Quality of open air, single-shot LIBS spectra from waste particles

This work investigates the ability of LIBS to produce quality spectra from small particles of concrete demolition waste with single-shot spectra in open air. The 2 − 8mm materials are rounded river gravel, green glass shards and plastic flakes. Considered are focal length, air, moisture, laser energy and laser angle of incidence. The research methodology is an experimental study using the observation depth and spectral abundance as quality indicators. The relation between ablation volume, breakdown threshold, signal strength and observation depth is captured in a simplified model to provide a better understanding of the dependence of the spectra on the laser incidence angle and material positioning in the laser beam. A 100mm lens provided a compromise between spectral abundance, level of air interference and achievable observation depth. The study indicates LIBS can yield good quality data, even in cases of up to 3mm surface roughness. Surface moisture did raise the percentage of bad spectra from an average 4% to 18%, but overall LIBS is still capable of providing quality data under challenging conditions.