Quantitative analysis of carbonaceous aerosols using laser-induced breakdown spectroscopy: a study on mass loading induced plasma matrix effects

摘要

We present results indicating mass loading induced plasma matrix effects on the application of quantitative laser-induced breakdown spectroscopy (LIBS) for estimation of carbon contents in aerosols. An in-house flow-controlled powder-dispersion system generated carbonaceous aerosols with varying bi-modal particle size distributions (～1 μm and 10 μm median diameters), thereby resulting in a wide mass loading range. For ease of chemical handling and to eliminate toxic effects, common talcum powder was used as our standard aerosol. Normalized atomic species concentrations of C, i.e., [C]/[Si] ratios, were calculated from atomic emission lines of C I (248 nm), Si I (252 nm), and plasma temperatures estimated from a series of Mg I lines. The results show a decrease in [C]/[Si] ratio to about 65% of the initial value as relative mass loadings increased (5.5-100%) due to the increase in number concentrations of larger sized particles (～ 10 μm median diameter). As a comparison, normalized ratio of [Mg]/[Si] did not exhibit any marked change with increased mass loading. The normalized total absorption of photon flux across the C I (248 nm) spectral line indicated a strong correlation to the percentage decrease in [C]/[Si] ratio. We used an impactor with a cut-off size of around 10 μm diameter to generate mono-modal aerosolized powders (～ 1 μm median diameter) that had lower relative mass loadings (0.32-0.16%). Similar LIBS analysis on these did not indicate any of the matrix effects. We conclude that for aerosol systems with widely varying mass loadings, quantitative LIBS analysis can be significantly affected by plasma matrix effects, specifically for the C I (248 nm) emission line as noticed in this study. This bears significance for the application of quantitative LIBS in the chemical characterization of all forms of carbonaceous aerosols.