Determination of trace elements in human milk by inductively coupled plasma sector field mass spectrometry (ICP-SFMS)

Abstract

The potential of high resolution inductively coupled plasma sector field mass spectrometry (ICP-SFMS) was evaluated to quantify reliably various toxic and essential elements (Al, Sc, Ti, V, Cr, Mn, Fe, Ni, Co, Cu, As, Ag, Pt, Au, Pb) in human milk and infant formulae with respect to sensitivity, spectral and non-spectral interferences, blank levels, relative stability and accuracy. Some elements of potential interest such as Ti, V, Cr, Mn, Fe, Ni, Co and As are significantly interfered by polyatomic ions and cannot be determined under routine conditions without using high mass resolution, since interference levels vary significantly with the composition of the milk matrix. A microconcentric nebuliser in combination with a membrane desolvation unit was tested with respect to signal enhancement and reduction of interferences. In general, an increase of the signal intensities up to a factor of 5 was observed, whereas not all spectral interferences can be reduced to a negligible amount (only oxide containing ions to some extent). Moreover the use of the N2 as the make-up gas did not improve the sensitivity but led to additional N-containing polyatomic interferences (e.g., for V, Cr and Mn). The use of a torch shielded by a Pt electrode did lead to the same signal intensities both for the conventional MCN100 and the MCN6000 with membrane desolvation. The investigation of In, Re, Ru, and Rh as internal standard showed that not all elements can be corrected for matrix suppression without using an additional correction factor derived from an approximate matrix composition. A series of milk samples was investigated using the optimised analytical set-up and compared to infant milk formulae. Trace element levels in instant milk formulae are significantly influenced by the quality of tap water used for preparation.