Study of the mechanism of chromium cluster formation by laser microprobe mass spectrometry. Correlation with theoretical computations

Abstract

Different stoichiometries of micrometric particles of powdered chromium oxides and salts are examined by time-of-flight laser microprobe mass spectrometry (TOF-LMMS). The negative cluster ion distributions show a good correlation with the stoichiometry of the chromium in the oxide. We have noticed a great spectral similarity between chromium(VI) oxide and hydrated chromium(III), salts leading to difficulties in differentiating these two kinds of compounds and determining the valency of chromium. The formation of CrO 4 − ions could be associated with product hydration, and could modify the fingerprint spectra of the chromium oxides and salts. We demonstrate that the CrO 4 − ion arises from collision between molecules present in the plasma generated by laser ablation. The mechanism of cluster formation is closely associated with the presence of neutral or ionized species (water, sulfate, nitrate, etc.). In particular, the hydration effect is very marked in the initial chromium salt. To confirm these results, an FT ion cyclotron MS investigation has been carried out, which allowed determination of the laser power dependence and relative stability of CrO −, CrO − 2 and CrO 3 −. Results from a theoretical study of these types of cluster ions are presented and compared with the experimental data.