Improvement in the Sensitivity and Reproducibility of Laser-Induced Soil Plasma Produced in a Microwave Environment

This study investigated the influence of microwave (MW) on signal enhancement, shot-to-shot repeatability of emission signals, and parameters of laser-induced plasma (LIP) of soil samples. A neodymium-doped yttrium aluminum garnet (Nd:YAG) laser working at its fundamental wavelength, with energy ranging from 40 to 260 mJ was utilized in conjunction with MW power varying from 400 to 1200 W to generate plasma. The plasma emissions were recorded and analyzed with a spectrometer (LIBS 2500 Plus, Ocean Optics) at varying detector gate delay (DGD) up to 5 µs. At optimized laser energy of 140 mJ and MW power of 1.2 kW, the emission signal intensities and the signal-to-noise ratio (SNR) were enhanced by up to seven-fold and nine-fold, respectively, compared to conventional LIBS. Furthermore, MW coupling with plasma significantly improved LIBS repeatability by reducing the relative standard deviation (RSD) of emission line intensities from 38% to 11% and from 18% to 4%, for Mg and Si, respectively. Additionally, the MW-assisted LIBS also decreased shot-to-shot fluctuations in plasma temperature (T_e) from 15% to 6%, and in electron density (N_e) from 14% to 7%. These improvements in sensitivity and stability due to MW-assisted LIBS pave the way for future quantitative analysis and the detection of trace elemental contaminants in soil at low concentration levels (e.g., ppm), which is a critical application in environmental monitoring.