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Anel. Chem. 1991, 63,1298-1303
the lipophilicity of the ionophore. CONCLUSION The sensitive and selective electrodes for the lead(I1) ion based on MBDiBDTC and TMBDiBDTC were developed by using the conventional membrane electrode and membranecoated carbon rod electrode. The MBDiBDTC membrane electrode exhibited good properties for a Nernstian slope of 28 mV/decade, linearity range, and selectivity factor of different metal ions, because of the ideal size of the C-shaped cavity fitting the lead(I1) ion. LITERATURE CITED Ross, J. W.; Frant, M. S. Anal. Chem. 1989. 41, 987. Hkata, H.; Hlgashlyama, K. Anal. Chkn. Acta 1971, 54, 415. Hlrata, H.; Hlgashlyama, K. Bull. Chem. Sa.Jpn. 1971, 44, 2420. Hkata, H.; Hlgashlyama, K. Tsknta 1072, 70. 391. Masclnl, M.; Llbertl, A. Anal. chkn.Acta 1972, 60, 405. Haensen, E. H.; Ruzlka, J. Anal. chkn.Acta 1974, 72, 385. Klvelo, P.; VManen, R.: Wlckstrom, K.; Wilson, M.; Pungor, E.; Horval, 0.;Toth, K. Anal. C h h . Acta 1976, 87, 401. Qruendler. P. Anal. Lett. PatiA 1961, 14, 183. Thlnd, P. S.;Slngh, H.; Blndal, T. K. Indlan J . Chem., Sect. A 1962, 2 7 , 295. Pungor, E.: Toth, K.; Nagy, G.; Polos. L.; Ebel. M. F.; Wernlsch, I. Anal. C h h . Acta 1989, 147, 23. Vlasov, Yu. 0.; Bychkov, E. A.; Legln, A. V. Zh.Anal. Khlm. 1985, 40, 1839. Vlamv, Yu. G.; Bychkov, E. A.; Legln, A. V. Sov. Ekbochem. (Engl. Transl.) 1987, 2 2 , 1379. Van Stadem, J. F. Fresonb' 2.Anal. Chem. 1969, 333, 226. Lal, S.; Chrlstlan, G. D. Anal. chkn.Acta 1970, 52, 41. Ruzlcka, J.; TJeii, J. C. Anal. C h h . Acta 1970, 40, 348.
(18) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34)
Sharp. M. Anal. chkn.Acm 1072, 50, 137. J a k , A. M.; Moody, 0. J.; Thomes, J. D. R. Anahf I S M , 113, 1409. oehme, M.; S l m , W. Anal. CMn. Acta 1978, 86, 21. Mebr, P. C.; Morf, W. E.; Laubll, M.; Slmon, W. Anal. C h h . Acta 1984, 156, 1. Simon, W.; Retsch, E.: Morf. W. E.; Ammnn, D.; Oesch, U.; Dlnten, 0.Analyst 1964, 109. 207. Khura, K.; Ishikawa. A.; Tamura, H.; Shono, T. J . Chem. Soc.,Perkh Trans. 1984, 2 , 447. Nlemen, T. A.; Horval, G. Anal. Chim. Acta 1085, 770, 359. Kltazawa, S.; Klmura. K.; Yam, H.; Shono,T. Analyst 1985. 170, 295. Llndec, E.; Tom, K.; Pungor, E.; Behm, F.; Ogoenfuss, P.; Wdtl, D. H.; Ammann, D.; Morf. W. E.; Retsch, E.; Simon, W. Anal. Chem. 1964, 56. 1127. Shpigun, L. K.; Novikov, E. A.; Zolotov, Yu. A. Zh.Anal. K h h . 1966, 41. 817. Novkov, E. A.; Shpigun, L. K.; Zolotov. Yu. A. Zh. Anal. KMm. 1967, 42. 885. Popova, V. A.; Volkov, V. L.; Podgornaya, I. V. Zavod. Lab. 1989, 55. 25. Kamta, S.; Higo, M.; Kamlbeppu, T.; Tanaka, I. Chem. Lett. 1062, 287. Kamta. S.; W w a , F.; Fukumoto, M. Chem. Lett. 1987, 533. Kamata. S.; Wale, A.; Uda, T. Chem. Lett. 1966, 1247. Kamta, S.; Bhale, A.; Fukunags. Y.; Murata, H. Anal. Chem. 1966, 60, 2464. Kamta, S.; Murata, H.; Kubo, Y.; Bhale, A. Analyst 1080, 714, 1029. IUPAC, Recommendations for Nomenclature of Ion-Selective Electrodes. h e Appl. Chem. 1076, 48, 127. Moody, 0. J.; Thomas, J. D. R. Talanta 1972, 10, 823.
