AOAC Methods for Review in Codex STAN 234_11-2018

AOAC Official Methods Listed in CXS 234 for Milk and Milk Products

38

9.1.02

( g ) Working standard solutions. —Prepare either separate or mixed working standard solution for Cd and Pb in the range 0.1–10 µ g/mL from standard solutions, ( e ) and ( f ), by mixing appropriate aliquots in 1% (v/v) HNO 3 . [ Note: Electrolyte solution, ( d ), and K 2 SO 4 solution, ( b ), may require further cleanup for sufficiently low reagent blanks. For stated quantitation limits, analyte concentrations in final cell solutions (electrolyte and test solutions) of reagent blank should not be >0.5 ng Cd/mL and >1 ng Pb/mL. Controlled potential electrolysis is recommended means of cleaning reagents.] D. Preparation of Material ( Note: Laboratory contamination control is important. Take all precautions possible to avoid contamination of laboratory and test samples, reagents, and equipment. Prepare at least 3 control reagent blanks which include any additional H 2 O and HNO 3 used for test portion ashing. Carry control reagent blanks through entiremethod.) Weigh 5.0–10.0 g homogenized test portion into ashing vessel, B ( b ). Use 5.0 g for dry materials such as cereals. Add 5.0 mLK 2 SO 4 ashing solution, C ( b ), and mix thoroughly, using glass stirring rod. If needed, add H 2 O to ensure test portion and ash aid are well mixed. Cover ashing vessel with glass cover and dry in 110 ° –120°C oven, B ( c ), until thoroughly dry (usually 2–3 h or, if desired, overnight). Place vessel in cold furnace, B ( d ), and set temperature at 500 ° –550°C. ( Caution : Do not heat >500°C if using Pyrex beakers, and avoid excessive overshooting of temperature.) Maintain set temperature ≥ 4 h (may be ashed overnight). Remove vessel from furnace, and cool. Ash should be white and essentially carbon-free. Wash down sides of vessel with minimum amount H 2 O and add 2.0 mLHNO 3 . Use glass stirring rod to break up solid particles. Dry thoroughly on hot plate, B ( e ), at low setting. If products such as sugars and cereals splatter on hot plate during HNO 3 treatment, dry under IR lamp instead. Increase hot plate setting to medium for several minutes to ensure dryness. Return vessel to 500°C furnace 30 min. Cool; if necessary, repeat HNO 3 treatment using 1 mL increments of HNO 3 , until white, C-free ash is obtained. Add 1.0 mLHNO 3 and ca 10 mL H 2 O to vessel, swirl to dissolve, and let stand 5 min. If residue remains undissolved, warm gently on 80 ° –90°C hot plate not >5 min. Minimize possible Sn(II) formation by heating dilute acid solution as little as possible. Small amount of white, siliceous-like precipitate may remain undissolved. Cool, and quantitatively transfer residue to 50 mL volumetric flask with aid of H 2 O. Dilute to volume with H 2 O and mix well. Let stand to allow any precipitate present to settle. Do not filter. Use clear supernate to determine analytes by either DPASV or LSASV below. E. Differential Pulse Anodic Stripping Voltammetry Transfer 5.0 mL aliquot of test solution to electrolysis cell containing Teflon-coated stirring bar and add 5.0 mL electrolyte solution, C ( d ), to cell. (Aliquot volume may be varied as long as 1:1 ratio is maintained between test solution and electrolyte.) pH of cell solution should be 4.3 ± 0.3. Room temperature should be constant ( ± 1°C/2 h) and between 20 ° and 30°C. Purge solution 5 min with N 2 , C ( c ). Adjust gas inlet to let N 2 flow gently above and across solution surface. If hanging Hg drop electrode is used, add fresh drop of Hg to capillary tip with micrometer or similar device to ensure reproducibility of drop. Turn on stirrer motor and electrolyze solution at –0.8 V vs saturated calomel electrode (SCE) or Ag/AgCl electrode. Deposition time may vary with instrument ( see manufacturer’s instructions). When using PAR 174 polarographic

AOAC Official Method 982.23 Cadmium and Lead in Food Anodic Stripping Voltammetric Method

First Action 1982 Final Action 1988

(Not applicable to fats and oils.) A. Principle Material is dry-ashed with K 2 SO 4

and HNO 3 at ca 500 ° C. Pb and Cd are determined by anodic stripping voltammetry (ASV). Estimated quantitation limits, based on 10 g test portion, are 0.005 mg Cd/kg and 0.010 mg Pb/kg. B. Apparatus [Thoroughly soak all glassware and plasticware in 20% (v/v) HNO 3 for ≥ 24 h and rinse with distilled, deionized H 2 O.] ( a ) Voltammetric analyzer .—Capable of ASV and equipped with necessary accessories, i.e., cells, electrodes, recorders, Hg capillaries, micrometer or similar device for adjusting drop size, stirring motor, etc. (Princeton Applied Research Corp., PO Box 2565, Princeton, NJ 08540, USA; Models 384A, 264A, and 303A, (current Model 394), or equivalent, for differential pulse anodic stripping voltammetry [DPASV] at hanging Hg drop electrode; Environmental Sciences Associates, 45 Wiggins Ave, Bedford, MA 01730, USA, Model 3010A, or equivalent, for linear sweep anodic stripping voltammetry [LSASV]). ( b ) Ashing vessels. —150–250 mL quartz, Vycor, or Pyrex beakers equipped with suitable glass covers (Fisher Scientific Co., No. 2-609A, or equivalent). Quartz is preferred. Vycor or Pyrex may be used if quartz beakers are not available. ( Note: For best results, quartz beakers should be fire-polished to retard etching.) ( c ) Drying oven. —Controllable within range 50 ° –150 ° C with ≤ 5 ° C variation. ( d ) Furnace. —Controllable within range of 100 ° –1000 ° C with ≤ 5 ° C variation. Check calibration of oven temperature control to ensure accurate temperatures. Furnace must be operated in suitable fume hood. ( e ) Controllable hot plate. ( f ) Micropipets. —10–100 µ L (Eppendorf, or equivalent). C. Reagents ( Note: Use only distilled, deionized H 2 O.) ( a ) Nitric acid. —J.T. Baker Chemical Co. No. 9598, or equivalent. ( b ) Potassium sulfate ashing solution. —10 g/100 mL. Dissolve 50.0 g K 2 SO 4 (J.T. Baker Chemical Co. No. 3278, or equivalent) in 400mLH 2 O containing 10mLHNO 3 . Dilute to 500mLwith H 2 O. ( c ) Nitrogen. —Prepurified, H 2 O-pumped. ( d ) Electrolyte solution. —1.7M in CH 3 COOH, 1.25M in sodium acetate trihydrate, and 0.1M in tartaric acid. Dissolve 170.0 g NaCH 3 COO ⋅ 3H 2 O (ACS) in 300 mL H 2 O. Add 97 mL glacial CH 3 COOH and 15 g tartaric acid (ACS). Dilute to 1 Lwith H 2 O. pH should be 4.7 ± 0.1. ( e ) Cadmium standard solution. —1.0 mg/mL. Dissolve 1.000 g Cd (99.99%) in 10 mL HNO 3 in 1 L volumetric flask. Dilute to volume with H 2 O. ( f ) Lead standard solution. —1.0 mg/mL. Dissolve 1.000 g Pb (99.99%) in 10 mL HNO 3 in 1 L volumetric flask. Dilute to volume with H 2 O.

 2005 AOAC INTERNATIONAL

10/9/2018