boulton has its parallel in many other areas. This section is followed by a further section on applications. The present volume maintains the standard set by its predecessors. R. A . Morzi-o.~

ISSN:0003-2654    

DOI:10.1039/AN9749900613

出版商:RSC    

年代:1974

数据来源: RSC