基于放电辅助激光诱导击穿光谱技术的水中痕量金属元素检测分析

doi:10.19595/j.cnki.1000-6753.tces.L10087

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摘要

重金属离子引起的水污染已经成为世界上一个严重的环境问题。激光诱导击穿光谱（LIBS）是一种基于原子发射光谱学的新型分析技术,可用于分析任何形态（固体、液体、气体等）的样品,因此被广泛应用于环境污染检测、食品安全、化工生产等领域。然而,在针对液体样品检测时,等离子体快速淬灭、液面不稳定对光学系统的污染以及等离子体有限的激光能量吸收,都制约着LIBS技术的进一步发展。为此,提出了一种用于检测水中痕量元素的放电辅助LIBS技术（D-LIBS）,并与传统单脉冲LIBS技术（C-LIBS）进行比较。使用中速滤纸对BaCl₂和NaCl₂混合溶液进行快速采样。利用纳秒激光在滤纸上产生含有待测元素的等离子体并利用放电辅助装置实现等离子体光谱增强。对比C-LIBS和D-LIBS时间积分光谱,发现采用额外的放电辅助装置可以有效增强LIBS光谱信号,Ba II 455.40 nm和Na 588.99 nm谱线分别增强了30倍和6倍。由于额外的电能的注入,等离子体被再次加热和激发,因此光谱强度得到增强。C-LIBS和D-LIBS光谱强度均和激光能量正相关,但光谱增强因子在小激光能量条件下较高,随着激光能量的增加,Ba Ⅱ谱线增强因子从30倍（20 mJ）降低至2倍（50 mJ）。这是由于激光能量越低,所烧蚀的样品量越小,因此等离子体受到额外的放电激励后,增强效果越明显。与C-LIBS中的光谱信噪比（SNR）相比,D-LIBS使SNR增强超过1个数量级,当激光能量为20 mJ时,Ba Ⅱ谱线和Na I谱线SNR增强倍数分别为56倍和16倍。此外,对比了不同激光能量条件下C-LIBS及D-LIBS技术对液体中痕量金属元素的检测能力。C-LIBS中Ba元素检出限随激光能量增大而降低,在50 mJ时达到0.823 mg/L。另外,当激光能量为20 mJ时,由于额外的放电辅助装置,Ba元素检出限从12.5 mg/L降低至0.26 mg/L,D-LIBS的检测灵敏度比C-LIBS提高了48倍。

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关键词 ：放电辅助LIBS, 滤纸采样, 水溶液, 痕量金属元素

Abstract：

Heavy metal ion-induced water pollution has become a severe environment in the world. Laser-induced breakdown spectroscopy (LIBS) is a novel analytical technique based on the atomic emission spectroscopy, which can be used for the target in any state. Thus, LIBS is widely applied in various applications, including environmental pollution monitoring, food safety, and chemical engineering production. However, for liquid sample detection, the rapid quenching of the plasma, the contamination of the optical system by the unstable liquid level, and the limited laser energy absorption of the plasma have limited the further development of LIBS technology. Here, a discharge-assisted LIBS technique (D-LIBS) for the detection of trace elements in solutions is proposed and compared with the conventional LIBS (C-LIBS).
Rapid sampling of mixed BaCl₂ and NaCl₂ solutions is performed using medium-speed filter paper. A Nd:YAG laser is implemented to generate plasmas containing the target elements on the filter paper. A discharge assistance consists mainly of a high voltage DC power supply, a capacitor, and a pair of rod electrodes. The capacitor is first charged by the high voltage DC power supply. The electrodes are respectively connected to the positive and negative terminals of the capacitor, and placed horizontally on the surface of the filter paper. Once the sample surface is ablated by the focused laser beam, the pre-existing plasma between the electrodes acts as the seed charged particles in the discharge gap, which rapidly ignites a strong discharge there.
Compared to the time-integrated spectra in C-LIBS and D-LIBS, the LIBS signal has been greatly increased by using the discharge assistance. Emission lines of Ba Ⅱ 455.40 nm and Na Ⅰ 588.99 nm are respectively enhanced by 30-fold and 6-fold. The spectral intensities in C-LIBS and D-LIBS are positively correlated with laser energy. However, the spectral enhancement factors show higher values at small laser energy conditions, and the Ba Ⅱ spectral enhancement factor decreases from 30 (20 mJ) to 2 (50 mJ) with increasing laser energy. Compared with the spectral signal-to-noise ratio (SNR) in C-LIBS, D-LIBS results in a SNR enhancement of more than 1 order of magnitude. When the laser energy is 20 mJ, the SNR enhancement factor of Ba Ⅱ spectral line and Na I spectral line is 56 and 16, respectively. In addition, the quantitative analysis performance of C-LIBS and D-LIBS for trace metal elements in solution under different laser energy conditions are comparatively studied. The limit of detection (LoD) of Ba is decreased along with the laser energy in C-LIBS, reaching 0.823 mg/L at 50 mJ. Moreover, under the optimal condition the LoD is reduced from 12.5 mg/L in C-LIBS to 0.26 mg/L in D-LIBS, and the detection sensitivity of LIBS technique is increased by 48-fold due to the discharge assistance.
The following conclusions can be drawn from the experimental results: (1) The spectral intensity, SNR can be enhanced by over 1 order of magnitude. The increases in the intensity and SNR of the plasma spectra are due to the additional electrical energy injection resulting in the reheating and excitation of the plasma. (2) The spectral intensity and SNR are positively correlated with laser energy. The enhancement factor reaches the maximum in the smallest laser energy condition. This is due to the fact that the lower the laser energy, the smaller the mass of sample ablated, and therefore the more significant the enhancement effect is when the plasma is excited by the additional discharge. (3) The quantitative analysis of LIBS can be efficiently enhanced over 1 order of magnitude by the discharge assistance. And the LoD is reduced to 0.26 mg/L which is 1/48 of the original level.

Key words： Discharge-assisted LIBS filter paper sampling water solution trace metal element

收稿日期: 2023-01-13

PACS:

O433

基金资助:

国家自然科学基金（52177166, 51877210）和陕西省自然科学基金（2020JM-309）资助项目

通讯作者: 汤洁男,1979年生,研究员,博士生导师,研究方向为大气压气体放电等离子体物理、激光等离子体物理、激光与物质相互作用及应用。E-mail：tangjie@opt.ac.cn

作者简介: 许博坪男,1996年生,博士,研究方向为激光诱导击穿光谱技术。E-mail：bopingx@outlook.com

引用本文:

许博坪, 刘颖华, 刘在浩, 尹培琪, 汤洁. 基于放电辅助激光诱导击穿光谱技术的水中痕量金属元素检测分析[J]. 电工技术学报, 0, (): 151-151. Xu Boping, Liu Yinghua, Liu Zaihao, Yin Peiqi, Tang Jie. Analysis of Trace Metal Elements in Water Based on Discharge-Assisted Laser-Induced Breakdown Spectroscopy. Transactions of China Electrotechnical Society, 0, (): 151-151.

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https://dgjsxb.ces-transaction.com/CN/10.19595/j.cnki.1000-6753.tces.L10087 https://dgjsxb.ces-transaction.com/CN/Y0/V/I/151

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