College of Optoelectronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing Municipal Level Key Laboratory of Photoelectric Information Sensing and Transmission Technology, Chongqing 400065, China
Abstract: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. Keywords:reheat double pulse/ Coptidis/ spectral enhancement/ limit of detection
脉冲间隔是影响双脉冲LIBS光谱强度的重要参数[24,25], 因为脉冲间隔会影响第二束激光与第一束激光诱导产生的等离子体相互作用的过程. 实验选择了800 mg/kg黄连样品作为研究对象, 通过控制数字脉冲延时发生器改变两束激光脉冲之间的时间变化, 主脉冲激光能量为E1 = 15 mJ, 再加热脉冲激光能量为E2 = 35 mJ, Cu I (324.46 nm)与Pb I (405.78 nm)特征谱线的ICCD探测延时分别为1.5 μs和1.7 μs, 激光脉冲频率为4 Hz, ICCD探测门宽为0.2 μs的条件下, 获得Cu和Pb元素光谱强度和信噪比随脉冲间隔的变化, 如图5所示. 分析图5可知Cu I (324.46 nm)谱线在0—0.8 μs之间光谱信号强度很高, 信噪比很低, 这是由于第一束激光诱导产生的等离子体随着脉冲间隔的变大, 体积增大, 进一步吸收了第二束激光能量, 使等离子体电子激发温度升高, 从而导致等离子体的轫致辐射增强, 但是随着脉冲间隔的继续变大, Cu I (324.46 nm)谱线特征信号开始减弱, 信噪比逐渐增大再减少. 而对于Pb I (405.78 nm)是该现象发生在1.2 μs之后. 综合考虑光谱强度和信噪比两个因素, 选择1.4 μs和1.6 μs分别作为Cu I (324.46 nm)与Pb I (405.78 nm)定量分析的脉冲间隔实验条件. 图 5 光谱强度和信噪比随脉冲间隔的变化 Figure5. Variations of signal intensity and SNR as a function of pulse interval time
为了对比SP-LIBS和RDP-LIBS技术的检测性能, 本文使用外标法分别建立了黄连标准参考样品的Cu I (324.46 nm)和Pb I (405.78 nm)定标模型. 使用下列公式分析计算Cu I (324.36 nm)和Pb I (405.78 nm)特征谱线的检测限:
$LOD = \frac{{3\sigma }}{k}, $
式中, $\sigma $为在特征谱线处背景光谱强度的标准偏差, $k$为定标曲线的斜率. 图7分别给出了采用SP-LIBS与RDP-LIBS技术对黄连中Cu和Pb元素检测的定标曲线, 具体的性能参数如表1所列. 图 7 SP-LIBS和RDP-LIBS下Cu, Pb元素定标曲线拟合图 Figure7. Linear fitting curves of Cu and Pb in SP-LIBS and RDP-LIBS.
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
表1特征谱线的检测限(LOD)和线性拟合度(R2)对比 Table1.Comparison of detection limits and relative standard deviations of characteristic lines.