激光诱导击穿光谱（LIBS）定量分析中的自吸收效应不仅会降低谱线强度和增加线宽, 而且使定标结果饱和, 从而影响最终的分析精度。 为了消除该效应的影响, 提出了一种基于共振双线与非共振双线选择的自吸收免疫激光诱导击穿光谱（SAF-LIBS）技术, 通过比较所测谱线强度比值和理论强度比值来确定等离子体的光学薄时刻, 并使用共振线与非共振线来拓展元素含量的可测量范围。 该技术可以分为定标和定量两个分析过程, 其定标过程为： 计算待测元素的共振双线及非共振双线的理论强度比, 通过对比不同待测元素含量样品的共振双线及非共振双线在不同延时下的强度比和理论比, 确定等离子体的光学薄时刻； 使用一系列标准样品建立LIBS非共振线的单变量定标曲线； 利用准光学薄谱线建立共振线和非共振线的SAF-LIBS单变量分段定标曲线。 其定量分析过程为： 先用非共振线和LIBS定标曲线确定未知样品所属的含量分段, 再用准光学薄谱线以及与所属分段的共振或非共振SAF-LIBS定标曲线完成定量分析。 对Cu元素的单变量定标结果表明, 对于共振线, 最佳延时随着样品含Cu量的增加而增加, 且只有当含Cu量低于0.05%时, 才可能获得准光学薄的共振线, 而随着Cu含量的增加, 自吸收变得非常严重, 以至于无法获得光学薄的共振线； 对于非共振线, 当含Cu量在0.01%~30%范围内, 均可获得准光学薄的非共振谱线, 而当Cu含量大于50.7%时, 将无法在等离子体寿命期内捕获到光学薄谱线。 对Cu元素的定量分析结果表明, 基于共振双线与非共振双线的自吸收免疫LIBS技术可以有效地避免自吸收效应的影响, 各分段定标曲线的线性度均大于0.99, 对两个未知样品中Cu元素含量的绝对测量误差分别为0.01%和0.1%, 探测限达到了1.35×10-4%, 最大可测量范围拓展至50.7%。

The self-absorption effect in quantitative analysis of LIBS not only reduces the spectral line intensity and increases its width, but also causes saturation effects in calibration, thus affecting the analytical accuracy. A resonance and non-resonance doublet based self-absorption-free laser-induced breakdown spectroscopy (SAF-LIBS) technique is proposed to eliminate its influence. The optically thin time is obtained by matching the measured lines intensity ratios with the theoretical one, and the applicable measurement range is expanded by utilizing the resonance and non-resonance lines. This technique can be divided into two analytical processes: calibration and quantification. The calibration process is: calculating the theoretical intensity ratio of the resonant doublet and non-resonant doublet of the element, and the optically thin time of plasma can be determined by matching these ratios with the measured values at different delay times. Using a series of standard samples to establish a univariate calibration curve of non-resonance line by conventional LIBS and using quasi-optically thin spectra to establish the univariate multi-segment calibration curve of resonance and non-resonance lines by SAF-LIBS. For quantitative measurements, the segment to which the unknown sample belongs is determined firstly by using the conventional LIBS calibration curve, and then the SAF-LIBS spectra and the resonance or non-resonance calibration curve that corresponds to the predetermined segment are used for implementing the quantitative analysis. The calibration results for Cu showed that the optimal delay time increased with the increase of the Cu content, and the resonance lines could be considered as quasi-optically thin only for Cu content no larger than 0.05%. With the increase of element content, the self-absorption effect became so serious that it was impossible to acquire any optically thin spectra. The non-resonance lines could be considered as quasi-optically thin over a wide content range of 0.01%~30%. However, when the content was larger than 50.7%, the optically thin lines could never be captured during the lifetime of plasma. The quantitative analysis of Cu showed that the resonance and non-resonance doublet based SAF-LIBS can effectively avoid the self-absorption effect. The linearity of each segment calibration curve is greater than 0.99, the absolute errors of two unknown samples are 0.01% and 0.1%, respectively, the limit of detection is 1.35×10-4%, and the maximum measurable range is extended to 50.7%.