| Laser-induced breakdown spectroscopy (LIBS) is one of elemental analysis techniques, which exhibits attractive features, such as sensitive multi-elemental detection, rapid response for online monitoring, easy sample preparation for in situ measurements, high spatial resolution for surface mapping, and standoff operation capacity for remote sensing. It thus demonstrates great potential for its applications in various domains. However, quantitative analysis of LIBS suffers from matrix effects due to the complex nature of the interaction between laser and material, which lead to poor accuracy and reproducibility and seriously impedes its practical application. The research on the specificity for quantitative analysis is focused and vigorously continued in the international community working on LIBS. This work presented the studies on the quantitative analysis of LIBS for specific application to environmental samples including oil, aqueous solution and soil. Some new experimental methods were proposed and demonstrated to improve the analytical performance of the technique.In this work, the possibility of using surface-enhanced LIBS (SENLIBS) was evaluated as a simple and fast method for trace elemental analysis for oil. The experiment was carried out with analysis by laser irradiation of oil gel layer coated on metallic substrates. Temperature higher than 12 000 K of the induced metal plasma allows considering sensitive detection of trace elements in oil film. The limits of detection (LoDs) were thus obtained from sub ppm to several ppm for 9 metallic elements (Mg, Cu, Ag, Ti, Si, Sn, Cr, Fe and Ni) in a reference oil. Ten kinds of engine oil or cooking oil were prepared and their calibration curves for Fe, Ni, Cr were compared, which shows the absence of matrix effects brought by different oil with SENLIBS. The concentrations of metallic elements in 5 used lubricating oils were finally determined.To address the matrix effect when the metallic SENLIBS was used for aqueous solutions, chlorine content brought by different salts in a composite powder was determined when the sample was prepared in pellet or first dissolved into solution and then dropped on the surface of a pure metallic target. The similar responses from chlorine for the solution samples showed absence of matrix effect for analysis with the SENLIBS configuration, which was further confirmed by the consistence of the measured temperatures and the electron densities of the produced plasmas. In contrast, the slopes of the chlorine calibration curves exhibited significant variation for different pellet samples corresponding to different powder mixtures, which is an indication of matrix effect in the LIBS analysis of the pellet samples.A fast and sensitive analysis for heavy metals in aqueous solution was realized by using improved SENLIBS with a laser-pretreated aluminum substrate. Solutions were concentrated in a laser-pretreated area of a polished aluminum target surface, wherein pretreated grooves enabled homogeneous distribution of the metallic solutions on the metallic surface, and laser ablation of the aluminum target produced unique plasma excitation of various metallic ions. Moreover, the laser-pretreated metallic micro structures kept the solution in the well-defined area and allowed more solution deposited with the help of a hot plate, which resulted in LODs of sub-ppb level for Cr and Ni and ppb level for Cd due to the enrichment effect. Compared with those by the optimal LIBS measurement method reported, it has been improved by about two orders of magnitude.In this work, we also demonstrated a new method to control and reduce the matrix effect by adjusting KBr percentage as a binder in the preparation of pellets with soil powders. Matrix effects were serious for quantitative analysis of soil due to the heterogeneous nature. The powders of nine types of geological samples (soils and rocks) were mixed with an amount of pure KBr powder with a percentage up to 80% in the pellet preparation. The results show that at a high binder percentage of 80%, the matrix effect is much reduced. And at the same time the emission intensity remains at the same level for the diluted samples thanks to an improvement of the ablation rate for the pellets prepared with binder, which was also proved by observation sizes of the ablated craters and the signal response for different KBr ratio in the mixture.The highlights of this work:1) The metallic SENLIBS method for liquid was realized, which improved the coupling efficiency of laser and liquid, the sensitivity for trace element analysis. Moreover matrix effects brought by different analytes can be minimized because the properties of the induced hybrid plasma depended on a same metal substrate.2) An improved SENLIBS method using a laser pretreated target was proposed, which was proved an improvement for sensitivity with about two orders of magnitude and better repeatability than conventional LIBS.3) A new method to control and reduce the matrix effect by adjusting the binder material percentage of KBr in the preparation of pellets with soil powders was demonstrated. |
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