Simple Defocus Laser Irradiation to Suppress Self-Absorption in Laser-Induced Breakdown Spectroscopy (LIBS)

Abstract

This study introduces a novel and extremely simple way for suppressing the self-absorption effect in laser-induced breakdown spectroscopy (LIBS) by utilizing a defocusing laser irradiation technique. It is claimed that defocusing laser irradiation produces more uniform laser plasma due to lower fluence than tight focus laser irradiation, hence greatly lowering the effect of self-absorption in the laser plasma. KCl and NaCl pellet samples were used to demonstrate this achievement. When the defocus position is adjusted to – 6 mm for KCl and NaCl samples, the self-reversal emission lines K I 766.4 nm, K I 769.9 nm, Na I 588.9 nm, and Na I 589.5 nm vanish. Meanwhile, the FWHM values of K I 766.4 and K I 769.9 nm are 0.29 nm and 0.23 nm, respectively, during -6 mm defocus laser irradiation, as opposed to 1.24 nm and 0.86 nm, under tight focus laser irradiation. Additionally, this work demonstrates that when the laser energy is changed in between 10 to 50 mJ, no self-reversal emission occurs when -6 mm defocus laser irradiation is applied. Finally, a linear calibration curve is generated using KCl at a high concentration ranging between K concentration from 16.6–29%. This simple change of defocus laser irradiation will undoubtedly contribute to the suppression of the self-absorption phenomenon, which disrupts LIBS analytical results.