Speciation of chromium compounds by laser ablation/ionization mass spectrometry and a study of matrix effects

Abstract

The capabilities of the laser ablation/ionization technique coupled to a Fourier transform ion cyclotron resonance mass spectrometer for the speciation of chromium compounds were evaluated and various analytical processes have been investigated. A better knowledge of the pathways of cluster ion formation allows us to propose a simple and reliable methodology for the identification of the chromium chemical state. In the negative ion mode, the Cr x O y − ions are generated by aggregative processes between CrO 2 − or CrO 3 − ions on the one hand and CrO 2 and/or CrO 3 neutrals on the other. The more oxygenated species are produced during the laser ablation of hexavalent chromium compounds. In addition, hydrogenated ions are formed in the study of hydrated trivalent chromium compounds. The choice of only two ion intensity ratios allows to distinguish unambiguously trivalent, hydrated trivalent, and hexavalent chromium compounds, respectively. This new methodology, based on the examination of nearly 20 chromium reference compounds, was evaluated in the study of simple system (pure chromium compounds and commercial pigment). The influence of various oxides, which are generally present in polyphasic and complex matrices, on the methodology proposed is finally evaluated. An antagonistic effect between first calcium and silicon oxides and second trivalent iron and zinc oxides is observed and allows to indicate the limitations of the method. It was demonstrated that the introduction of this kind of oxide induces important modifications in the distribution of the species in the gas cloud after the laser ablation and consequently in the processes of cluster ion formation. Silicon and lime involved competitive reaction with chromium species and lead to less oxygenated Cr x O y − ions whereas zincite and hematite assisted the formation of highly oxygenated chromium species.