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空气中含有的大量的氮气、氧气以及少量水蒸气,会对样品光谱造成影响,因此需要对空气的光谱进行测量。
图2所示为激光击穿空气的光谱图,包括H (656.2 nm),O (777.2 nm、777.6 nm和777.7 nm)和N (742.4 nm、744.6 nm和746.8 nm),O在777 nm处有三重峰,近似当成一个波峰进行研究,N取746.8 nm处的谱线。如果气压降低等离子体膨胀速度也会变快,等离子尺寸、电子数密度、特征谱线强度等[15]也会发生变化,因此应该选择一个合适的气压进行实验。
如图3所示,随着真空腔内的气压不断变低,空气含量也会逐渐降低,光谱强度也会变得越来越弱。激光能量为90 mJ时,当气压达到4×104 Pa时,空气中H、N和O原子的光谱强度基本消失,认为在该气压下空气的干扰已经排除。
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图4所示为两种有机爆炸物TNT (C7H5N3O6)和RDX (C3H8N6O6)的分子结构。
空气条件下对两种样品进行实验,在激光大能量作用下有机物会与空气发生复杂反应,会出现CN (421.3 nm)和C2 (516.2 nm)两组分子谱线,图5为空气中RDX的分子特征谱线。
对有机物的光谱进行测量时,碳原子的谱线强度基本相同,所以碳原子在分析和鉴别有机爆炸物中的价值很低[6],最终选择CN (421.3 nm)、C2 (516.2 nm)、H (656.2 nm)、N (746.8 nm)和O (777.2 nm)作为特征谱线进行分析。
在空气中分别对RDX和TNT进行激光诱导击穿得到光谱图,二者特征元素的谱线强度对比图如图6 所示。
由图6可以观察到两种有机爆炸物的光谱有明显差异,在空气中RDX的C2谱线强度低于TNT的C2谱线强度,C2分子来自芳环中的碳碳双键以及C原子的重组[6]。RDX的分子结构里面没有碳碳双键,所以C2谱线强度很弱,而TNT分子结构含有一个苯环,在激光高能量作用下苯环裂解开,形成新的分子,所以TNT的C2谱线强度更高。
在空气中RDX的CN谱线强度低于TNT的CN谱线强度,CN分子的形成主要以下几个原因:苯环被裂解形成的C2分子和空气中的N2重组形成CN;有机物被裂解开以后,C原子和N原子直接重组形成CN;有机物被裂解开以后,C原子和空气中的N2重组形成CN;有机物中原有的CN键直接被裂解出来。因为CN分子中携带的C原子来自于样品,分子式中C原子越多CN净信号值应该越强,所以TNT的CN净信号值高于RDX的CN净信号值。
空气中RDX和TNT的H谱线强度与分子式中H原子占比并没有很好地对应。N的谱线强度与分子式中N原子占比也没有很好地对应,空气中的N2含量非常高,N的光谱受空气影响非常大,因此N的光谱信息并无太大价值。O的光谱也和N的光谱类似,应该谨慎用于分析和鉴别有机爆炸物。基于以上分析,单纯用原子谱线去进行分析和鉴别有机爆炸物是很困难的,因此C2和CN是用于爆炸物识别最重要的两条谱线[6]。
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为了消除空气影响,更好地得到样品的光谱信息,我们将两种样品置于低气压条件下。在低气压条件下,空气干扰已经被排除,所得光谱信息即为样品光谱信息,图7为低气压条件下RDX分子特征谱线。
图8为低气压条件下RDX和TNT特征元素谱线强度对比,在低气压条件下,C2和CN谱线强度规律和空气中相似,H的谱线强度很好地反应了两种样品分子式中H原子的占比。N和O与在空气中的光谱信息类似,都不能很好地反应出样品信息,因此有机物中N和O的谱线不能对有机物识别提供帮助。
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如图9所示,低气压条件下RDX和TNT特征谱线强度均有所下降,因为低气压排除了空气的干扰。RDX的C2谱线强度在低气压条件下略微所降低,TNT的C2谱线强度基本没有变化。C2分子来自芳环中的碳碳双键以及C原子重组,RDX不含碳碳双键,所以RDX中的C2分子全部来自于C原子重组,低气压条件下RDX的C2谱线强度降低,可以认为低气压对C原子重组造成了一定影响。TNT含有碳碳双键,而TNT的C2谱线强度在两种条件下基本没有变化,因此认为TNT中的分子主要来源于芳环中的碳碳双键。与空气条件相比,RDX的CN谱线强度在低气压条件下变化很小,而TNT的CN发射谱线在低气压条件下变化很大,结合CN分子的形成原因,认为RDX的CN分子形成过程中氮气参与比例比较小,而TNT的CN分子形成过程中氮气参与比例很大。
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相对标准偏差(Relative Standard Deviation , RSD),用以表示检测结果的精密度。该实验需要对样品进行多次采集取平均值,因此选择具有分析价的CN分子谱线进行相对标准偏差计算,如表1所示。
表 1 不同实验条件下RDX和TNT的10组CN谱线强度
Table 1. Ten groups of CN spectral line intensities of RDX and TNT under different experimental conditions
RDX 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th Air 592 584 621 611 628 631 681 625 665 668 Infrabar 611 596 573 592 607 609 596 597 599 606 TNT 1 st 2 nd 3 rd 4 th 5 th 6 th 7 th 8 th 9 th 10 th Air 830 895 886 839 988 1329 1118 1056 890 1001 Infrabar 624 606 661 656 639 617 636 633 640 650 将表中的实验数据代入到相对标准偏差公式中:
