Improving the stability of LIBS for chromium in soil based on the model of micro-linear spectrum

Abstract

The intensity of the induced spectral signal is determined by the laser energy density. In order to reduce the influence of laser energy on plasma signal intensity and improve the stability of element spectral line identification, the model of micro-linear spectrum (MLS) has been derived. According to the Fresnel equation and Snell's theorem of light waves at different wavelengths, the mechanism of wavelength action on the transmittance in the process of plasma radiation has been explained by using the polarization separation technique at Brewster's angle combined with the transition characteristics of discrete spectral lines between bound energy levels. The elements Cr, Cu, Pb and Fe in soil were selected as the research objects, the measurement models of laser-induced breakdown spectroscopy (LIBS) and MLS were used to collect the characteristic spectral lines induced by seven different energies, and 20 soil samples with different concentrations of Cr element were measured. Combined with the linear fitting analysis function, the linear correlations between spectral line intensities and excitation energies were constructed to characterize the stability of spectral line identification. The results show that the line intensity obtained by the MLS model has a better linear relationship with the laser energies. Compared with the recognition curve of LIBS, the stability of MLS is increased by 3.9%. The model is located at the end of the signal measurement device, which is used to analyze the captured original spectral information. Without changing the original optical path measurement system, it has better feasibility and reliability.