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Coptidis plays an important role in the field of traditional Chinese medicine. However, it is easily polluted by heavy metals in environment (water and soil), and thus can affect human health. In order to detect the heavy metal elements Cu and Pb in Coptidis, which was purchased from the Chinese herbal medicine market in Chongqing, the reheated double-pulse laser-induced breakdown spectroscopy (RDP-LIBS) is investigated. In order to reduce the experimental error caused by the irregular shape, it is necessary to pretreat the Coptidis samples prior to the determination step. The Coptidis samples are dried, milled, and sieved to form thin cylindrical tablets each with a diameter of 13 mm and thickness of 2 mm, which are formed under a mechanical press of 10 MPa for 2 min. The influences of the main experimental parameters, such as double-pulse LIBS detection delay, double-pulse LIBS laser energy, and double-pulse LIBS pulse interval are optimized. According to the LIBS signal intensity and signal-to-background ratio, the optimal laser energy set of the characteristic line Cu I (324.46 nm) covers E1 = 15 mJ and E2 = 35 mJ, and the pulse interval and detection delay time are 1.4 μs and 1.5 μs respectively; the laser energy set of Pb I (405.78 nm) also covers E1 = 15 mJ and E2 = 35 mJ, and the pulse interval and detection delay time are 1.6 μs and 1.7 μs, respectively. Comparing with the scenarios of single-pulse laser-induced breakdown spectroscopy, it can be seen that the spectral intensity of Cu I (324.46 nm) increases from 5779 counts to 12749 counts, i.e. it increases about 2.2 times; the spectral intensity of Pb I (405.78 nm) characteristic line increases from 4703 counts to 15838 counts, i.e. it increases about 3.3 times. It is shown that the second laser pulse re-excites the plasma which is generated by the first laser pulse, thus making the plasma emission spectrum stronger. The detection performances of heavy metal elements in Chinese medicinal materials are evaluated by RDP-LIBS and SP-LIBS. The results show that the detection limit of Cu decreases from 5.13 mg/kg to 1.91 mg/kg, and the detection limit of Pb decreases from 10.87 mg/kg to 3.03 mg/kg. There was observed a noticeable difference in the limit of detection between Cu and Pb, which meets the requirements of the Green Industry Standard for Import and Export of Medicinal Plants. Moreover, the linear curve fitting degree of RDP-LIBS is higher than that of SP-LIBS, which indicates that the RDP-LIBS technology has better detection performance in Chinese herbal medicine.
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Keywords:
- reheat double pulse /
- Coptidis /
- spectral enhancement /
- limit of detection
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图 1 正交RDP-LIBS实验装置
Figure 1. Schematic diagram of the experimental setup for orthogonal re-heating DP-LIBS.
图 2 黄连样品
Figure 2. Coptidis Chinensis samples.
图 3 光谱强度和信噪比随探测延时的变化规律
Figure 3. Evolutions of spectral intensity and signal-to-noise ratio (SNR) at different delay times.
图 4 光谱强度随激光能量的变化
Figure 4. Variation of spectral intensity in different energy groups.
图 5 光谱强度和信噪比随脉冲间隔的变化
Figure 5. Variations of signal intensity and SNR as a function of pulse interval time
图 6 SP-LIBS和RDP-LIBS光谱强度对比
Figure 6. Comparison of spectral intensity between SP-LIBS and RDP-LIBS
图 7 SP-LIBS和RDP-LIBS下Cu, Pb元素定标曲线拟合图
Figure 7. Linear fitting curves of Cu and Pb in SP-LIBS and RDP-LIBS.
表 1 特征谱线的检测限(LOD)和线性拟合度(R2)对比
Table 1. Comparison of detection limits and relative standard deviations of characteristic lines.
特征谱线 Cu I 324.46 nm Pb I 405.78 nm LOD/mg·kg–1 SP-LIBS 5.13 10.87 RDP-LIBS 1.91 3.03 GB/T 5009 20 5 R2 SP-LIBS 0.9738 0.9287 RDP-LIBS 0.9931 0.9926 
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表 2 Cu和Pb检测能力对比
Table 2. Comparison of detection ability between Cu and Pb.