RECEIVED for review December 10,1990.Accepted March 14, 1991. This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture.
Microwave Acid Digestion and Preconcentration Neutron Activation Analysis of Biological and Diet Samples for Iodine Raghunadha R.Rao and Amares Chatt*
Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4Jl
A sbnple precomxntratkn neutron actlvatbn andyrb (PNAA) mothod has been developed for the determlnatlon of low levels of lodlne In blologlcal and nutritional materials. The method Involves dludutlon of the samples by mlcrowave dgodon Inthe prosonce of add8 In closedTeflon bombrand prmmconkatkm of total kdbn, after rdudkm to k d h wfth hydrazkn d a t e , by copndpltakn wtth #muth adflde. The effects of dMerent factors wch as acldlty, time for complete preclpltatlon, and concentratlonr of blrmuth, sutflde, and divot" knr on the quantltatlve recovery of kdick have been studied. The absolute detectJon #nil of the PNAA method b 5 ng of kdhn. Reddon af mea8uratnent, expressed In term6 of relatlve standard devlatlon, Is about 5 % at 100 ppb and 10% at 20 ppb kvok of Iodine. The PNAA method has beon applkd to several bbbgkal rdwance matwials and total dkt samples.
INTRODUCTION Iodine is considered an essential element. The daily dietary safe and adequate intake range of iodine for adults is reported to be 150-200 pg (1,2). Excessive iodine intake can contribute to certain thyroid disorders in susceptible individuals. In many countries, regulations require the control of the level of daily iodine intake through diet. The accurate determi-
nation of iodine in materiale such as diets, excreta, and tissues is of much interest in medical, nutritional, and epidemiologic research. The analytical techniques most commonly employed for measuring iodine levels are colorimetry (3, 41, ion-selective electrode (5-7), isotope exchange (4,8),gas chromatography (2, 9), and neutron activation analysis (NAA). The last technique has an excellent intrinsic sensitivity for iodine. However, low levels of iodine in biological materials cannot be easily measured by conventional reactor-flux instrumental NAA (INAA) because of interferences from the high activities of thermal neutron activation products of Na, K, Mn, Br, and C1 which are generally present in high amounts. Epithermal INAA (EINAA) can reduce some of these interferences but does not eliminate them completely. Thermal INAA involving combustion and gas-phase separation of iodine on hydrated manganese dioxide has been applied to determine iodine in biological (10) and total diet ( 1 1 ) samples. Applications of EINAA methods using boron, cadmium, and some combinations of boron and cadmium as thermal neutron shields to biological and geological materials have been reported ( 1El 9). The EINAA methods have been applied for the analysis of iodine in milk (20,21),infant formula (221, blood (231, biological reference materials (24-28), and food samples (29). Radiochemical NAA (RNAA) has been employed for the estimation of iodine in plant material (30),reference materials (31-33), and rock samples (34).
