Voigt函数拟合的LIBS谱线自吸收校正方法在磷矿镁元素分析中的应用

A LIBS Spectral Self-Absorption Correction Method Using Voigt Profile Fitting for the Application of Magnesium Analysis in Phosphorus Ore

MgO含量是磷矿浮选过程中最为关注的指标之一。实现浮选过程中MgO含量的快速检测,对于优化浮选过程、提高效率、降低成本有着非常实际的意义。因此, LIBS技术被引入到了磷矿中镁元素含量的分析。常用于LIBS分析的Mg元素特征谱线(如MgⅡ279.6 nm,MgⅡ280.3 nm, MgⅠ285.2 nm)多为共振线,谱线强度易受自吸收效应影响,导致谱峰强度下降,影响分析准确性。提出一种基于近似Voigt函数的拟合方法:首先通过近似函数对Voigt函数进行简化;再通过低含量样本确定谱峰中心和理想条件下的半峰宽;进而通过计算谱峰所在区域线型的斜率,确定用于拟合的谱翼区域;最终通过对谱峰两翼受自吸收效应影响较小的光谱进行拟合,得到更接近理论线型的谱线。在定量分析磷矿样品中的镁元素含量应用中,使用拟合后的MgⅠ285.2 nm谱线区域面积作为分析谱线强度,以拟合的Si 288.2 nm谱线区域面积作为参考谱线强度,使用内标法对镁元素含量进行了标定。对比未拟合直接内标的方法,该方法的标定确定系数(R^2)由0.923提升至0.998,均方根误差(RMSE)和平均相对误差(ARE)分别由0.96, 38.65%下降到0.16, 2.79%,说明该方法使磷矿镁元素定量分析的整体测量准确性得到了有效的提升。

The concentration of MgO is one of the most important parameters in the phosphate ore flotation. The fast detection of the concentration of MgO has great significance for the optimization of the flotation, the improvement of the efficiency and the reduction of the cost. Therefore, LIBS(Laser Induced Breakdown Spectroscopy) is introduced into the analysis of Mg in phosphate. But the common used strong lines of Mg(Mg Ⅱ 279.6 nm, Mg Ⅱ 280.3 nm, Mg Ⅰ 285.2 nm) are resonance lines in LIBS analysis. For the effect of the self-absorption, the spectral intensity of the resonance line is lower than the theoretical value, and this intensity reduction will reduce the accuracy of the analysis. In this work, a method based on an approximate function of Voigt profile was proposed. Firstly, simplified the Voigt profile function with the approximate function;secondly, determined the center of the spectral line and the full width at half maximum(FWHM) under ideal conditions with the low concentration samples;then, determined the wing area for fitting by calculating the slope of the spectral area near the spectral peak;finally, obtained the fitting profile closer to the theoretical one, by fitting the approximate function with the wing area selected above. In the application of the quantitative analysis of Mg in phosphate ore, the internal standard method was used forthe calibration. The fitting spectral areas of Mg Ⅰ 285.2 nm and Si Ⅰ 288.2 nm were chosen as the analytical line intensity and the reference line intensity, respectively. Comparing the internal standard method without fitting, the determination coefficient(R^2) with the proposed method was improved from 0.923 to 0.998;the root-mean-square error(RMSE) and the average relative error(ARE) were reduced from 0.96 and 38.65% to 0.16 and 2.79%, respectively. The calibration results proved that with the proposed method, the measurement accuracy can be improved significantly for the application of magnesium analysis in phosphorus ore.