Comparison of mineralization methods for fluorimetric determination of selenium in milks

Abstract

Methods for the mineralization of milk samples (whole milk, casein and the total protein fraction) were studied. The dry ashing method was eliminated because low recoveries (<10%) of Se were obtained. Acid-induced digestion was assayed using several mixtures of acids: HNO3–HClO4, HNO3–HClO4– H2SO4, HNO3–HClO4–TCA (where TCA is trichoroacetic acid). The proportions and total amounts of the acids were also varied and the effect of this investigated. Mixtures HNO3–HClO4 and HNO3–HClO4–H2SO4 were not adequate to induce complete mineralization of the milk samples. The mixture HNO3–HClO4–TCA (3:1:2) was able to digest skimmed milk. Two successive digestions with this mixture (HNO3–HClO4–TCA (3:1:2)] were necessary to induce the complete digestion of casein and the total protein fractions and, in the latter case, a recovery of 94.5±5.2% (100 μg Se/l) was achieved.