$$ RS D = \dfrac{{\sqrt {\dfrac{{\displaystyle\sum\nolimits_{i = 1}^n {\left( {{x_i} - \overline x } \right)^2} }}{{n - 1}}} }}{{\overline x }} \times 100 {\text{%}} $$ 计算得出RDX在大气条件下的CN谱线强度的相对标准偏差为5.1 %,在低压条件下的相对标准偏差为1.8 %,TNT在大气条件下CN谱线强度的相对标准偏差为15.7 %,在低气压条件下的相对标准偏差为2.7 %。通过两种有机物CN谱线强度在不同条件下的对比,发现在低气压条件下,RDX和TNT的CN谱线强度的相对标准偏差都有了降低,即在低气压条件下检测结果精密度更好。
Spectral characteristics of laser-induced breakdown of organic explosives at low atmospheric pressure
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摘要: 在恐怖袭击中,爆炸袭击为最常见的恐怖袭击方式,爆恐袭击已经严重威胁公众的日常生活,因此对爆炸物的检测越来越受到关注。通过激光诱导击穿光谱技术在空气和低气压条件下分别对RDX和TNT两种有机爆炸物进行检测,检测到原子谱线和分子谱线两种特征谱线,发现CN (421.3 nm)和C2 (516.2 nm)是有机爆炸物最有研究价值的两条谱线。研究结果表明:谱线强度与样品分子式比以及分子结构有关,分子谱线比原子谱线更具有研究价值。与空气条件相比,低气压环境下RDX的相对标准偏差由5.1 %降低到1.8 %,TNT的相对标准偏差由15.7 %降低到2.7 %,低压环境可以有效提高LIBS检测有机物光谱的分析精密度,增加光谱分析准确性,为LIBS对有机爆炸物的检测和分析精密度提高提供了帮助。
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关键词:
- 激光诱导击穿等离子体 /
- 低气压 /
- 有机爆炸物 /
- 分析精密度
Abstract: In terrorist attacks, explosive attacks are the most common terrorist attacks. Explosive attacks have seriously threatened the daily life of the public, so the detection of explosives has attracted increasing attention. Through laser-induced breakdown of RDX and TNT at atmospheric pressure and low pressure, it was found that two characteristic spectral lines of atomic lines and molecular lines, CN (421.3 nm) and C2 (516.2 nm), were the most valuable among all the spectral lines of organic explosives. The results show that the spectral line intensity is related to atomics proportion and molecular structure of the sample, and molecular lines are more valuable than atomic lines. Compared with the atmospheric pressure, the relative standard deviation of RDX decreases from 5.1% to 1.8%, and the relative standard deviation of TNT decreases from 15.7% to 2.7% under low pressure. A low pressure environment can effectively improve the analytical precision of LIBS to detect organic and increase the accuracy of spectral analysis, which helps LIBS to improve the detection and analysis precision of organic explosives. -
表 1 不同实验条件下RDX和TNT的10组CN谱线强度
Table 1. Ten groups of CN spectral line intensities of RDX and TNT under different experimental conditions
RDX 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th Air 592 584 621 611 628 631 681 625 665 668 Infrabar 611 596 573 592 607 609 596 597 599 606 TNT 1 st 2 nd 3 rd 4 th 5 th 6 th 7 th 8 th 9 th 10 th Air 830 895 886 839 988 1329 1118 1056 890 1001 Infrabar 624 606 661 656 639 617 636 633 640 650 -
