DETERMINATION OF ULTRATRACE LEVELS OF LEAD IN INFANT FORMULA BY ISOTOPE-DILUTION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

A simple method was developed for the accurate and precise determinati on of low- and sub-ppb (ng/g) concentrations of lead in infant formula by isotope dilution inductively coupled plasma mass spectrometry usin g ultrasonic nebulization. After addition of a known amount of Pb-207, samples were microwave digested and the ratio Pb-207/Pb-208 was measu red in the digests. Agreement with certified values for lead in milk p owder standard reference materials was good, and isotope dilution anal ysis using Pb-206 yielded identical results for the standard reference materials. Lead concentrations determined for several infant nutritio nal products were verified by an independent method. Typically, relati ve standard deviations of <4% were obtained with this method for lead concentrations above 2 ppb. The recovery of 2 ng of lead from an aqueo us standard carried through the microwave digestion was 104 +/- 4%. In fant formula (containing 0.6 ppb lead) to which 0.4 ng of natural-abun dance lead had been added, to simulate a formula containing 0.9 ppb le ad, was analyzed by isotope dilution, and the result was 96 +/- 18% of the theoretical value. Thus, differences of 0.3 ppb lead could be cle arly distinguished, and the detection limit was estimated to be 0.1 ng lead per gram of infant formula. The keys to accuracy for this method are minimizing contamination and accurately determining the concentra tion of lead in the isotopically enriched standard.