AOAC Methods in Codex STAN 234 (Preliminary Methods Review)

J. ASSOC. ()Ff. ANAL. CHEM. (VOL. 65, NO. 4, !'l82)

979

CAPAR F.T At.:

r orks. Corning, NY, PC-35, or equiv. (fl :\1icropipets.-10 thru 100 µL (Eppendorf, l'quiv.), Reagents Nt>le: Use only distd, deionized H2O. (a) Nitric acid. -J .T. Baker Chemical Co. No. 593, or equiv. fb) Potassium sulfate ashing so/n.-10 g/100 mL. :Dissolve 50.0 g K25O 4 (J.T. Baker Chemical Co. No. 3278, or equiv.) in 400 mL H 2 O contg 10 mL H\\13. Dil. to 500 mL with H 2 0. (cl Nitrogen. -Prepurified, H 2 O-pumped. (d) Electrolyte soln.-.l.7M in HOAc, 1.25M in 'N,1 ,1cetate trihydrate, and 0.01Min tartaric acid. b,ssolve 170.0 g NaOAc•3H2O (ACS) in 300 mL :lip. Add 97 mL glacial HOAc and 1.5 g tartaric 'bi (ACS). Di I. to 1 L with H2O. pH should be 4. ± 0.1. (e) Cadmium std soln.-1.0 mg/ml. Dissolve 000 g Cd (99.99%) in 10 mL HNO3 in 1 L vol. sk. Di!. to vol. with H 2 O. (f) Lead (Pb) std soln.-1.0 mg/mL. Dissolve 1 OOfl g Pb (99.99%) in 10 mL HNO 3 in 1 L vol. Ha,k, Oil. to vol. with H20. " (g) Working std solns. -Prep. either sep. or IJ'lhed working std soln for Cd and Pb in the l'ang~ 0.1-10 µg/ mL from std solns (e) and (f) by di~$OJving appropriate aliquots in 1% (v/v) .fiNO,. Nt1/~: Electrolyte soln (d) and K 2 SO 4 soln (b) 111ay require further cleanup for sufficiently low !l!agN1t blanks. For stated quantitation limits, in,1lytc concns in final cell solns (electrolyte and !.lmple solns) of reagent blank should not be }Q.:; ng Cd/mL and >l ng Pb/mL. Controlled kntial elecrolysis is recommended means of c!eanlng reagents. Preparation of S,1mple Nolt·: Laboratory contamination control is llmportant. Take all precautions possible to "il'(l1d contamination of samples, reagents, and uipmcnt. Prep. at least 3 control reagent nks which include any addnl H 2O and HNO 3 ed for sample ashing. Carry control reagent la:1.ki; thru entire method. Weigh 5.0-10.0 g homogenized sample into ~hing vessel (b). Use 5.0 g for dry materials ch as cereals. Add 5.0 mL K2SO 4 ashing soln ,lbland mix thoroly, using glass stirring rod. If 11e~ded, add H2O to ensure sample and ash aid re well mixed. Cover ashing vessel with glass er and d.ry in 110-120° oven (c) until thoroly (usually 2-3 h or, if desired, overnight). c~vessel in cold furnace (d) and set tempera-

ture at 500-550°. Caution: Do not heat >500° if using Pyrex beakers, and avoid excessive ov– ershooting of temp. Maintain set temp. 2:4 h (may be ashed overnight), Remove vessel from furnace, and cool. Ash should be white and es– sentially carbon-free. Brownish-red color in the ash (possible Fe 2 O 3 ) is acceptable and does not require the following HNO:1 treatment. If ash contains C particles (i.e., ash is grey or blac.k instead of white), wash down sides of vessel with H2O and add 2.0 ml HNO 3 . Use glass stirring rod to break up solid particles. Dry thoroly on hot plate (e) at low setting. If samples such as sugars and cereals splatter on hot plate during HNO 3 treatment, dry under IR lamp in– stead. Increase hot plate setting to medium for several minutes to ensure dryness. Return ves– sel to 500° furnace 30 min. Cool; if necessary. repeat HNO 3 treatment using l mL increments of HNO 3 , until white, C-free ash is obtained. Add 1.0 mL HNO3 and ca 10 mL H2O to vessel and, if necessary, heat on hot plate at low heat until sample ash is dissolved. Small amt of white, siliceous-like ppt may remain undis– solved. Cool, and quant. transfer sample to 50 mL vol. flask with aid of H 2 0. Dil. to vol. with H 2 O and mix well. Let stand to allow any pp! present to settle. Do not filter. Use clear su– pernate to det. analytes by either DPASV or LSASV below . 25.C0S Differential Pulse Anodic Stripping Voltammetry Transfer 5.0 mL aliquot of sample soln to electrolysis cell containing Teflon-coated stirring bar and add 5,0 mL electrolyte soln (d) to cell. (Aliquot vol. may be varied as long as 1:1 ratio is maintained between sample soln and electro– lyte.) pH of cell soln should be 4.3 ± 0.3. Room temp. should be const-a11t (±1 ° /2 h) and between 20 and 30°. Purge soln 5 min with N (c). Adjust gas inlet to let N flow gently above and acros!-; soln surface. If hanging Hg drop dectrode is used, add fresh drop of Hg to capillary tip with micrometer or similar device t<) ensure repro– ducibility of drop. Turn on stirrer motor and electrolyze soln at -0.8 V vs satd calomel elec– trode (SCE) or Ag/ AgCl electrode, Deposition time may vary with instrument (see manufac– turer's instructions). When using PAR 174 po– larographic analyzer, 1-2 min is sufficient, de– pending on level of analytes of interest in cell soln. Stop stirring and let soln equilibrate 30 s. Linearly increase applied voltage anodically. Follow manufacturer's instructions for rate of scan, e.g., 2-6 mV/s. Measure wave ht at peak

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