AOAC Methods in Codex STAN 234 (Preliminary Methods Review)

CAl'AR F.T AL.: J. ASSOC. Ol'F. ANAi.. C.:HEM. (VOL. 6S, NO. 4, 1982)

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to det. µ.g of !'!ach analyte in sample soln aliqu11t. Cale. ppm, using equation in 25.C0S. 25.C07 lnterfere11ce Tl may interfere with Pb detn, but its occur– rence in food is unlikely. If Tl interference is suspected, treat as follows: Transfer 5.0 mL ah· quot of sample soln to electrolysis cell and make basic with 3.0 m L NaOH. Det. elements of in– terest in this soln by ASV in the usual manner Plating potential is - 1.0 V vs SCE or similar ref. electrode. Strip deposited elements by anodi· ca!ly scanning from - 1.0 to -0.3 V vs SCE. Jn this manner, Cd and Pb peaks shift to -0.78 ± 0.05 V and -0.73 ± 0.05 Vvs SCE, resp. Tl peak remains at -0.47 V vs SCE. Collaborative Study Three laboratories representing the Food and Drug Administration (FDA), the National Ma· rine Fisheries Service (NMFS), and the National Food Processors Association (NFP A) cooperated in conducting this collaborative study Twenty-two laboratories representing govern– ment and industry agreed to participate in tlw study. Results were received from 20 of these laboratories. Laboratory 1 did not finish becauSt' of instrument failure. Laboratory 22 did not finish because of an unforeseen emergency. Six commodities representing the more im· portant food classes were selected for analysis· green beans (strained), beef (baby food), fish (mackerel), infant formula (milk base), apple juice, and cereal (wheat farina). The collabora– tive samples were prepared by homogenizing sufficient amounts (ca 2 kg) of each commodity, for effective homogenization and fortification, a 40% cereal mix and a 50% fish mix were pre· pared with water. All analytical results o~tained for the cereal and fish commodities are based on the diluted samples. Each homogeneous com· modity was split into 3 portions. One portion, Level 0, was not fortified. The other 2 portiom were each fortified with cadmium and lead. Level 1, the low fortification, ranged from O.Q3 to 0.08 ppm cadmium and from 0.05 to 0.15 ppm lead, depending on the commodity. Level 2, the high fortification, ranged from 0.12 to 0.28 ppm cadmium and from 0.24 to 0.45 ppm lead. Lev~] 0 was split into s·ubsamples of approximately 250 g each. Levels 1 and 2 were split into subsam– ples of approximately 12 g each, which were placed in 20 mL plastic scintillation vials and frozen until the time of analysis. These samples were randomly numbered with 6 digits con-

pokntials for Cd at -0.62 :I: 0.05 Vand for Pb at -0.45 ± 0.05 V vs SCE or Ag/ AgCI. For widely varying concns of Cd and Pb, change current sensitivity to appropriate range by momentarily stopping stripping scan at end of Cd peak, switching to appropriate sensitivity setting for Pb, and then continuing scan bt!fore Pb peak begins. Quantitate total amts of Pb and Cd in cell soln by using method of std addns in cell as follows: Record voltammogram from known vol. of cell soln. From working std soln (g), add known amts of Pb and Cd, using appropriate micwpipets (f) and being certain to add amt of each element sufficient to generate peak hts ca twice those given by sample cell soln. Repeat with 2 more similar addns of working std soln to cell soln. For each analyte, plot µ,gadded on x-axis vs peak ht in µ,A current on y-axis. Extrapolate linear plot to x-axis intercept to det. total amt of analyte in sample aliquot. If available, use computer program based on method of least squares to calc. regression line and det. amt of analyte in sample aliquot. Similarly, det. amt of each analyte in reagent blank aliquots, using same Vt>!. of ali· quots for reagent blank as for sample. Cale. ppm analyte in sample as follows: ppm (µg/g) == [(B - C)/AJ X (50/W) where A = mL sample soln taken for analysis; B = /,Lg analyte in sample soln aliquot; C = av. µ,g analyte in reagent blank soln aliquots; and W = total g ::;ample. Transfer 2.0 ml aliquot of sample soln to electrolysis cell and add 3.0 ml electrolyte (d). pH of cell soln should be 4.3 ± 0.3. Deposit el– ements of interest onto composite Hg graphite electrode (CMGE) at -0.9 V vs Ag/ AgCl ref. electrode for 30 min. Bubble N through cell soln during entire deposition period. Linearly in– crease applied voltage anodically at 60 mV /s from -0.9 to -0.2 V vs Ag/ AgCl ref. electrode. Measure peak current (µA) for each analyte. Run reagent blank in same manner using same size aliquot as for sample and dct. peak current (µA) for each analyte. For each analyte, make std addn to cell soln and measure peak current (µ.A). Cale. conversion factor, µ,g / µ,A, for each analyte as µ,g of addn divided by diff. between peak current before and after addn of analyte std. Verify conversion factors periodically. Multiply sample peak current (µA) by conversion factor 25.C06 Determination by Linear Sweep Anodic Stripping Voltammelry

Codex Trial Method Review