Particle size limits for quantitative aerosol analysis using laser-induced breakdown spectroscopy: Temporal considerations

The temporal evolution of the Si atomic emission signal produced from individual silica microspheres in an aerosolized air stream was investigated using laser-induced breakdown spectroscopy (LIBS). Specifically, the temporal evolution of Si emission from 2.47 and 4.09-micrometer-sized particles is evaluated over discrete delay times ranging from 15 to 70 μs following plasma initiation. The analyte signal profile from the microspheres, taken as the silicon atomic emission peak-to-continuum ratio, was observed to follow the same profile of silicon-rich nanoparticles over the range of delay times. The ratio of analyte signals for the 2.47 and 4.09-micrometer particles was observed to be approximately constant with plasma decay time and less than the expected mass ratio, leading to the conclusion that further vaporization and enhanced analyte response do not continue with increasing delay times for these microsphere sizes. While recent research suggests that the temporal component of analyte response is important for quantitative LIBS analysis, the current study does confirm earlier research demonstrating an upper size limit for quantitative aerosol particle analysis in the diameter range of 2 to 2.5 μm for silica microspheres.