祁文科, 张启磊, 周 彧, 郑贤锋, 崔执凤
(安徽师范大学原子与分子物理研究所, 芜湖 241000)
混合水溶液中多金属元素的激光诱导击穿谱
祁文科, 张启磊, 周 彧, 郑贤锋, 崔执凤
(安徽师范大学原子与分子物理研究所, 芜湖 241000)
使用自制的液相射流装置测定了含有多种微量金属元素混合水溶液的激光诱导击穿光谱.研究了激光脉冲能量和ICCD门延时、门宽、增益等对LIBS谱线强度和信噪比的影响,以提高信噪比为标准对实验参数进行了优化.在优化后的实验参数下,确定了金属元素Cr,Cd,Fe,Ca,Al,Mn和Pb的分析谱线,由实验测定定标曲线得到了各元素的检测限,其数值在0.018 ppm(Ca)到41.231 ppm(Cd)区域内.
激光诱导击穿谱; 液体射流; 检测限; 多金属元素; 混合水溶液
随着工业的迅速发展,工业废水污染日趋严重,直接威胁到人类的健康和安全.监控工业废水中重金属元素的种类和含量是对其进行治理的重要前提,迫切需要一种可以快速而准确地检测废水中重金属含量的分析技术.现有的分析方法主要有原子荧光光谱法[1]、原子吸收光谱法[2]、电感偶合等离子体-质谱法[3]、色谱法[4]、溶出伏安法[5]等,这些方法测量精度高,但都需要对样品进行复杂的预处理、分析时间长,不具备快速、实时、在线以及多元素同时分析能力,难以满足水污染在线监测的需要. 激光诱导击穿光谱(Laser Induced Breakdown Spectroscopy,LIBS)是光谱分析领域中一种新的微量元素分析技术[6],他是利用高功率密度激光烧蚀样品表面形成激光等离子体,通过测定等离子体中各元素的发射光谱,实现对样品中各微量元素的定性定量分析.激光诱导击穿光谱技术具有时间、空间分辨率高、可分析元素种类多且可同时分析、分析时间短、无需对样品进行预处理等独特优势,有望实现LIBS技术应用于工业废水中重金属的快速、实时、在线监测,对进一步提高水质的在线检测能力有促进作用.
相比于LIBS用于固相基质中痕量元素分析,由于激光烧蚀引起的液体溅射、液面波动的影响, LIBS技术应用于液相基质痕量元素分析,在检测灵敏度和重复性上有待进一步提高.Cremers 和Rasziemski使用激光光束直接作用于液体,探测到了Li、Na、K、Rb、Cs、Be、Mg、Ca、B 和Al 元素的谱线,得到了各元素的检测限[7].当前, LIBS技术应用于液相基质痕量分析需要解决的关键问题是提高检测灵敏度,主要方法有选取合适的样品取样方式、激光与取样样品之间的作用方式、实验参数优化,样品取样方式有静态液面[8,9]、液相射流[10-12]、电子喷雾[13]和固体基质吸附[14-15]等,激光作用方式上有单脉冲LIBS[16,17]、双脉冲LIBS和与其它辅助激发手段相结合的LIBS[18,19],激光种类有纳秒激光[20]和飞秒激光[21].在实验参数优化方面,研究较多的是激光脉冲能量、探测延时、环境条件等对LIBS信号强度和信背比(SBR)的影响,依据光谱分析原理和分析过程,SBR值对提高LIBS检测灵敏度没有直接影响,因为背景信号可以在数据处理过程中直接扣除,真正决定LIBS检测灵敏度的是信号强度和信噪比(SNR),实验参数的优化应以提高LIBS信号强度和SNR值为目标.
本工作采用液相射流取样系统,运用单脉冲激光诱导击穿光谱技术对混合水溶液中含有的Cr、Cd、Fe、Ca、Al、Mn、Pb等七种金属元素同时进行定量分析.以提高LIBS信号强度和信噪比为目标,对激光脉冲能量、ICCD门延时、门宽和ICCD增益等实验参数进行了优化,在最优实验参数下,确定了各金属元素的分析谱线,由实验测定的定标曲线得到了各元素的检测限.
实验装置主要由激光光源、射流取样系统、光谱检测和数据系统三部分组成.实验方框图如图1所示.
