Codex STAN 234 Working Group-Method Reviews 1-23-18

9.2.19

acid mixture, C ( b ), for each g dry matter and let stand ≥ 2 h. Heat under hood or H 2 O vacuum manifold system until flask contains only H 2 SO 4 and inorganic salts. ( Note: Take care to avoid material loss from foaming when heat is first applied, and when foaming occurs soon after material chars. Remove heat and swirl flask before continuing digestion. Add HNO 3 , if necessary.) Cool digest few min. (Digest should be cool enough to add ca 15 mLH 2 O safely, but hot enough to boil when H 2 O is added.) Wash while still hot into 40–50 mL tapered-bottom centrifuge tube and swirl. Let cool, centrifuge 10 min at 350 × g , and decant liquid into waste beaker. (Film-like precipitate on surface may be discarded.) Dislodge precipitate by vigorously stirring with eccentric-coupled stirring motor. To complete transfer, add 20 mL H 2 O and 1 mL 0.5M H 2 SO 4 to original flask and heat. Do not omit this step even though it appears transfer was complete in first wash . Wash hot contents of original digestion flask into centrifuge tube containing precipitate. Swirl to mix, cool, centrifuge, and decant liquid into waste beaker. Dislodge precipitate by stirring vigorously, add 25 mL saturated (NH 4 ) 2 CO 3 solution (ca 20%), and stir until all precipitate is dispersed. Let stand 1 h, centrifuge, and decant liquid into waste beaker. Repeat (NH 4 ) 2 CO 3 treatment. After decanting, invert centrifuge tube on paper towel and drain all liquid. Add 5 mL1MHNO 3 (use larger volume 1MHNO 3 in both test solution and blank if > 25 µ g Pb is expected), stir vigorously to expel CO 2 or use ultrasonic bath 2–3 min, let stand 30 min, and centrifuge if precipitate remains. (Use same technique for all test solutions.) E. Determination Set instrument to previously established optimum conditions, using air–C 2 H 2 oxidizing flame and 217.0 or 283.3 nm resonant wavelength. Determine A of test and blank solutions and ≥ 5 standards within optimumworking range (10–80% T ) before and after test readings. Flush burner with 1M HNO 3 and check 0 point between readings. Determine Pb from standard curve of A against µ g Pb/mL: Concentration Pb, mg / kg ( g Pb / mL) (mL 1M HNO 3 = × µ ) g test portion Reference: JAOAC 55, 737(1972). CAS-7439-92-1 (lead)

AOAC Official Method 972.25 Lead in Food Atomic Absorption Spectrophotometric Method

First Action 1972 Final Action 1976

A. Principle Organic matter is digested and Pb released coprecipitates with SrSO 4 . Soluble sulfate salts are decanted, and precipitate is converted to carbonate salt, dissolved in acid, and determined by AA at 217.0 or 283.3 nm. B. Apparatus ( a ) Atomic absorption spectrophotometer. —Operated at 217.0 or 283.3 nm. ( b ) Stirring motor. —With eccentric coupling for stirring centrifuge tubes. C. Reagents (Age all new glassware and all glassware which has contained high Pb concentration in boiling HNO 3 before washing. Never let used glassware dry before washing, and always include final HNO 3 rinse followed by deionized H 2 O rinse.) ( a ) Strontium solution. —2%. Dissolve 6 g SrCl 2 ⋅ 6H 2 O in 100 mL H 2 O. ( b ) Ternary acid mixture. —Add 20 mL H 2 SO 4 to 100 mL H 2 O, mix, add 100 mL HNO 3 and 40 mL 70% HClO 4 , and mix. ( c ) Nitric acid 1M .—Add 128 mL redistilled HNO 3 to 500–800 mL distilled or deionized H 2 O and dilute to 2 L. Redistilled HNO 3 (GFS Chemical, Inc., No. 63) may be diluted and used without redistillation. ( d ) Lead standard solutions .—( 1 ) Stock solution .—1000 µ g/mL. Dissolve 1.5985 g Pb(NO 3 ) 2 , recrystallized as in 935.50B ( see 9.2.12), in ca 500 mL 1MHNO 3 in 1 L volumetric flask and dilute to volume with 1M HNO 3 . ( 2 ) Working solutions .—Prepare 100 µ g Pb/mL by diluting 10 mL stock solution to 100 mL with 1M HNO 3 . Dilute 1, 3, 5, 10, 15, and 25 mL aliquots of this solution to 100 mL with 1M HNO 3 (1, 3, 5, 10, 15, and 25 µ g Pb/mL, respectively). D. Separation of Lead Accurately weigh test portion containing ≤ 10 g dry matter and ≥ 3 µ g Pb. Place in 500mLboiling or Kjeldahl flask and add 1mL2% Sr solution, C ( a ), and several glass beads. Prepare reagent blank and carry through same operations as test portion. Add 15 mL ternary

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