Speciation and isotope pattern deconvolution for inductively coupled plasma-mass spectrometry quantitative studies of mineral metabolism and supplementation*

Human breast milk can be considered as “ideal” food for the correct development of newborn babies and, for those that are not breast-fed, formula milk has to be used instead. Ideally, the composition of such formula milk preparations should closely resemble that of maternal human milk. Considerable differences between both in the total content of trace elements such as Fe, Cu, Se, Zn, and I and in their chemical form in both milk types have been demonstrated. Speciation analysis in milk whey was carried out first by high-performance liquid chromatography (HPLC) with inductively coupled plasma-mass spectrometry (ICP-MS) elemental detection and showed that the observed element distribution patterns were very different in the investigated human and formula milks. Using complementary molecular mass techniques (i.e., MALDI-TOF), the identity and chemical characterization of some biomolecules (e.g., protein) with which metals are associated in each fraction was also established (by a typical heteroatom-tagged proteomics protocol). Attempts to assess the nutritional value of elemental supplements in formula milk with the aid of quantitative chemical speciation, using stable isotopes in combination with ICP-MS and isotope pattern deconvolution (IPD), proved to be successful to differentiate and quantify endogenous (natural) and exogenous (supplemented) Se or Fe trace levels. In particular, the application of such ICP-MS based techniques to study Se bioavailability from formula milk and metabolism in Se-supplemented lactating rats is discussed in detail. Quantification of selenospecies of endogenous (natural) and exogenous (supplement) Se in rat’s urine is demonstrated and relevant information on possible Se biotransformations and its final catabolism from such results is discussed.