1. Institute of Laser Spectroscopy, State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China
2. Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Abstract：The self-absorption effect in quantitative analysis of LIBS not only reduces the spectral line intensity and increases its width, but also causes saturation effects in calibration, thus affecting the analytical accuracy. A resonance and non-resonance doublet based self-absorption-free laser-induced breakdown spectroscopy (SAF-LIBS) technique is proposed to eliminate its influence. The optically thin time is obtained by matching the measured lines intensity ratios with the theoretical one, and the applicable measurement range is expanded by utilizing the resonance and non-resonance lines. This technique can be divided into two analytical processes: calibration and quantification. The calibration process is: calculating the theoretical intensity ratio of the resonant doublet and non-resonant doublet of the element, and the optically thin time of plasma can be determined by matching these ratios with the measured values at different delay times. Using a series of standard samples to establish a univariate calibration curve of non-resonance line by conventional LIBS and using quasi-optically thin spectra to establish the univariate multi-segment calibration curve of resonance and non-resonance lines by SAF-LIBS. For quantitative measurements, the segment to which the unknown sample belongs is determined firstly by using the conventional LIBS calibration curve, and then the SAF-LIBS spectra and the resonance or non-resonance calibration curve that corresponds to the predetermined segment are used for implementing the quantitative analysis. The calibration results for Cu showed that the optimal delay time increased with the increase of the Cu content, and the resonance lines could be considered as quasi-optically thin only for Cu content no larger than 0.05%. With the increase of element content, the self-absorption effect became so serious that it was impossible to acquire any optically thin spectra. The non-resonance lines could be considered as quasi-optically thin over a wide content range of 0.01%~30%. However, when the content was larger than 50.7%, the optically thin lines could never be captured during the lifetime of plasma. The quantitative analysis of Cu showed that the resonance and non-resonance doublet based SAF-LIBS can effectively avoid the self-absorption effect. The linearity of each segment calibration curve is greater than 0.99, the absolute errors of two unknown samples are 0.01% and 0.1%, respectively, the limit of detection is 1.35×10-4%, and the maximum measurable range is extended to 50.7%.
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