Understanding the laser-induced aerosol ablation of sub-micron liquid particles via size-resolved spectral and image analyses

Abstract

Laser-induced breakdown spectroscopy of aerosols is associated with large signal variability and lower sampling rates. In this study, aerosol ablation and signal variability associated with aerosol size were studied by size-resolved spectral and image analyses of submicron sized particles using a high-density stream of monodisperse liquid aerosols. The rise in the particle size from 0.16 μm to 0.96 μm for a similar aerosol mass concentration showed a size-dependent intensity increase. The initial increase in the analyte emission was due to the enhanced analyte concentration inside the plasma volume, whereas for larger aerosols, the stronger ablation of the larger liquid particles resulted in elevated emission intensity. The electron number density increased from 1.6 × 10¹⁷ cm⁻³ to 2.6 × 10¹⁷ cm⁻³ with an increase in the particle size; however, the temporal delay of number density was faster for the larger particles due to higher plasma–ambience interaction. The analyte signal variation reduced to 3%, calculated using ensemble averaging, for particles larger than 0.5 μm. Plasma image analysis showed that the signal variation stemmed from inhomogeneity in the initial breakdown process and grew with the increasing delay. The variation in the plasma image area increased from 8% to 16% for 0.16 μm particles and 12% to 28% for 0.96 μm particles at the respective delays of 0 and 3 μs. Thus, two different sources of signal variability were determined. For smaller particles, the major source of variation was the variable sample scarcity inside the plasma, whereas for larger particles, plasma variation contributed more to the analyte signal variability from the beginning.