再加热双脉冲激光诱导击穿光谱技术对黄连中Cu和Pb的定量分析

Quantitative analysis of Cu and Pb in Coptidis by reheated double pulse laser induced breakdown spectroscopy

基于单脉冲激光诱导击穿光谱(single pulse laser-induced breakdown spectroscopy, SP-LIBS)实验装置,搭建了再加热双脉冲激光诱导击穿光谱(re-heating double pulse laser-induced breakdown spectroscopy, RDPLIBS)系统,实现了黄连中重金属元素Cu和Pb的检测.固定激光脉冲频率为4 Hz,两束激光的总能量为50 mJ,实验优化了增强电荷耦合器件探测延时、脉冲间隔和双激光脉冲能量组合等参数对Cu I (324.46 nm)和Pb I(405.78 nm)光谱强度的影响,得特征谱线Cu I (324.46 nm)的最佳激光能量组为(E1=15 mJ,E2=35 mJ),脉冲间隔和探测延时分别为1.4μs和1.5μs;Pb I (405.78 nm)的激光能量组也为(E1=15 mJ,E2=35 mJ),脉冲间隔和探测延时分别为1.6μs和1.7μs.为实现RDP-LIBS技术对中药材重金属元素检测性能的评估,在最佳实验参数下,分别对Cu和Pb进行SP-LIBS技术和RDP-LIBS技术定量分析,检测限分别为1.91 mg/kg和3.03 mg/kg,较SP-LIBS技术,检测限均有所降低,满足《药用植物进出口绿色行业标准》的要求,且RDPLIBS的线性曲线拟合度都优于SP-LIBS,说明RDP-LIBS技术在中药材检测中具有更佳的检测性能.

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 are1.4 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.