AOAC Methods for Review in Codex STAN 234_11-2018

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

39

F. Determination by Linear Sweep Anodic Stripping Voltammetry

analyzer, 1–2 min is sufficient, depending on level of analytes of interest in cell solution. Stop stirring and let solution equilibrate 30 s. Linearly increase applied voltage anodically. Follow manufacturer’s instructions for rate of scan, e.g., 2–6 mV/s. Measure wave height at peak potentials for Cd at − 0.62 ± 0.05 V and for Pb at − 0.45 ± 0.05 V vs SCE or Ag/AgCl. For widely varying concentrations 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 before Pb peak begins. Quantitate total amounts of Pb and Cd in cell solution by using method of standard additions in cell as follows: Record voltammogram from known volume of cell solution. From working standard solution, C ( g ), add known amounts of Pb and Cd, using appropriate micropipets, B ( f ), and being certain to add amount of each element sufficient to generate peak heights ca twice those given by test cell solution. Repeat with 2 more similar additions of working standard solution to cell solution. For each analyte, plot µ g added on x -axis vs peak height in µ A current on y -axis. Extrapolate linear plot to x -axis intercept to determine total amount of analyte in test aliquot. If available, use computer program based on method of least squares to calculate regression line and determine amount of analyte in test aliquot. Similarly, determine amount of each analyte in reagent blank aliquots, using same volume of aliquots for reagent blank as for test solution. Calculate concentration of analyte (mg/kg) in test portion as follows: Concentration, mg/kg = B C A 50 W − × where A = mL test solution taken for analysis; B = µ g analyte in test solution aliquot; C = average µ g analyte in reagent blank solution aliquots; and W = total g test portion.

Transfer 2.0 mL aliquot of test solution to electrolysis cell and add 3.0 mL electrolyte, C ( d ). pH of cell solution should be 4.3 ± 0.3. Deposit elements of interest onto compositeHggraphite electrode (CMGE) at − 0.9 V vs Ag/AgCl reference electrode for 30 min. Bubble N 2 through cell solution during entire deposition period. Linearly increase applied voltage anodically at 60 mV/s from − 0.9 to − 0.2 V vs Ag/AgCl reference electrode. Measure peak current ( µ A) for each analyte. Run reagent blank in same manner using same size aliquot as for test solution and determine peak current ( µ A) for each analyte. For each analyte, make standard addition to cell solution and measure peak current ( µ A). Calculate conversion factor, µ g/ µ A, for each analyte as µ g of addition divided by difference between peak current before and after addition of analyte standard. Verify conversion factors periodically. Multiply test peak current ( µ A) by conversion factor to determine µ g of each analyte in test solution aliquot. Calculate ppm ( µ g/g), using equation in E . G. Interference Tl may interfere with Pb determination, but its occurrence in food is unlikely. If Tl interference is suspected, treat as follows: Transfer 5.0 mL aliquot of test solution to electrolysis cell and make basic with 3.0 mL NaOH. Determine elements of interest in this solution by ASV in the usual manner. Plating potential is − 1.0 V vs SCE or similar reference electrode. Strip deposited elements by anodically scanning from − 1.0 to − 0.3 V vs SCE. In this manner, Cd and Pb peaks shift to − 0.78 ± 0.05 V and − 0.73 ± 0.05 V vs SCE, respectively. Tl peak remains at − 0.47 V vs SCE. References: JAOAC 65, 970, 978(1982); 70, 295(1987).

CAS-7440-43-9 (cadmium) CAS-7439-92-1 (lead)

 2005 AOAC INTERNATIONAL

10/9/2018