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经过多次实验, 对比选取了具有代表性的LIBS光谱。为了解决光谱数据不同波段波长漂移现象,减少实验误差,在标定元素的特征谱线前,通过美国国家标准与技术研究院(National Institute of Standards and Technology, NIST)数据库中不同元素的特征谱线的波长与本实验中所得的光谱数据对比,校准光谱波长的漂移。
校准后,对216nm~255nm,266nm~290nm,360nm~410nm,530nm~630nm,700nm~880nm波段的光谱进行元素标定,标定结果如图 2所示。经过整理,得出了电烙铁焊锡烟雾的光谱谱线的鉴别表,见表 1。烟雾中观察到Sn, Pb, Fe, Na, K等金属元素,还检测到N, O, C等非金属元素。
表 1 Characteristic spectral lines of the main elements in smoke
element characteristic spectral lines/nm Sn 235.48, 242.95, 270.65, 283.99, 286.33 Pb 220.35, 283.30, 363.95, 368.34, 373.95, 405.78 Fe 238.20,239.56,248.32,274.93,275.57 N 742.36,744.22,746.83 Ca 393.36,396.98,558.87,612.22,616.21,854.2 K 766.48,769.89 Na 588.99,589.59 Mg 279.55,280.27,285.21 O 777.19, 844.63 在363.95nm, 368.34nm, 373.99nm, 405.78nm处发现Pb的特征谱线,并满足NIST数据库中实验得出的铅元素谱线强度关系,验证了电烙铁焊接含铅锡线时产生的烟雾中含有重金属铅的猜想。进行电烙铁焊接时需要精细操作,这意味着人们需要靠近焊接点,则铅元素会通过人们的呼吸道进入人体,不仅影响人体血红蛋白的合成,诱发溶血,而且破坏消化系统的粘膜,造成萎缩性胃炎[17-18]。
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仅仅从使用含铅锡线的电烙铁工作产生的烟雾中检测出铅元素还不能说明含铅锡线对工作人员健康的危害,实验中要对烟雾中的铅元素进行定量分析,大致估算出烟雾中铅元素的含量,以此来证明使用电烙铁焊接含铅锡线的危险。实验中采用的是元素内标法[19-20]。
将样品分成3份浸泡在不同质量分数的(CH3COO)2Pb·3H2O溶液中,在40℃的低温下烘干。经过计算,3份样品中铅元素的质量分数为5×10-5,10×10-5,20×10-5。通过LIBS技术能够得到各个样品的光谱。以含铅元素质量分数为20×10-5的样品的光谱为例:通过比较添加质量分数为20×10-5铅元素的样品与未添加任何物质的原始样品的光谱,选取340nm~420nm这一段光谱分析。如图 3所示。
由图 3可知, 363.95nm, 368.34nm, 373.99nm, 405.78nm这4条为铅的特征谱线。对铅元素的定量分析就由此为基础。
由Lomakin-Scheibe公式:
$ I=a w^b $
(1) 式中,I为谱线的观察强度, a是实验常数, w是目标元素的质量分数, b是自吸收系数。若忽略自吸收,即b=1,则(1)式可以改写为:
$ I=a w $
(2) 由于在不同的实验中,a的值是不同的, 但是使用内标法可以消除实验常数a对实验的影响。由于电烙铁烟雾中没有钙元素,且在不同实验中,钙元素是不变的, 因此,本实验中选取CaⅡ393.36nm为参考谱线,则公式可以改写为:
$ \frac{I_{\mathrm{Pb}}}{I_{\mathrm{Ca}}}=\frac{a_{\mathrm{Pb}} w_{\mathrm{Pb}}}{a_{\mathrm{Ca}} w_{\mathrm{Ca}}} $
(3) 进一步可化简为:
$ I_{\sum i}=A w_{\mathrm{Pb}} $
(4) 式中, I∑i为4条铅元素特征谱线的相对光强的和,wPb为铅元素的质量分数,A为常数。以I∑i为y轴,以wPb为x轴,用不同质量分数的样品重复实验,得到图 4。由图 4可知,拟合曲线与特征谱线光强有较好的线性关系,因此根据定标曲线,结合实时的空气颗粒物质量分数, 即可得到空气中的铅元素质量分数。
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检出限(limit of detection, LOD)是样品中检测出待测物质的最小质量分数,是评价一个检测方法的重要指标。本实验中使用的元素内标法对待测样品中铅元素的含量有一定的要求,若铅元素质量分数过小,会导致背景光与样品中铅元素的特征谱线混淆,增大实验误差。因此,通过LIBS的检出限公式,结合铅元素的定标曲线,计算出铅元素的检出限:
$ L_{\mathrm{LOD}}=3 \sigma / K $
(5) 式中,σ是340nm~350nm波段多次实验求得的背景光强度的标准差, K是上文计算的铅元素定标曲线的斜率。经过计算,得出烟雾中铅元素的检出限为19.35×10-5。
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当等离子区域需要满足局域热平衡(local thermal equilibrium,LTE)态,即粒子需要满足Boltzmann分布时,样品中的铅元素质量分数与其特征谱线的强度才回近似线性关系。因此,为了验证本实验对铅元素进行定量分析的可行性,需要判断等离子区域是否在实验中处于LTE态[21]。
利用McWhirter准则:
$ n_{\mathrm{e}} \geqslant 1.6 \times 10^{12} \times T^{\frac{1}{2}} \times \Delta E^3 $
(6) 式中,ne代表等离子体的电子数密度(单位为cm-3),T代表等离子温度(单位为K),ΔE代表所选相关元素相邻能级间最大能级差(单位为eV)。当实验光谱满足(6)式时,光谱有效。
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使用Boltzmanna直线法可以计算出(6)式中的T。由Saha-Boltzmann方程[22]:
