A simplified spectrum standardization method for laser-induced breakdown spectroscopy measurements

Abstract

Relatively high uncertainty (or low repeatability) is one of the main bottlenecks for wide application of LIBS quantitative measurements. The change of plasma temperature and electron number density from pulse to pulse weakens the correlation between the ablation mass and total or part of the spectral area for the same sample, making the normally applied normalization method not effective enough for uncertainty reduction. In the present work, it was assumed that there existed a standard state for samples with similar matrix, where there is a standard plasma temperature, electron number density, and total number density of the element of interest. Therefore, Taylor expansion can be applied near the standard plasma condition to obtain the standard state value of the characteristic line intensity from theory. The temperature variation was regarded to be proportional to the variation of the logarithm of the ratio of two spectral line intensities of the interested element, the variation of electron number density was regarded to be proportional to the variation of the full width at half maximum (FWHM), and the variation of total number density was regarded to be proportional to the variation of the sum of the multiple spectral line intensities of the measured element. Based on these assumptions, the calibration model was established. The results show that measurement precision and accuracy can be greatly improved by the application of this normalization method in measuring the Cu concentration for 29 brass alloy samples. The average relative standard deviation (RSD) value, the coefficient of determination (R²), the root mean square error of prediction (RMSEP), and average value of the maximum relative error were 2.92%, 0.99, 1.46%, 8.42%, respectively, while the values for normalization with the whole spectrum area were: 8.61%, 0.95, 3.28%, 29.19%, respectively, showing significant improvement.