Laser-induced breakdown spectroscopy (LIBS) has been recognized as a “future star " in the field of compositional analysis for its rapid, in-situ, remote characteristics. However, the self-absorption effect leads to the decrease of the spectral lines and the nonlinear relationship between the element contents and the intensities of the spectral lines, which results in poor accuracy of detection. Conventional self-absorption correction method can only modify the self-absorption spectral line to a certain extent, and cannot obtain the plasma source free from self-absorption. Therefore, it is hard to eliminate the influence of self-absorption on the accuracy of LIBS analysis at the origin.
The research group of Wuhan National Laboratory for Optoelectronics made in-depth investigations on the influence of self-absorption effect on the quantitative analysis, and found that the plasma source free from self-absorption could be obtained by reducing the air pressures. A simple calculation method of self-absorption coefficient (SA) is proposed based on the exponential fitting coefficient of calibration curve. As an example, the quantitative analyses of Mn and Cu in steels were carried out. The physical mechanism was discussed by analyzing the evolution of plasma morphology, plasma temperature and electron density at different air pressures. The results show that the calibration curves of Mn and Cu exhibit a severe nonlinearity due to self-absorption effect at atmospheric pressure. However, when the pressure drops to 1 kPa, the plasma temperature and electron density decrease rapidly with the plasma expanding, which generates an optical thin plasma free from self-absorption with SA close to 1. Furthermore, the calibration curves established by several characteristic lines of both elements have a good linear correlation, and the determination coefficients R2 of the calibration curves all can reach morn than 0.99.
This work has been published in Optics Express (Z. Q. Hao, L. Liu, M. Shen, X. Y. Yang, K.H. Li, L. B. Guo, X. Y. Li*, Y. F. Lu, and X. Y. Zeng, “Investigation on self-absorption at reduced air pressure in quantitative analysis using laser-induced breakdown spectroscopy,” 24(23), 26521-26528, (2016)).
The research was financially supported by the National Special Fund for the Development of Major Research Equipments and Instruments (No. 2011YQ160017) and the National Natural Science Foundation of China (No. 61378031, 51429501, and 61575073).
Calibration curves and plasma images： (a) Calibration curves of Mn I 403.07/Fe I 400.52 under different air pressures, (b) Calibration curves of Mn lines with different characteristic wavelengths, (c) Lateral images of the plasma plumes under different delay times and different air pressures.