【作者】 吴江来；

【导师】 郑荣儿；

【作者基本信息】 中国海洋大学 ， 光学工程， 2008， 硕士

【摘要】 现有海水中金属含量的分析方法主要有原子吸收分光光度法、电感耦合等离子光谱法及一些传统的化学分析方法。虽然测量精度很高,但都需要对海水进行采样,并对样品进行预先的处理,分析时间较长,不具备快速、实时、在线分析的能力,难以满足海洋原位探测的需要。激光诱导击穿光谱技术(LIBS)作为一种原位、实时、连续、无接触检测技术,常用于一些异常环境下的化学成分检测,其检测对象主要是金属元素。目前,LIBS技术已被广泛地应用于矿业、冶金业、环境分析等多个领域。然而在液体环境中,受液体压力、吸收等因素的影响,光谱探测难度增加,使得LIBS技术应用到水下探测面临着极大的挑战。因此,作为LIBS海洋原位探测技术研究的实验室预演,本课题瞄准LIBS技术在水下应用的技术关键进行实验探索,以期为基于LIBS的海洋原位探测系统原理样机的开发提供有价值的技术参数与方案。论文首先介绍了选题的背景和意义,然后从LIBS技术的基础原理、现状及发展趋势、仪器等方面介绍了本论文的理论依据和实验方法。作者的主要工作包括:①搭建气-液界面LIBS实验系统,对气-液界面激光诱导等离子的光谱特性进行了分析;②搭建水下LIBS实验系统,对水下激光诱导等离子的光谱特性进行了分析;③搭建LIBS & Raman光谱联合探测系统,对将两种光谱技术结合的可行性进行了探讨。运用气-液界面LIBS实验系统,对竖直流动的CuSO4和Pb(NO3)2水溶液样品表面的LIBS光谱特性进行了观测分析。通过对Cu元素的LIBS信号随时间及能量演化的研究,确定了该系统脉冲激光在气-液界面的击穿阈值约为10 mJ,诱导的等离子体寿命约为1400 ns;检测Cu元素时的最佳延时为700 ns,最佳脉冲能量为40 mJ。在进一步优化系统的其它工作参数后,对信号随浓度的演化进行了探测,初步确立了系统对Cu元素的检测限为31 ppm。在对Cu元素检测的基础上,对溶液中的Pb元素进行了探测,结果表明两者有着几乎相同的时间及能量演化趋势;为了减少空气击穿时氧信号对Pb元素信号的影响,实验在探测Pb元素时,将击穿点从液柱的前表面移到了后表面,优化工作条件后,系统对Pb的检测限由在前表面击穿时的200 ppm降到了50 ppm,得到了很大的改善。运用水下LIBS实验系统,对激光在CaCl2溶液中诱导的等离子体特性进行了研究。通过对Ca I 422.7 nm、Ca II 393.4/396.8 nm特征谱线强度随时间演化的研究,得到了532 nm和1064 nm脉冲各自在溶液中诱导的等离子体寿命分别约为600 ns及1200 ns,确定了有利于信噪比改善的探测延时。通过对Ca的特征信号随能量演化的研究发现,在脉冲能量高于击穿阈值时,激光诱导等离子体的寿命趋于恒定,并且增加脉冲能量会使等离子体辐射的连续背景大大增强,影响元素特征辐射的探测,选择较低的单脉冲能量激发将使信号得到改善。开展上述工作之后,对实验条件进行了优化,初步确定了在采用532 nm及1064 nm脉冲激发时,所搭建的LIBS系统对溶液中Ca元素的最低检测浓度分别为50 ppm和25 ppm。通过对水下LIBS实验系统的改进,引入Raman光谱的探测,对LIBS与Raman光谱技术的结合进行了初步的探讨。实验通过两种工作方式同时获得了Na元素的LIBS信号及SO42-的Raman信号,肯定了将LIBS和Raman技术结合的可行性,但所探测到信号的信噪比都较差,有待对激光器及探测器等方面进行改善。在总结所做工作的基础上,论文最后对开发一种基于LIBS的海洋原位探测系统,应用于海洋中金属通量监测的可行性进行了分析;对今后的工作开展进行了展望;并浅谈了进一步的努力方向。更多还原

【Abstract】 Standard analysis methods, including atomic absorption spectroscopy, inductively coupled plasma atomic emission spectrometry, X-ray emission spectroscopy etc., have been used to assess the elemental composition of water samples. However, these methods have suffered from a unique set of practical and technical limitations due to it needs to pre-handle the sample. Laser-induced breakdown spectroscopy (LIBS) has been shown to be a useful technique in elemental analysis with many advantages including rapid analysis, simultaneous multi-element detection, in-situ and stand-off analysis capability. LIBS has been widely used in many areas such as mining, metallurgy, environmental analysis and numerous other fields, but the application of LIBS for water samples remains interesting in the laboratory due to low ablation efficiency and short lifetime of laser induced plasma in water. So in this thesis, to evaluate its potential application for on-line metals elements monitoring in ocean, the spectroscopic characteristics of the laser induced plasma of water samples has been investigated in this work.The thesis begins with a brief background introduction of this work. After that is the description of fundamental mechanical of LIBS, a detailed review of the status quo and the tendency of this technique. A detail introduction of the LIBS systems used to carry out the work was given in Chapter 3. The bulk of author’s contribution, within the general group effort, was described in Chapter 4 and Chapter 5.The experimental investigation of laser induced plasma on a flowing water solutions surface has been presented in Chapter 4. The Cu and Pb in CuSO4 and Pb(NO3)2 water solutions has been detected. The temporal characteristic of the laser induced plasma and the power dependence of LIBS signal had been investigated. The operation condition was improved with the optimal ablation pulse energy and the delay time for LIBS signal detection. The ablation location has been varied to achieve better LIBS signal. The optimized ablation location for lead was found to be different from that for copper due to the breakdown of the ambient air. The detection limit of metal ion in water solution under the optimized operation conditions was found to be 31 ppm for copper and 50 ppm for lead.After the above work, the characteristics of the laser induced plasma in water have been investigated with calcium chloride water solution samples in this work and it has presented in Chapter 5. The lifetime of the laser induced plasma underwater is determined to be about 600 ns for 532 nm laser pulses, 1000ns for 1064nm laser pulses by the study of the temporal characteristic of the laser induced plasma and the optimal detector delay for signal detection is determined to be about 150ns to 500ns when 532nm laser pulses have been used. Through the study of the power dependence of the LIBS signal, it was found that the lifetime of the laser induced plasma doesn’t change with the pulse energy and when the pulses energy is above some level, the increased pulse energy cannot improve but deteriorate the signal. The detection limit of Ca in water solution under the optimized operation conditions was found to be about 25 ppm. The potential and obstacles of LIBS for on line metal elements monitoring in ocean has been point out.Along with the ongoing Raman Project in the laboratory, the feasibility of combine LIBS and Raman spectroscopy together as a system has been evaluated. The work of joint analysis of sodium sulfate by LIBS and Raman spectroscopy has also been carried out. The LIBS signal of sodium and the Raman signal of sulfate have been detected at the same time in two different ways with the same spectra system. This work is presented in the last part of Chapter 5. The results suggest that the two technique can integrated into a system with the improvement of the LIBS and Raman signal.In the last part of this thesis, a general discussion of the work and some suggests of possible future developments have been given in chapter 6.更多还原