0003-2700/91/0383-1298$02.50/0 @ 1991 American Chemlcal Socbty
ANALYTICAL CHEMISTRY, VOL. 63,
The Community Bureau of Reference (BCR) in their interlaboratory comparison studies u t i l i z i different analytical techniques has reported a spread of iodine results of skim milk powders by a factor of 3 at about 0.2 ppm level (2). The problem of iodine determination in biological materials like food appears critical at concentrations below 0.5 ppm. The low iodine contents of food, complexity of the matrix, chemical interferences, possibility of contamination, and loss during sample dissolution are some of the potential limiting factors for iodine determination by many of the reported analytical methods. This paper presenta a new method for the determination of traces of iodine in biological materials. The method involves microwave acid digestion of the sample followed by preconcentration as well as separation of iodine from the interfering elements by bismuth sulfide coprecipitation. EXPERIMENTAL SECTION Reagents. The followinghigh-purity chemicals were purchased from the suppliers shown in square brackets: AR Grade bismuth nitrate, ammonium dihydrogen phosphate, and thioacetamide [Mallinckrodt Chemicals]; Puriss grade hydrazine sulfate, cupferron (N-nitrosophenylhydroxylamineammonium salt) [Fluka]; ultrapure ammonium iodide (SPEX]; and ultrapure nitric acid and ammonium hydroxide [Seastar]. Separate stock solutions of bismuth nitrate (40mg/mL Bi)' in 0.1 M nitric acid and thioacetamide ([S%] = 8.5 mg/mL), cupferron (100 mg/mL), and ammonium dihydrogen phosphate = 100 mg/mL) in deionized distilled water were prepared. Iodide Comparator Standards. A 0.5 mg/mL iodide stock standard solution was prepared by dissolving ammonium iodide in 5% v/v ammonium hydroxide. A working standard of iodide was prepared by diluting a required amount of the stock solution with deionized distilled water. The comparator standards were prepared by depositing microliter portions containing nanogram quantities of iodide onto precleaned 0.45-ctm pore size and 47mm-diameter Gelman Sciences GA-6S membrane filters that were preloaded in precleaned 1.2-mL polyethylene irradiation vials. Particular care was taken in preparing these comparator standards to ensure the same geometry as those of the samples. Reference Materials and Samples. Several standard reference materids (SRM), cert8ed reference materials (CRM) and reference materials (RM) were obtained from the U.S. National Institute of Standards and Technology (NIST), International Atomic Energy Agency (IAEA), and the Japanese National Institute for Environmental Studies (NIES). These materials were selected to cover a wide range of iodine concentrations that might exist in different biological and diet samples. All materials were dried according to the procedures prescribed by the issuing agencies. Many candidate reference materials and diet samples were also analyzed for iodine. Microwave Digestion of Samples. About 250 mg of the sample material was accurately weighed into the precleaned 45mL Teflon sample cup of a microwave acid digestion bomb (Parr Instrument Company), 5 mL of concentrated nitric acid was added, and the bomb was immediately closed. The bomb was placed at the center of a 675-W solid-state Admiral microwave oven and heated at full power for 35 8. A preliminary study on the microwave digestion of the sample as a function of time revealed that a heating period of 30 s was sufficient for dissolution of the sample. A heating period of 45 s and higher using the above loading caused an excessive pressure buildup inside the sample cup when the safety valve released. The bomb was removed from the oven and cooled for 20 min in a refrigerator. The bomb cap was unscrewed carefully. The sample cup was then removed from the bomb cylinder, and the lid was carefully opened. After checking for sufficiently complete digestion, the contents were poured into a precleaned 250-mL beaker containing 1 g of hydrazine sulfate. The sample cup and its lid were rinsed with 3 X 5-mL aliquota each of a 5% hydrazine sulfate solution and deionized distilled water to ensure that any iodine species that could have formed and deposited on the inner walls of the cup and lid were reduced to iodide and t r a n s f e d to the beaker. The washings were combined with the sample solution. Experiments
NO. 13, JULY 1, 1991 1299
