Comparison between digestion procedures for the multielemental analysis of milk by inductively coupled plasma mass spectrometry

Elements (As, Ca, Cd, Cr, Cu, Mg, Mn, Mo, Ni, Pb, Rb, and Zn) were determined by inductively coupled plasma mass spectrometry in Certified Reference Material (A-11 milk powder, International Atomic Energy Agency), human breast milk, and a representative variety of infant formulas. The effects of digestion procedures and the mass of reference milk samples on the recovery, precision, and accuracy of multielemental analyses for milk were examined. High-pressure microwave bomb and closed-vessel ashing on a hot plate were used as the means for digestion. Elements were either preconcentrated on a Chelex 100 resin or determined directly from the diluted digest. The closest comparisons between measured and reference values for the A-11 reference milk were obtained using a 0.05-g sample digested in 1.0 mL of concentrated HNO₃ on a hot plate set at 70°C for 5 days and measured directly. Concentrations of elements in human milk and formula were, in general, in good agreement with literature values, where available.