Microgram and submicrogram quantities of molybdenum, iron, and copper in bovine milk and intestinal xanthine oxidase fractions were determined by atomic absorption spectroscopy using the standard addition method. Dialysis fluids consisting of 0.1-10 m<i>M</i> potassium phosphate buffer, <i>p</i>H 7.4, containing 0.01-1 m<i>M</i> ethylenediaminetetraacetate served as blanks and diluents for both samples and standards. The sensitivities of the method for iron, molybdenum, and copper were 0.02-0.05, 0.01-0.02, and 0.01-0.02 (μg/ml)/1% absorption, respectively. Coefficients of variation of 8.2, 6.6, and 4.2% were obtained with samples containing 0.11, 0.056, and 0.021 μg/ml of iron, molybdenum, and copper, respectively. Occasional interferences encountered with either concentrated or partially purified fractions could be completely eliminated by direct dilution of the sample. Known quantities of all three metals added to various enzyme preparations were quantitatively recoverable. Dry ashing prior to analysis had no effect on the analytical results.

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