图1 实验装置框图Fig. 1 Experiment setup
Nd: YAG脉冲激光二倍频532 nm激光(Spectra-Physics,LAB170-10)作为光源,脉宽和重复频率分别为10 ns、10 Hz,单脉冲能量在300 mJ以下可调.实验中采用液体射流进样技术,混合水溶液经过一个直径为0.5 mm的喷嘴形成稳定的液相射流,通过蠕动泵控制液体的流速,实验结果表明液体流速为35 ml/min时射流最稳定.为了避免液体溅射对光学元件的污染,通入氮气在射流周围形成保护气体幕布.脉冲激光经过焦距为30 cm的透镜(L1)垂直聚焦于位于喷嘴下方6 mm处的射流前表面,对样品进行烧蚀,产生的LIBS信号经焦距为5 cm的透镜(L2)成像于光纤探测探头,经焦距为750 mm、分辨率为 0.023 nm的单色仪(Princeton Instruments,ACTON SP 2300i)分光后,由增强型电荷耦合器件ICCD探测(Princeton,PIMAX1024),最后由计算机完成探测信号的采集与处理.
按表1所示数据配制混合水溶液400 ml作为基准样品,分析时所需要的低浓度样品溶液由基准样品进一步稀释得到,实验中的样品溶剂为二次蒸馏水.
4.1 实验参数的优化
实验测定了基准样品在348 nm~411 nm区域内的LIBS光谱,如图2所示.通过与 (NIST)[22]光谱数据库的比对以及对谱线强度分布规律的分析,对实验测定的光谱区域内的谱线进行了元素归属,同时根据归属谱线的特征情况(谱线强度、相邻谱线干扰)确定了用于元素分析的分析谱线,结果如表2所示.
表1 基准样品中各元素的浓度及所需的质量
Table 1 Amounts and concentrations of various elements in standard sample
Elementcompoundmolecularformulaamount(mg)concentration(ppm)CrchromicchlorideCrCl3·6H2O204.96100CdcadmiumchlorideCdCl2·2.5H2O24383000FeirontrichlorideFeCl3·6H2O1930.651000CacalciumchlorideCaCl22.7752.5AlaluminiumchlorideAlCl3148.1575MnManganoussulfateMnSO4·H2O245.76200PbleadnitratePb(NO3)2693.401000
图2 基准样品在348 nm-411 nm区域内的LIBS光谱Fig. 2 LIBS spectra of the standard sample in the region 348 nm-411 nm
ElementMeasuredspectralines(nm)Analysisline(nm)Cr357.86 359.35 360.53357.86Cd361.05361.05Fe356.54 357.01 358.12 361.88363.15 364.78 371.99 373.48374.55 374.94 375.82 376.37381.58 382.04 382.58 383.42384.10 385.63 385.99 387.85388.62 404.58 406.36371.99Ca393.36 396.84393.36Al394.40 396.15396.15Mn403.08 403.30 403.44403.08Pb363.95 368.34 405.78405.78
为了提高LIBS信号的信噪比(SNR),首先将基准样品稀释到1/10浓度,对脉冲激光能量、ICCD探测延时、门宽和ICCD增益等实验参数进行优化,得到了七种元素分析谱线LIBS信号强度和信噪比与相应实验参数的依赖关系,其中LIBS信号强度是10个相同条件下所测定信号强度的平均值,其绝对标准偏差均在100~200之间,结果如图3所示.
由图3(a)可以看出当ICCD探测延时在3600 ns~4400 ns这个区间内变化时,信噪比有一个先上升后下降的趋势,在探测延时取4200 ns时, LIBS信号的SNR值最大.由图3(b)可知,在1500~5000 ns范围内,随着ICCD门宽的增加,LIBS信号强度逐渐增加,当门宽大于3000 ns时,信号强度增加的幅度非常小而信噪比有显著的减小,SNR值最大时的最佳ICCD门宽为3000 ns.
由图3(c)可以看出随着激光脉冲能量的增大,激光功率密度增大,激光与样品之间的相互作用增强,样品中处在激发态的原子数目增多,LIBS信号持续增强.SNR值在能量为30 mJ时达到最大值,但如果能量继续增大,导致保护气体击穿和样品的光散射效应增强,导致SNR值直线下降.ICCD增益的功能是将采集到LIBS信号强度值按一定比例增强.由图3(d)可知,随着ICCD增益增大,信号强度逐渐增大,信号的SNR值先增大后减小,在170附近达到最大值.最后经过综合考虑七种元素的LIBS信号强度与SNR值,确定了最佳实验参数为:脉冲激光能量30 mJ、ICCD探测延时4200 ns、门宽3000 ns、ICCD增益170.
