AOAC Methods in Codex STAN 234 (Preliminary Methods Review)

970

J. ASSOC. OPF. ANAL. CHEM. (VOL 65, NO. 4, 1982)

GAJAN ET AL.:

Determination of Lead and Cadmium in Foods by Anodic Stripping Voltammetry: I. Development of Method RAYMOND J. GAJAN,l STEPHEN G. CAPAR, CHERYL A. SUBJOC and MARION SANDERS 2 ' Food and Drug Administration, Division of Chemical Technology, Washington, DC 20204

Food samples are dry ashed at 500 ± S0°C with a 10% aqueous K2S04 solution used as an ashing aid. The ashed sample is dissolved in 50 ml 2% HN0 3 • An– odic stripping voltammetry is used to determine lead and cadmium in a mixture of the sample solution and an acetate electrolyte at pH 4.3 ± 0.3. The estimated quantitation limits, based on a 10 g food sample, are 0.005 ppm for cadmium and 0.010 ppm for lead. Over the last several decades, regulatory officials have been aware of the potential hazards asso– ciated with toxic metals such as cadmium and lead in foods (1-7). The sources of lead, in par– ticular, and the health concerns arising from its presence in the food supply were recently dis– cussed by the Food and Drug Administration (FDA) in the Federal Register (8). The Official Methods of Analysis of the AOAC (9) now includes several methods for the deter– mination of cadmium and lead in foods. How– ever, a need still exists for an accurate, precise, and sensitive method capable of determining these elements in a wide variety of foods at con– centration levels lower than can be determined by the current official methods. Recently 16 laboratories participated in an AOAC collaborative study of a dry ash anodic stripping voltammetric (ASV) method for the determination of cadmium, copper, lead, and :line in foods (10). Although 12 of the 16 labo– ratories finished the study, only 4 submitted statistically acceptable results. Consequently, the Associate Referee ruled that the method failed to meel AOAC requirements and recom– mended that the method be further studied (11 ). Most of the collaborators objected to the amount of sulfuric acid ashing aid required. Using sul– furic acid as the ashing aid often caused splat– tering with certain sample matrices during either thl:' drying or the ashing step of the method. Several collaborators commented that prolonged This is p.irt of the report of the Associate Referee, R. J. Gajan, wh,ch was presl•nted at the Symposium on Analytical Meth· \>r 2, 1981. ~ Assnmtt' Rcf~ref k~r Pol_aro~rap~y (Metals). . • Nati1"1~I Mannl' h sheries Service, Southeast hsheries ~ \ ·ntl'r, ('h,1rll'st11n. SC 29412.

use of the sulfuric acid ashing aid caused rapid corrosion of their furnaces, and we have experi· enced the same problem. Because of these disadvantages, we restudied the method and also re-evaluated other wet and dry ash methods used for the determination of cadmium and lead in biological materials. We found problems of various degrees with all of the methods studied, especially at the lower con– centration levels of interest, i.e., 0.025 ppm cad· mium and 0.050 ppm lead. We also wanted tu avoid using the various perchloric acid digestion methods because special apparatus, i.e., hoods and/ or digestion apparatus, are required to use this hazardous reagent safely. Yeager et al. (12), in a modification of a method described by Bambach and Burkey (13), used potassium sulfate as the ashing aid for the de· termination of lead in air and biological materi· als. Potassium sulfate is also the ashing aid used in the U.S. Public Health Service method de– scribed by Keenan and coworkers (14) for the determination of lead in air and biological ma– terials. We obtained good results by using 10 g K2S04/IOO ml 2% (v /v) HN03 as the ashing aid. We also found that using this ashing aid instead of sulfuric acid permitted relaxation of tht- usual stringent temperature requirements associated with the use of the latter. Consequently, ashing times were faster and any slight overshooting of furnace temperature was better tolerated. Also, because we did not need to boil off sulfuric acid, we could reach the maximum temperature more rapidly. The developmental work for the method was done with differential pulse anodic stripping voltammetry (DPASV) at the hangi'.'.lg mercurv drop electrode (HMDE), The method was alsu evaluated less extensively with linear sweep anodic stripping voltammetry (lSASV) at a composite mercury graphite electrode (CMGE), results were comparable to those obtained with the HMDE. Summary of Method The method summarized here was collabora– tively studied. The complete method and tht:>

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