The influences of ambient humidity on laser-induced breakdown spectroscopy

Abstract

The poor long-term repeatability of laser-induced breakdown spectroscopy (LIBS) is a barrier to its general adoption. To date, its causes and the mechanisms involved are poorly understood. Despite the potential impact of variations in ambient operating conditions such as ambient temperature and humidity, these factors have not been thoroughly investigated, especially ambient humidity. This study investigates the influence of ambient humidity on LIBS spectra. The ambient humidity of a pure copper sample was adjusted in real-time and the LIBS spectra were acquired. The results indicate that the H I 656.28 nm intensity increased with humidity, whereas the variation of the Cu I line intensity with humidity depended on the laser energy. The intensity of the Cu I 510.55 nm decreased with humidity at 8 mJ and 13 mJ, and increased with humidity at 18 mJ, 30 mJ, and 50 mJ. An investigation of the underlying mechanism suggested that at a high ambient humidity, sample ablation increased, but the lifetime of the induced plasma was reduced. The two mechanisms increased and decreased the spectral intensity, respectively, and determined the variation in the Cu I line intensity with humidity. This study demonstrates that ambient humidity affects the LIBS spectral intensity and should be considered especially for long-term LIBS measurements or open-air measurements.