4.2 LIBS定标曲线
分别配置浓度为基准样品浓度的1/10、1/5、3/10、2/5、1/2等5种分析样品,在最佳实验参数下,测定了分析样品中Cr、Cd、Fe、Ca、Al、Mn、Pb等七种元素的LIBS信号强度,如图4所示.
由实验测定的不同浓度下各元素分析线相对应的LIBS信号强度,得到了这些元素的LIBS定标曲线,如图5所示.
根据检测限的计算公式:
其中σ是背景信号的标准偏差,S是定标曲线的斜率,根据实验测定的σ值和由各元素定标曲线所确定的曲线斜率S,得到这些元素的LIBS检测限,结果如表3所示.
本工作利用单脉冲激光诱导击穿光谱技术对混合水溶液中多种金属元素进行了定量分析.以提高LIBS信号的信噪比为标准,对各实验参数进行了优化,得到的最佳实验参数为:ICCD探测延时4200 ns、ICCD增益170、ICCD门宽3000 ns、激光能量30 mJ,由此得到的七种元素的LIBS检测限远低于文献报道过的含单一金属元素溶液中该元素的LIBS检测限, 为LIBS技术应用于工业废水的检测提供理论依据和实验参数.
a(1) a(2)Gate width: 1700 ns ICCD gain: 100 Pulse energy: 40 mJ
b(1) b(2)Gate delay: 4200 ns ICCD gain: 100 Pulse energy: 40 mJ
c(1) c(2)Gate delay: 4200 ns ICCD gain: 100 Gate width: 3000 ns
d(1) d(2)Gate delay: 4200 ns Pulse energy: 30mJ Gate width: 3000 ns图3 实验参数对LIBS信噪比与信号强度的影响, (a)ICCD门延时(b)ICCD门宽(c)激光能量(d)ICCD增益Fig. 3 The influence of experiment parameters on the LIBS SNR and signal intensity, (a) gate delay(b)gate width(c)laser pulse energy(d)ICCD gain
ElementCrCdFeCaAlMnPbAnalysisline(nm)357.86361.55371.99393.36396.15403.08405.78LOD(ppm)0.42741.2314.1550.0180.5300.6826.056
图4 不同浓度下混合水溶液七种元素的LIBS谱Fig. 4 LIBS spectra of the seven elements in the mixed aqueous solution with different concentrations
(Fe)和(Pb) (Cr)
(Cd) (Ca)
(Al) (Mn)图5 单脉冲下七种元素的LIBS定标曲线Fig. 5 LIBS calibration curves of the seven elements
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Laser induced breakdown spectroscopy of multi-metal elements in mixed aqueous solution
QI Wen-Ke, ZHANG Qi-Lei, ZHOU Yu, ZHENG Xian-Feng, CUI Zhi-Feng
(Institute of Atomic and Molecular Physics, Anhui Normal University, Wuhu 241000, China)
Laser-induced breakdown spectroscopy (LIBS) of trace multi-metal elements in mixed aqueous solution were measured by a homemade liquid jet device. The influence of laser pulse energy, ICCD gate delay and width, ICCD gain on signal intensities and signal to noise ratio(SNR) of LIBS were investigated. The experiment parameters were optimized in order to enhance the SNR value. The analysis lines of Cr, Cd, Fe, Ca, Al, Mn, and Pb metal elements were confirmed under the optimum experiment condition. The limits of detection(LOD) of LIBS for these elements were obtained by the measured calibration curves. The measured LOD values range from 0.018 ppm(Ca) to 41.231 ppm(Cd).
Laser induced breakdown spectroscopy; Liquid jet; Limit of detection; Multi-metal elemental; Mixed aqueous solution
103969/j.issn.1000-0364.2015.12.017
2014-02-27
国家自然科学基金(11074003)
祁文科(1988—),男,湖北黄冈人,硕士研究生,主要研究领域为激光光谱技术.
崔执凤. E-mail: zfcui@mail.ahnu.edu.cn
O562.3
A
1000-0364(2015)06-1013-05