| 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. |
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