performed with spiked standard iodide and lBI radiotracer solutions showed no detectable loss of iodine during the sample digestion and transfer steps. The rapid digestion-cycle time (approximately 25 min including cooling) allowed digestions to be performed in multiple steps, if desired, with a choice to add more acid to ensure sufficiently complete decomposition of the sample with respect to iodide and to permit its subsequent preconcentration. Recently a study on the evaluation of completeness of digestion by wet ashing of biological materialsin nitxic acid using microwave heating has been reported (35). When a microwave digestion is contemplated, the user must take adequate precaution in selecting appropriate power level, reaction time, and filling level of the sample cup depending on the nature of sample and digestion medium to avoid any danger of explosion due to excessive temperature and pressure buildup in the bombs. An investigator may obtain a preliminary estimate of the heating rate of a microwave oven by noting the time required to bring a small amount of the sample and digestion medium in an open sample cup to boiling at a given power level. The operating instructions, information on sample cup filling levels, and vapor pressure characteristicsof commonly used acids, etc., generally provided by the supplier of the bombs can be useful in estimating the expected pressure in a bomb. Any deformations of the sample cup and O-ring seal and/or leakage of the digestion medium in closed microwave digestions are indications of exceasive temperature and pressure generation in the bomb. After digeation, the microwave bomb must be suffkientlycooled for safety reasons before making any attempt to open the bomb. This cooling procedure can be monitored by observing the retraction of the retaining screw in the bomb cap to its preheating position. The cooling process is slow and may take a considerable length of time, which can be shortened by either directing a jet of cold air against the bomb or placing it in a refrigerator. A preliminary study done in our laboratory on the rate of cooling of a bomb in a refrigerator showed that a 20-min cooling period was more than adequate. The cooling also facilitates the deposition of any volatile iodine species that might have formed during the digestion step. Consequently, a 20-min cooling period was used in all subsequent experiments. General Preconcentration Scheme. This scheme was followed for the optimization of various factors that can influence the recovery of iodide. In general, a solution containing 100 mL of deionized distilled water and varying amounts of nitric acid and ammonium iodide was used. Either ammonium dihydrogen phosphate, cupferron, or thioacetamide was added to the above solution. The mixture was stirred with a magnetic stirrer and the precipitate was allowed to settle for different time periods at room temperature. It was then filtered through a precleaned Gelman Sciences membrane filter under vacuum suction. The precipitate and the beaker were washed several times with a solution containing 1% hydrazine sulfate and of same acidity as the sample solution. Finally, the filters were kept in precleaned 1.2-mL polyethylene vials and heat-sealed, maintaining the same geometry as those of the comparator standards. Irradiations and Counting. The Dalhousie University SLOWPOKE2 reactor (DUSR) total neutron flux can be varied by 3 orders of magnitude (between 10l2and 10'' n cm-2 s-l) and stabilized at a desired flux (36). The thermal, epi-cadmium, and fast neutron fluxes in all irradiation sites of DUSR have been studied in detail (19,36,37). All samples and standards of iodine were irradiated for 30 or 60 min in an epi-cadmium flux of 5.2 X 1@ n cm-2 s-l. For a few samples with very low levels of iodine, an epi-cadmium flux of 1X 1Olo n cm-2s-l was used. There was no need to irradiate either the comparator standard, reference material, or flux monitor every time a sample was irradiated in the DUSR facility because of the exceptional stability and homogeneity of the neutron flux (19,37).The samples and standards for iodine were counted for 30 or 60 min after a decay period of 1 min. An Aptec 25-cm3active volume hyperpure Ge detector (FWHM of 2.08 keV at the 1332-keV photopeak of)C o@ ' connected to a Link high count rate pulse processor coupled with a Nuclear Data ND 66 analyzer was used for counting purposes. RESULTS AND DISCUSSION Preconcentration techniques involving a solid phase fol-
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Table I. Effect of Acidity on the Recovery of Iodide
acidity, M
4.0 2.0 1.o 0.8-0.2 0.1 1 x 10-2 1 x 10-8 1 x 104 1 x 104 1 x 10" 1 x lo-' 1 x 10" 1 x 10+ 1 x 10-10 1 x 10-11
hydroxide method
3 87.3 92.8
100 100 100 100 100 100 100
% recovery of iodide phosphate cupferron method method