[1] Yang Yanwei, Zhang Lili, Hao Xiaojian, et al. Classification of iron ore based on machine learning and laser induced breakdown spectroscopy [J]. Infrared and Laser Engineering, 2021, 50(5): 20200490. (in Chinese) [2] Ma Weizhe, Dong Meirong, Huang Yongru, et al. Quantitative analysis method of unburned carbon content of fly ash by laser-induced breakdown spectroscopy [J]. Infrared and Laser Engineering, 2021, 50(9): 20200441. (in Chinese) [3] Yu Dan, Sun Yan, Feng Zhishu, et al. Effects of the combination of sample temperature and spatial confinement on laser-induced breakdown spectroscopy [J]. Chinese Optics, 2021, 14(2): 336-343. (in Chinese) doi: 10.37188/CO.2020-0118 [4] Li Chenyu, Qu Liang, Gao Fei, et al. Composition analysis of the surface and depth distribution of metal and ceramic cultural relics by laser-induced breakdown spectroscopy [J]. Chinese Optics, 2020, 13(6): 1239-1248. (in Chinese) doi: 10.37188/CO.2020-0112 [5] Ren Jia, Gao Xun. Detection of heavy metal Pb in soil by filament-nanosecond laser induced breakdown spectroscopy [J]. Optics and Precision Engineering, 2019, 27(5): 1069-1074. (in Chinese) doi: 10.3788/OPE.20192705.1069 [6] Yang Xindong. Research on identification technology of organic explosives deposited by organic substrate based on LIBS[D]. Chengdu: University of Electronic Science and Technology of China, 2019. (in Chinese) [7] Zhu Shuiquan. Application of laser induced breakdown spectrometry (LIBS) in the field of plastic recycling [J]. Plastic Science and Technology, 2020, 48(1): 155-158. (in Chinese) [8] Osman A E, Marouf A, Ahmed M M. Study of drug bottles using Laser Induced Breakdown Spectroscopy (LIBS) [J]. International Journal of Scientific Research, 2020, 7(3): 442-450. [9] Vadas S. Quantitative analysis of pharmaceutical products by laser-induced breakdown spectroscopy [J]. Spectrochimica Acta Part B: Atomic Spectroscopy, 2002, 57: 1131-1140. [10] Zeng Q, Sirven J B, Gabriel J, et al. Laser induced breakdown spectroscopy for plastic analysis [J]. Trends in Analytical Chemistry, 2021, 140: 116280. doi: 10.1016/j.trac.2021.116280 [11] Frank C, Lucia D, Russell S, et al. Laser-induced breakdown spectroscopy analysis of energetic materials [J]. Applied Optics, 2003, 42: 6184-6152. [12] Jennifer L, Gottfried, F D, Miziolek A, et al. Laser-induced breakdown spectroscopy for detection of explosives residues: A review of recent advances, challenges, and future prospects [J]. Analytical & Bioanalytical Chemistry, 2009, 395: 283-300. [13] Tran M, Sun Q, Smith B W, et al. Determination of C: H: O: N ratios in solid organic compounds by laser-induced plasma spectroscopy [J]. Journal of Analytical Atomic Spectrometry, 2001, 16(6): 628-632. doi: 10.1039/B009905H [14] Jo Cáceres, Jt López, Telle H H, et al. Quantitative analysis of trace metal ions in ice using laser-induced breakdown spectroscopy [J]. Spectrochimica Acta Part B Atomic Spectroscopy, 2001, 56(6): 831-838. doi: 10.1016/S0584-8547(01)00173-2 [15] He Yaxiong, Zhou Wenqi, Ke Chuan, et al. A review of laser induced breakdown spectroscopy in gas detection [J]. Spectroscopy and Spectral Analysis, 2021, 41(9): 2681-2687. (in Chinese)