元素 误差分析 Cu Pb 实际值/mg·kg–1 测量值/mg·kg–1 相对误差/% 精密度/% 实际值/mg·kg–1 测量值/mg·kg–1 相对误差/% 精密度/% SP-LIBS 80 67.7 15.4 9.8 800 638.3 20.2 5.4 300 244.4 18.5 7.6 6400 7163.5 11.9 4.2 RDP-LIBS 80 69.5 13.1 7.8 800 693.9 13.2 3.6 300 323.5 7.7 3.1 6400 5936.4 7.2 3.8
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[1] Yuan X D, Ling K W, Keuing C W 2009 Phytochem. Anal. 20 293
Google Scholar
[2] Arpadjan S, Celik G, Taşkesen S, Gucer S 2008 Food Chem. Toxicol. 46 2871
Google Scholar
[3] Zhang X H, Li H, Qin K M, Cai H, Liu X, Zheng L J, Gu J, Cai B C 2014 Anal. Lett. 47 1589
Google Scholar
[4] Guo Y M, Deng L M, Yang X Y, Li J M 2017 J. Anal. Atom. Spectrom. 32 2401
Google Scholar
[5] Zhu Z H, Li J M, Guo Y M, Cheng X, Tang Y, Guo L B, Li, X Y, Li, X Y, Lu Y F, Zeng, X Y, 2017 J. Anal. Atom. Spectrom. 32 205
Google Scholar
[6] Zheng P C, Liu H D, Wang J M, Shi M J, Wang X M, Zhang B, Yang R 2015 J. Anal. Atom. Spectrom. 30 867
Google Scholar
[7] 赵法刚, 张宇, 张雷, 尹王保, 董磊, 马维光, 肖连团, 贾锁堂 2018 67 165201
Google Scholar
Zhao F G, Zhang Y, Zhang L, Yin W B, Dong L, Ma W G, Xiao L T, Jia S T 2018 Acta Phys. Sin. 67 165201
Google Scholar
[8] Wang J M, Shi M J, Zheng P C 2017 J. Appl. Spectrosc. 84 188
Google Scholar
[9] Wang J M, Xue S W, Zheng P C, Chen Y Y, Peng R 2017 Anal. Lett. 50 2000
Google Scholar
[10] 吴宜青, 刘津, 莫欣欣, 孙通, 刘木华 2017 66 054206
Google Scholar
Wu Y Q, Liu J, Mo X X, Sun T, Liu M H 2017 Acta Phys. Sin. 66 054206
Google Scholar
[11] Wang Q Q, Liu K, Zhao H 2012 Chin. Phys. Lett. 29 044206
Google Scholar
[12] 刘晓娜, 史新元, 贾帅芸, 赵娜, 吴志生, 乔延江 2015 中国中药杂志 40 2239
Liu X N, Shi X Y, Jia S Y, Zhao N, Wu Z S, Qiao Y J 2015 China J. Chin. Mater. Med. 40 2239
[13] 李占锋, 王芮雯, 邓琥, 尚丽平 2016 红外与激光工程 45 1006003
Li Z F, Wang R W, Deng H 2016 Infrared Laser Eng. 45 1006003
[14] 李占锋, 王芮雯, 邓琥, 尚丽平 2016 发光学报 37 100
Li Z F, Wang R W, Deng H, Shang L P 2016 Chin. J. Lumin. 37 100
[15] Wang J M, Liao X Y, Zheng P C, Xue S W, Peng R 2018 Anal. Lett. 51 575
Google Scholar
[16] Skrodzki P J, Becker J R, Diwakar P K, Harilal S S, Hassanein A 2016 Appl. Spectrosc. 70 467
Google Scholar
[17] Lei W Q, Motto-Ros V, Boueri M, Ma Q L, Zheng L J, Zeng H P, Yu J 2009 Spectrochim. Acta B 64 891
Google Scholar
[18] Wang Z Z, Deguchi Y, Liu R W, Ikutomo A, Zhang Z Z, Chong D T, Yan J P, Shiou F J 2017 Appl. Spectrosc. 71 2187
Google Scholar
[19] St-Onge L, Detalle V, Sabsabi M 2002 Spectrochim. Acta B 57 121
Google Scholar
[20] Ahmed R, Iqbal J, Baig M A 2015 Laser Phys. Lett. 12 066102
Google Scholar
[21] 王金梅, 郑慧娟, 郑培超, 谭癸宁 2018 中国激光 45 0702006
Wang J M, Zheng H J, Zheng P C, Tan G N 2018 Chin. J. Lasers 45 0702006
[22] Yu J, Ma Q, Mottoros V, Lei W Q, Wang X C, Bai X S 2012 Front Phys.-Beijing 7 649
Google Scholar
[23] 余洋, 赵南京, 方丽, 孟德硕, 谷艳红, 王园园, 贾尧, 马明俊, 刘建国, 刘文清 2017 光谱学与光谱分析 37 588
Yu Y, Zhao N J, Fang L, Meng D S, Gu Y H, Wang Y Y, Jia Y, Ma M J, Liu J G, Liu W Q 2017 Spectrosc. Spect. Anal. 37 588
[24] de Giacomo A, Dell'Aglio M, Bruno D, Gaudiuso R, de Pascale O 2008 Spectrochim. Acta B 63 805
Google Scholar
[25] Song C, Gao X, Shao Y 2016 Optik 127 3979
Google Scholar
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