11 6 6 6 4 4 2 2 2 2 2
9 9 9 5 5 5 5 5 6 6 20 24 29
sulfide
method 44.1 63.8 94.3 100 100 100 98.6 98.1 94.1 89.5 88.2 87.1 82.4 63.0 40.4
lowed by quantitation with neutron activation require that the collector matrix should not give rise to any intense activity on irradiations. Since bismuth does not get activated on short irradiations and Bi3+can be quantitatively precipitated by cupferron, hydroxide, phosphate, and sulfide, we have attempted to coprecipitate iodide as Bi13 (Kapof Bil, = 8.1 X with one of these agents. The influence of different parameters on the quantitative coprecipitation of Bil, is discussed below in detail. Effect of Acidity. Onehundred-milliliter aliquots of 0.1-4 M nitric acid solutions were used to study the effect of acidity on the recovery of iodide ions. Onemilliliter portions of each of the working standard iodide (500ng/mL) solution and stock solutions of bismuth nitrate and one of the complexing agents, namely ammonium dihydrogen phosphate, cupferron, or thioacetamide solutions, were added in sequence while stirring. To study the recovery of iodide from alkaline medium, the acidity of sample solution was adjusted to the desired value with ammonium hydroxide by using a Ross combination electrode in conjunction with Fisher Accumet digital pH meter. The solution was allowed to stand for 60 min at room temperature for complete precipitation and later taken through the general preconcentration scheme described earlier. The percentage recovery of iodide with different coprecipitants at various acidities was determined. The results are presented in Table 1. The recovery of iodide was quantitative in the acidity range of (1-80) X M with sulfide and 1 X 10a-l X lo-" M with hydroxide; it was extremely poor with phosphate and cupferron over the entire range of acidity used (Table I). The highest recovery of 20-29% iodide between 1 X lo4 and 1 X lo-" M acidity with cupferron could be due to the formation of bismuth hydroxide. Selection of Coprecipitating Agent. The results of iodide recovery as a function of acidity suggest that interstitial coprecipitation (sulfide and hydroxide ions with close ionic size as that of iodide) may be predominant over adsorption coprecipitation (cupferrate and phosphate with larger ionic size) in scavenging iodide ions from solution. It was observed that the filtration of bismuth hydroxide was time consuming compared to the sulfide counterpart. Moreover, the general tendency of BiC13 and BiBr, in alkaline solutions to decompose to BiOCl and BiOBr, respectively,may lead to their coprecipitation with Bi(OH)& On the other hand, BiOCl and BiOBr are soluble in acidic solutions and hence they effectively neither coprecipitate with sulfide nor interfere in the estimation of iodide. For these reasons, sulfide coprecipitation procedure a t 0.2 M acidity was selected for further investigation of the optimum conditions for the quantitative recovery of iodide.
Table 11. Effect of Bismuth Ion Concentration on Recovery of Iodide" Bi3+ion added, mg
recovery of iodide, %
0.10 0.25 0.50 0.75 1.0 2.5 5.0 7.5-120
17.5 32.6 51.9 65.5 81.7 94.1 100 100
"Volume of sample solution = 100 mL; iodide ion added = 500 ng; acidity of solution = 0.2 M; sulfide ion added = 8.5 mg. Table 111. Effect of Sulfide Ion Concentration on Recovery of Iodide" sulfide ion added, mg
recovery of iodide, %
0.0213 0.0534 0.1067 0.1601 0.2134 0.3201 0.4268 1.067 2.134 3.201 4.268 8.536 17-127
7.4 19.2 34.0 44.0 64.2 72.9 86.0 91.0 95.4 98.0 100 100 100
a Volume of sample solution = 100 mL; bismuth ion added = 40 mz: aciditv of solution = 0.2 M. iodide ion added = 500 ne.
Effect of Bismuth Ion Concentration. Bismuth ion concentrations were varied between 0.1 and 120 mg in a 100-mLsample solution of 0.2 M acidity. The concentrations of iodide and sulfide ions were kept constant a t 500 ng and 8.5 mg, respectively. The preconcentration scheme was used, and the recoveries obtained are presented in Table 11. The results show a gradual increase in recovery with increasing bismuth ion concentration. Though the recovery started even with 0.1 mg of Bi3+ion, it was not quantitative until 5 mg of Bi3+ion was added. It remained quantitative between 5 and 120 mg Bi3+ ion concentration. In subsequent studies, the concentration of Bi3+ion was maintained at 40 mg to enable complete precipitation of even a few micrograms of iodide. Effect of Sulfide Ion Concentration. The recovery of iodide ions as a function of sulfide ion concentration was studied by varying the sulfide concentration between 0.02 and 128 mg, keeping acidity, bismuth, and iodide ion