Application of Response Surface Methodology for Optimizing Test Parameters of Laser-Induced Plasma in Soil

The selection and optimization of the test parameters is one of the important steps in spectrochemical analysis based on laser-induced breakdown spectroscopy(LIBS). Appropriate test parameters can guarantee the accuracy of the later spectral data analysis. Here, LIBS technology was employed to study the influence of different test parameters of LIBS on the spectral characteristics of main elements in soils, and the universal soil testing parameters were obtained. Based on single variable test, the experiment of laser energy (LE), delay time (DT), and lens to sample distance (LTSD) three factors quadratic central composite design was carried out using the respond surface method (RSM). According to the mainelements(Si, Fe, Mg, Ca, Al, Na, K, etc. ) in soil, the combined signal-background-ratio (SBR) of characteristic spectral lines from main elements was named as the objective function (Y-SBR). The interaction influences among three factors on soil plasma characteristics were explored and the optimized parameters of LIBS were summarized. Results revealed as follows : the factor LE showed a remarkable linear effect to Y-SBR, and factors of DT and LTSD exhibited an opposite result. The interaction of three factors displayed a non-significant relationship. Meanwhile, the quadratic terms of LE2, DT2 and LTSD2 had a significant surface relationship. Through the RSM analysis, the optimized experimental parameters were : LE : 103. 09 mJ; DT : 2. 92 mu s; LTSD : 97. 69 mm; and a peak value Y-SBR of 198. 602 could be obtained. These optimized test parameters are the prerequisite for the LIBS data analysis in the late stage, which can offer important reference value for the soil LIPS detection in the field.