Atomic Absorption Spectrophotometric Determination of Lead, Cadmium, Zinc, and Copper in Clams and Oysters: Collaborative Study

Abstract In the method studied, clams and oysters are dry ashed at 475°C, the ash is dissolved in dilute HNO3, and Pb, Cd, Zn, and Cu are determined by flame atomic absorption spectrophotometry (AAS). Sample solutions containing levels of Pb and Cd too low for direct AAS are extracted as the 1-pyrrolidinecarbodithioate complex into n-butyl acetate before AAS. The method was subjected to a collaborative study to determine the precision and accuracy of the analysis. Concentration ranges in the samples studied were approximately 0.06–2 ppm Pb, 0.03–2 ppm Cd, 40–600 ppm Zn, and 5–50 ppm Cu. Within these ranges, Cd levels <0.5 ppm and Pb at all levels required extraction prior to AAS. The average coefficients of variation for Pb, Cd, Zn, and Cu were, respectively, 66.0, 93.2, 8.5, and 20.5% for the lowest levels and 13.5, 3.8, 6.7, and 6.8% for the highest levels. Recoveries were computed by using the sum of the average collaborative results for the totally unfortified samples plus the fortification levels as the expected concentration. The average recoveries of Pb, Cd, Zn, and Cu were, respectively, 94.0, 99.7, 99.3, and 100.2 % for the lowest fortification level and 94.8, 95.6, 96.5, and 97.5% for the highest level. High levels of Cu and/or Zn caused incomplete extraction of Pb and Cd, with Cd being affected more severely than Pb. Data obtained also suggest that background corrections may be needed for determining Zn and Cd with the direct aspiration technique. No recommendation is made for official action.