LIBS for soil analysis: Effects of sample heating and high-voltage re-excitation on plasma characteristics

Abstract

While LIBS is a proven technique for monitoring elemental traces in soils, its signal-to-noise ratio is lower than comparable methods. Enhancement techniques such as sample heating and applying a high-voltage (HV) discharge have shown potential to increase emission and improve LIBS sensitivity. This study investigates the combined effect of these techniques on plasma morphology, physical parameters, and the validity of local thermodynamic equilibrium (LTE). Plasma was produced by a nanosecond Nd:YAG laser beam focused on loam soil samples at 22 and 400 °C. A high-voltage discharge lasting about 10 μs was applied 1.5 μs after the laser irradiation. Plasma morphology was studied using fast photography; electron density was obtained through Stark broadening and temperature from Saha-Boltzmann plots, both with spatial and temporal resolution. LTE was tested by McWhirter criterion and two criteria for nonhomogeneous and transient plasmas. Results showed that sample heating doubled plasma lifetime and increased emission, while the HV discharge enhanced emission but induced turbulence, accelerating plasma disruption. Plasma temperature increased by 16 % when the discharge was applied while an increment of 10 % was obtained with sample heating alone; electron density rose by 80 % in both cases. Combining both techniques, the temperature increased by approximately 20 % and electron density by 130 %. Along the axial direction, increases were almost uniform. Radially, higher increases occurred near the cathode. The differential displacement of the ion and neutral clouds suggests that the HV discharge induced species polarization. LTE criteria were satisfied for all studied scenarios, in the range from 500 nm to 10 μs, after the laser onset. Our findings suggest that increased electron density and temperature result in the enhanced spectral emission observed. Finally, a laser-induced plasma may sustain LTE even under re-excitation or external perturbations.