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经过上述基础实验研究,优化后的实验条件为:激光能量为32 mJ,延迟时间为1.0 μs,LTSD取96 mm对压片土壤样品进行定量分析。在LTSD为97 nm时对纯Al基底的土壤样品进行定量分析。每个样品取5个不同的位置,每个位置采集10个过普信号,每个光谱信号为10次连续击打采集的激发信号的平均值。对含不同浓度Ni元素土壤标样(0.02%、0.05%、0.35%、0.65%和1.15%)的LIBS谱线进行采集。采用谱线强度比分别建立Ni I: 373.68 nm的定标曲线,Fe I: 309.27 nm作为内标线,Fe I: 309.27 nm由于不是共振线,避免了自吸收产生的影响,确保了谱线信号强度的稳定性;同时该谱线的上下能级差与Ni I: 373.68 nm的上下能级差接近,从而尽可能保证它们有相近的激发状态[14-15]。实验中笔者所在课题组分别测量了两种不同土壤样品中Ni I: 373.68 nm的谱线强度,然后以Fe I: 309.27 nm作为内标线建立基于内标法的Ni元素标准曲线。如图7所示为两种土壤样品中Ni的内标法建立的定标曲线。压片土壤样品中Ni的定标曲线的相关系数R2为0.992,纯Al基底土壤样品中Ni的定标曲线的相关系数R2为0.997。计算两种土壤样品谱线最大相对标准偏差(RSD)纯Al基底土壤样品为4.34%,压片土壤样品为3.47%。纯Al基底定标曲线中相关系数相对较高,但实验的稳定性较差,与土壤样品的松软程度有关,压片土壤较为紧实,纯Al基底土壤样品紧实度较差一些,因此稳定性较差。
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在最佳实验条件下对两种土壤样品中不同含量的样品(0.2%、0.85%)进行定量分析(表1),每个样品同样采集50个光谱叠加求平均。选择Ni I: 373.68 nm作为分析线。结果表明,采用内标法对两种样品进行定量分析时,得到压片土壤样品Ni元素最大相对误差为6.1%,纯Al基底土壤样品Ni元素最大相对误差为4%。
Ni Standard concentration Experimental fitted value Relative error 0.20% 0.210% 5.0% Soil tablet 0.85% 0.902% 6.1% 0.20% 0.192% 4.0% Al substrate 0.85% 0.880% 3.5% Table 1. Comparison results of internal standard method for quantitative analysis of two soil samples
LIBS experimental study of eliminating the interference of Al element in soil base based on background subtraction method
doi: 10.3788/IRLA20200136
- Received Date: 2020-08-21
- Rev Recd Date: 2020-10-25
- Available Online: 2021-01-22
- Publish Date: 2021-01-22
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Key words:
- laser-induced breakdown spectroscopy /
- spectral line interference /
- background subtraction /
- quantitative analysis
Abstract: The pollution of heavy metals in the soil has seriously affected agriculture and food safety. Therefore, efficient and accurate detection of heavy metal pollution is a problem that needs to be solved urgently. When using laser induced breakdown spectroscopy (LIBS) to quantitatively analyze the Ni element in the soil, it was found that the characteristic peak of the Ni element with a wavelength of 373.68 nm in the soil would be affected by the spectral line of the Al element at 373.39 nm. Therefore, the spectra of pure aluminum-based soil and tableted soil were compared and measured. A method of using pure aluminum as the substrate and subtracting the spectral line of the Al element in the soil background to eliminate the interference of the Al element in the soil background to the Ni element was proposed. This method was called the background subtraction method. The experiment determined that the optimal delay time for both soil samples was 1.0 μs, and the lens-to-sample distance (LTSD) was 97 mm and 96 mm, respectively. The internal standard method was used to quantitatively analyze Ni in the two soil samples. The calibration curve fitting effect of the Ni element in the pure aluminum-based soil samples was good, the correlation coefficient R2 was 0.997, and the maximum standard deviation (RSD) was 4.34%. The relative error of the Ni element in the soil sample after the base background subtraction method was reduced to 4%. The experimental results show that: when using LIBS technology to measure the content of heavy metal elements in the soil, under the condition that the characteristic line of the element is limited, in order to avoid the interference of the line and improve the detection accuracy, the background subtraction method can effectively eliminate the line between the elements Interference.