$ \ln \left(\frac{I_{i j} \lambda}{A_{i j} g_k}\right)=-\frac{1}{K_{\mathrm{B}} T} E+\ln \left[\frac{h c N_s}{U_s(T)}\right] $
(7) 式中, λ和Iij分别表示特征谱线波长和强度,Aij是原子或者离子的跃迁几率,gk是电子或者离子跃迁至上能级的统计权重,k表示上能级的序号, KB为Boltzmann常数,T是等离子温度,E是电子或者是离子跃迁至上能级所需的激发能,c是真空中的光速,h是Planck常数,Ns是电子数密度,Us(T)是电子或者是离子的配分函数, 下标s是电离次数。
选取铅元素质量分数为20×10-5的样品,根据LIBS中的357.27nm,363.95nm,368.34nm,373.99nm,405.78nm这5条为铅的特征谱线的波长λ和强度Iij,在NIST数据库中查寻这5条铅元素特征谱线各自跃迁的机率Aij、跃迁至上能级的统计权重gk和激发能E,通过线性拟合的方法得到等离子温度T。如图 5所示,经过计算得温度T=7143K。横坐标Ek表示上能级能量。
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Stark展宽是电子能级在外电场作用下发生能级分裂而出现的能级展宽,与电子数密度有关。因此可以通过谱线的半峰全宽w0和电子数密度ne之间的关系求得ne:
$ w_0 \approx 2 \omega \frac{n_{\mathrm{e}}}{10^{16}} $
(8) 式中,ω表示碰撞展宽系数。选取405.78nm铅元素的特征谱线,得到半峰全宽为0.295nm,计算得电子数密度为1.68×1017cm-3。
实验中,5条铅元素特征谱线中最大能级差ΔE=3.4693eV,等离子体温度T=7143K,得出电子数密度阈值为8.038×1015cm-3,小于1.68×1017cm-3。因此,实验中的等离子体满足(6)式,即本实验中得到的光谱为有效光谱。
利用LIBS技术对电烙铁的烟雾进行在线分析
Online detection of smoke from the electric iron by LIBS
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摘要: 为了研究电烙铁工作时焊接含铅锡线产生的烟雾对局域空气的污染, 设计了一套基于激光诱导击穿光谱技术(LIBS)的实验系统, 对电烙铁焊接含铅锡线的烟雾进行在线分析, 在烟雾的光谱中发现了重金属元素铅的特征谱线。使用元素内标法对铅元素进行定量分析, 通过分析铅元素的等离子体温度与电子数密度的相关特性, 验证了实验所得光谱的有效性。结果表明, 通过拟合曲线获得铅元素的检出限为19.35×10-5; 对比传统化学的化验方法, 基于激光诱导击穿光谱的电烙铁焊接锡线的场景检测实验系统和方法具有在线、原位、快速的优越性。该研究对解决电烙铁工作时室内空气的污染, 减轻对使用者健康造成的危害是有帮助的。Abstract: In order to explain the local air pollution caused by the smoke produced by soldering leaded tin wire with electric soldering iron, an experimental system based on laser induced breakdown spectroscopy was designed to analyze the smoke produced by soldering leaded tin wire with electric soldering iron, and the characteristic spectral line of heavy metal lead was found in the smoke spectrum. Lead was quantitatively analyzed by internal standard method, and the detection limit of lead was 19.35×10-5 by fitting the curve. By analyzing the correlation between the plasma temperature and electron number density of lead, the validity of the experimental spectrum was verified. The results show that the experimental system and method of scene detection of electric soldering tin wire based on laser-induced breakdown spectrum have advantages of on-line, in-situ and fast when compared with the traditional chemical test method.
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Key words:
- spectroscopy /
- laser-induced breakdown spectroscopy /
- quantitative analysis /
- smoke
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表 1 Characteristic spectral lines of the main elements in smoke
element characteristic spectral lines/nm Sn 235.48, 242.95, 270.65, 283.99, 286.33 Pb 220.35, 283.30, 363.95, 368.34, 373.95, 405.78 Fe 238.20,239.56,248.32,274.93,275.57 N 742.36,744.22,746.83 Ca 393.36,396.98,558.87,612.22,616.21,854.2 K 766.48,769.89 Na 588.99,589.59 Mg 279.55,280.27,285.21 O 777.19, 844.63 -
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