为促进LIBS技术在土壤微量重金属元素检测中的应用, 提高特征谱线的光谱强度和信背比, 对实验参数进行优化, 并对Cr元素进行分析。 首先对激光器激发能量、 样品距透镜距离和光谱仪采集延时等实验参数进行优化。 对比激光器能量从60 mJ到110 mJ的谱线强度和信背比, 当选用90 mJ的激发能量时可以得到最佳实验结果。 其次, 选择不同样品到透镜的距离, 对比从焦前5 mm到焦后5 mm得到的实验结果, 得出样品与透镜距离为焦后1 mm(即聚焦位置121 mm)时, Cr元素的特征谱线和信背比达到最佳。 最后, 分析对比光谱仪采集延时对谱线强度和信背比的影响, 结果显示, 与能量对等离子辐射强度的影响趋势大致相同, 当采集延时为1 000 ns时, 实验结果最佳。 在最佳实验条件下(即激光器能量90 mJ、 聚焦位置121 mm、 采集延时1 000 ns), 对12种含有重金属Cr元素的土壤样品进行了光谱检测, 为减弱外界环境的干扰, 对同一样品的10个激光烧蚀位置得到的光谱做平均值预处理, 选择Cr(Ⅰ)357.86 nm, Cr(Ⅰ)425.44 nm, Cr(Ⅰ)427.49 nm为特征谱线, 通过建立样品掺杂浓度和光谱强度的定标曲线, 得到了三条谱线的检测限LOD分别为74.62, 64.07和67.49 mg·kg-1, 拟合优度值R2分别为0.98, 0.97和0.99, 均方根误差值RMSE分别为0.41, 0.33和0.35。 同时, 引入偏最小二乘法及支持向量机算法进一步提高了定标模型精度。 研究表明, 通过对实验参数进行优化及改善LIBS技术对微量元素的定量探测参数, 得到了最优的光谱强度和信背比, 并通过对Cr元素进行定量分析, 计算定标曲线的Lorenz拟合得到检测限、 拟合优度和均方根误差等实验参数, 提高了LIBS对土壤中重金属元素的检测精度, 这对于利用LIBS技术检测微量重金属元素具有重要的参考意义。

In order to improve the spectral intensity and the signal-to-back ratio of the characteristic spectral lines, promote the application of LIBS technology in the detection of trace heavy metals in soil. The experimental parameters in the process of soil analysis were optimized, and the element of Cr was analyzed. The Nd∶YAG laser with an output wavelength of 1 064 nm, the pulse width of 10 ns and pulse frequency of 1~10 Hz was used as the light source to focus the pulse laser on the surface of soil samples to generate laser plasma. Experimental parameters such as laser excitation energy, sample distance from lens and spectrometer collection delay were optimized. Firstly, the spectral intensity and the signal-to-back ratio of the laser energy from 60 to 110 mJ were compared. It was found that the plasma radiation intensity rises first and decreases, and the best experimental results can be obtained when 90 mJ excitation energy is selected. Secondly, the variation of spectral intensity from 5 mm before coke to 5 mm after coke is compared. It was found that when the distance between the sample and the lens was 1 mm after the focus (i. e. the focus position was 121 mm), the characteristic spectral lines and the information to back ratio of Cr elements reached the best. Finally, the influence of the acquisition delay of the spectrometer on the spectral line strength and the signal-to-back ratio were analyzed. The results show that the influence trend of energy on plasma radiation intensity is roughly the same, and the experiment result is best when the collection delay is 1 000 ns. Under the optimum experimental conditions (that is, the laser energy 90 mJ, focus position 121 mm, the acquisition delay 1 000 ns), 12 soil samples containing heavy metal Cr were detected by spectroscopy. Meanwhile, in order to reduce the interference of the external environment, the average values of the spectra obtained from 10 laser ablation positions of the same sample were pretreated. Chromium (Ⅰ) 357.86 nm, chromium (Ⅰ) 425.44 nm and chromium (Ⅰ) 427.49 nm were selected as characteristic lines. The calibration curves of doping concentration and spectral intensity were established. The detection limits of the three lines were 74.62, 64.07 and 67.49 mg·kg-1, respectively. The goodness-of-fit values R2 were 0.98, 0.97 and 0.99, respectively. RMSE was 0.41, 0.33 and 0.35, respectively. At the same time, partial least square method and support vector machine algorithm are introduced to improve the accuracy of calibration model further. The results show that the optimization of experimental parameters improves the quantitative detection parameters of trace elements by LIBS technology. The optimal spectral intensity and signal-to-back ratio are obtained. Good experimental results are obtained by the Lorenz fitting calculation of calibration curve, which has important reference significance for the detection of trace heavy metal elements by LIBS technology.