concentrations constant at 0.2 M, 40 mg, and 500 ng, respectively, in a 100-mL sample solution. The recovery increased with increasing sulfide ion concentration and was quantitative above 4.3-mg sulfide ion concentration (Table 111). For all practical purposes, 8.5-mg sulfide ion concentration was selected as an adequate amount. Effect of Time. f i r the addition of bismuth, iodide, and sulfide ions to a 1WmL sample solution of 0.2 M acidity, the recovery of iodide ion was estimated as a function of time (1-60 min) allowed for complete precipitation before fdtration. It was found that quantitative recovery was attained in just 10 min after the addition of all reagents. A 20-min settling period was allowed in subsequent experiments. Effect of Diverse Ions. Stock solutions of potentially interfering ions, namely A13+,As3+,Co2+,CP+, Cu2+,Fe3+,K+, Mn2+,Na+, Ni2+,Pb2+,Sb3+,Zn2+,Br-, C1-, and Sod2-,at 100 mg/mL were separately prepared from ultrapure standards. For each ion, a sample solution was made consisting of a given
ANALYTICAL CHEMISTRY, VOL. 83, NO. 13, JULY 1, 1991
Table IV. Effect of Diverse Ionsa ion Pbz+ Cuz+, Fez+,Asa+, SbS+
Fea+,CrS+ A P , ' Co2+, Mn2+ Ni2+,Zn2+ Na+, K+
so,%
decontamination factor
>102 >lo4
>1P >104 >loa
tolerance limit 40 mg
10 mg 40 ma 5 mg40 mg 100 m i
C1-, Bra Volume of sample solution = 100 mL; acidity of solution = 0.2 M; bismuth ion added = 40 mg; sulfide ion added = 8.5 mg; iodide ion added = 100 ng.
amount of that ion, 40 mg of bismuth, 100 ng of iodide, and 8.5 mg of sulfide ions in a total volume of 100 mL at 0.2 M acidity. Separate sample solutions with varying amounts of that ion were prepared. These solutions were taken through the general preconcentration scheme, and the resulting precipitates were analyzed. In each case, the recoveries of iodide were measured by the EINAA method given earlier and those of the added ions by an INAA method described below. Concentrations of these ions were measured by irradiating the precipitates in a total reactor neutron flux of 5 X 10" n cm-2 s-l. The irradiation, decay, and counting times used varied between 1 min and 7 h, 1min and 7 days, and 10 min and 1 h, respectively, depending on the half-lives of the nuclides and concentrations of the elements. The nuclides and y-rays (keV) used were as follows: %Al(1779),76As(559),82Br (777), W 1 (1642), oC@ ' (1332), W r (3201, @Cu (1039), 69Fe (1099),'% (1525), 68Mn(1811), %Na (1369), @Ni(1482), laSb (564,693), and =Zn (1115). The tolerance limit was defined in this study as the amount of the added foreign ion that caused a deviation of 5% in the expected iodine value in the precipitate. Decontamination factor was calculated as the ratio between the amount of added foreign ion and its recovered amount in the precipitate along with iodide. The tolerance limits and decontaminationfactors calculated for various ions are given in Table IV. The decontaminationfactors and tolerance limits for all common ions which are present in large amounts in most biological and diet materials are appreciably high. Thus, the method is highly selective for the determination of traces of iodine in them. Reagent Blanks. Reagent blanks must be kept to a minimum in all preconcentrationtechniques for trace element analysis. For this reason, all glassware and polyethylene bottles were first cleaned by using soap, tap water, and deionized distilled water and then soaked in 2 M nitric acid for at least 1day, washed, and then stored in a covered bath containing 0.5 M nitric acid in a separate room away from the working laboratory. Immediately prior to use, the glassware was rinsed with deionized distilled water and used without drying. All reagents were stored in stoppered precleaned polyethylene bottles. The polyethylene irradiation vials and the Gelman Sciences membrane filters were separately soaked in 1M nitric acid for 2 days, rinsed with deionized distilled water, dried, and stored in a separate place. Materials and chemicals used in the procedure were analyzed for iodine content. The results obtained (Table V) show that contributions from these sources are critical only a t iodine concentrations below 10 ng/g. Recommended Preconcentration Procedure. The digested sample solution was diluted to 100 mL to keep the final acidity at 0.2 M. To this solution, 1-mL aliquots of each of bismuth nitrate and thioacetamide stock solutions were added dropwise one after the other while slowly stirring the solution with a magnetic stirrer. The contents were allowed to settle
1901
Table V. Concentration of Iodine in Materials and Chemicals material deionized distilled water
