Trivalent and hexavalent chromium oxides have been examined by time-of-flight laser microprobe mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). The positive-ion spectra show a strong correlation with the valency of chromium in the oxide. Thus, the positive cluster ion distribution and the presence of characteristic chromium cluster ions allow us to distinguish the two oxides. Metallic compounds present in industrial environments have been widely studied over the last ten years because of their toxicity. Among them are the chromium compounds, the toxicity of which depends on the metal oxidation state. Indeed, while chromium III is essential as a trace element for the normal functioning of living organisms (50-200 µg is necessary per day), chromium IV is known to have toxic effects on biological systems.1 In particular, it can cause DNA damage, cancer or allergic reactions. Thus, the valency determination of chromium is of first importance from a health point of view and many techniques are utilized for the determination of the stochiometry of chromium compounds. The most commonly used are UV/visible spectroscopy, 3-5 atomic absorption spectroscopy, 6 electrochemical techniques, 7,8 electrophoretic techniques 9 and ion chromatographic analysis.10 However, all these techniques use liquid samples, and oxidation or reduction reactions can occur during the dissolving of the chromium compounds.To overcome this problem, 'in situ' techniques such as Raman spectroscopy 11,12 or X-ray absorption spectroscopy 13 have been used; they require solid samples and thus give results more representative of the valency of the chromium in the original sample. Mass spectrometry coupled with the laser ablation/ionization process is another 'in situ' technique. Poitevin et al. and Hachimi et al. 14,15 have previously established a methodology to discriminate between trivalent and hexavalent chromium compounds by studying the relative intensities of negative cluster ions. Our purpose was to transpose this methodology to the study of positive cluster ions by time-of-flight laser microbe mass spectrometry (TOF-LMMS). In addition, we used the Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) technique with the aim of obtaining a similar discrimination of both oxides and getting a better understanding of the precursor ions, in particular the CrO + ion, present before the occurrence of clusters.
EXPERIMENTAL SETUPTime-of-flight laser microprobe mass spectrometry TOF-LMMS experiments were performed using a LAMMA 500 instrument (Leybold-Heraeus, Germany) in a transmission arrangement. In our modified system 16,17 the second harmonic of a Nd-YAG laser (532 nm) pumps a dye laser (TDL50 D of Quantel, France) which is used for the ionization step. Harmonic frequencies of the dye laser are selected by a holographic network and can be doubled by a doubling crystal cell (DCC). The various wavelengths are separated by a quartz separation prism by adjusting its orientation. The alignment of...