A novel method for correcting the effect of the lens-to-sample distance change on the signal intensity in laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) is a promising technique for real-time online coal analysis. The lens-to-sample distance (LTSD) significantly affects the obtained signal intensity as well as the accuracy of the element quantitative analysis by LIBS. In this study, a new method is proposed to correct the effect of the change in LTSD on the signal intensity. A correction formula that fits the relationship between the obtained signal intensity and the deviation () between the LTSD and the focal length is constructed through a series of experiments based on 18 standard coal samples and validated with three types of unknown coal samples. The results show that compared with the original signal intensity in the experiments, the relative errors between the corrected signal intensity and the signal intensity when the LTSD is equal to the focal length decreased by a factor of more than ten for almost all the elements of C, H, O, and N in the three samples. In particular, when takes -4 to 2 mm, the mean relative errors after correction are all below 10%. This indicates that the proposed method can be used to correct the signal intensity of the elements C, H, O, and N when is -4 to 2 mm in real-time online coal analysis.