Study On High-precision Analysis Methods Of Laser-Induced Breakdown Spectroscopy And Its Application

Laser probe, also known as laser-induced breakdown spectroscopy (LIBS), have unique features such as simple sample preparation, micro-damage, fast, simultaneous multi-element analysis and standoff sensing capability. So it has become an analytical technique with great potential for production and quality testing in many fields. However, the technique must be further improved in its analytical sensitivity, precision and accuracy due to spectral interferences, matrix effects, self-absorption effects and instrument drift, etc. For this purpose, this thesis took steel and iron ore as the research object, and systematically studied some methods for improving the sensitivity, precision and accuracy of quantitative LIBS analysis. Finally, based on the self-developed laser probe analyzer, two key problems are solved in the instrument analysis to establish a robust calibration models and the compensation of the instrument drift. The detailed contents are as follows:The mobile spatial-confinement plasma technique was used to enhance spectral intensity. By overcome the deep-craft effect of fixed spatial-confinement technique, the precision and accuracy of steel quantitative analysis with LIBS were improved to a certain extent. The results shown that the confinement performances mainly depend on the factors such as laser wavelength, laser energy and cavity size. The shorter the wavelength, the better the enhancement effect. With optimized laser energy and the cavity size, a maximum enhancement factor of 5.22 can be obtained. The further results confirmed that about 2-fold increase in precision and 1-5% accuracy improvement with the mobile spatial-confinement technique. The reason is that the plasmas are compressed by the reflected shock wave with the spatial confinement, which causes the increase in electron density and temperature of the plasmas. Thus the plasmas can reach the condition of local thermodynamic equilibrium more easily, and the precision and accuracy of quantitative analysis can be improved.A ring magnet was proposed to spatially and magnetically confine the plasmas at the same time, and than the technique was use in the quantitative analysis of the steel samples. The maximum intensity enhancement of 3.1 times and maximum signal-to-noise ratio increase of 2.9 times can be obtained using the ring magnet with a maximum magnetic field of 4090 G. In addition, the determination coefficients of the calibration curves of the trace elements (V, Mn, Cr, Ti, etc.) in steel samples all increased to more than 0.99, and the detection sensitivities had 2 to 4 folds of increase.An improved partial least squares regression (PLSR) algorithm was presented to establish the multiple regression model for the detection of iron ore, the high-precision quantitative analysis of 8 kinds of oxides, total steel (TFe) and acidity of iron ore powder were realized. A pellet preparation method suitable for LIBS detection was developed, and the problems in direct pellet making such as hard die-release and brittleness were solved. To solve the matrix effect and self-absorption issues faced by conventional internal standard method, PLSR multivariate regression model was established by using the spectral data of 10?20 nm, which included multiple characteristic lines of the analyzed elements. Base on these models, eight oxides such as CaO, MgO, Al2O3, SiO2, MnO, TiO2, Na20, K2O were analyzed quantitatively with high precision. The average relative errors (ARE) are 2.90 to 15.46%. The predicted root mean square error (RMSE) and ARE of TFe reached 0.5134 wt.% and 0.69%(lower than industry permissible deviation of 1%), respectively. The RMSE and ARE of iron ore acidity reached 0.0048 and 3.65%, respectively.Finally, based on the self-developed laser probe analyzer, two critical problems of the instrument analysis are solved in the establishment of a stable calibration model and the compensation of the instrument drift. The analytical performance of the JGTZ-2 was evaluated carefully using low-alloy steels as test samples according to the characteristic parameters such as precision, accuracy, and repeatability. Based on analysis- and reference-lines optimizing, outlier rejecting, high-order polynomial fitting, and subsection calibration, etc., the calibration curves of the trace elements in low-alloy steels were established for JGTZ-2. Two-point standardization method was introduced to compensate for instrument drift, the detecting repeatability of JGTZ-2 was improved effectively. The results show that the analysis deviations of JGTZ-2 for Mn, Cr, Ni, Cu, Si, Mo, Ti, V, and Al elements in low-alloy steels are equal or better than the national standard. The relative standard deviations (RSD) of repeated measurements are generally less than 2%. The detecting relative errors for Mn, Cr, Ni, Cu, Si, Mo, and Ti elements are less than 8%. The accuracy is comparable to the direct-reading spectrometer.