Chromium speciation in aerosols by rapid single-particle mass spectrometry

On-line speciation of chromium in aerosol particles by rapid single-particle mass spectrometry is discussed. Aerosols are sampled directly into the mass spectrometer where individual particles are detected by light scattering from a continuous laser beam. The scatter pulse from each particle triggers an excimer laser which ablates the particle in flight. Ions produced from the ablation event are analyzed by time-of-flight mass spectrometry. The negative ion spectra of particles containing chromium are dominated by chromium oxide clusters. The energy distribution of these ions suggests that they are produced by complex bond formation and dissociation processes in the ablation plume. Since these processes are strongly affected by the particle composition, the distribution of cluster ions can give a unique signature of the chromium species that are present, including a qualitative assessment of the relative amounts of Cr(III) and Cr(VI). However, these processes are also influenced by experimental variables such as particle size, water content, laser irradiance, and the identities of impurity and/or counterions, which must be controlled or accounted for in an analysis.