Substrate-assisted LIBS For Detection Of Traces Metal Elements In Liquid Samples

Laser-induced breakdown spectroscopy (LIBS) is a newly developed technique forelement analysis. It is based on the analysis of spectral emission from laser inducedplasma produced by the ablation of investigated substance with a high power laser pulse.LIBS has drawn growing attention in the past tens of years as a rapid, on-line, sensitive,and multi-elements analysis method with no or little sample preparation. It has beenextensively used for a wide range of scientific and industrial applications with differentdegrees of success. However, due to complex process of the ablation in bulk, the lifetimeof plasma is relatively short which result in poor detection sensitivity compared with thatin air. In this thesis, in order to improve the detection sensitivity of trace metal elementsin liquid samples, two enhanced approaches named substrate-assisted LIBS weredeveloped.Paper enrichment assisted LIBS was developed for detection of heavy metals in aqueoussolutions. As aqueous solutions were dipped into paper substrate, the ablation conditionwas improved, avoiding the adverse factors for the direct detection of liquid samples. Thewater was then evaporated from the substrate, so the sampling and detection sensitivitywere improved by the enrichment effect of paper substrate. Sub-ppm detection of heavymetal elements in aqueous solutions was obtained using this approach. Limits of detection(LODs) of almost elements were down to the highest concentration of each element inwastewater discharged according to the national standards. In order to evaluate the matrixeffect existed in mixed solution of many metals, this approach was also used to detectsimultaneously heavy metal elements in aqueous solutions. The results showed that thecalibration curve and limit of detection were affected for each element by matrix effect.However, the measured concentration of heavy metal elements in mixed solutions had agood agreement to the real value and the relative error was about5%, which exhibitedgood measurement accuracy of this approach. The approach was also used to detect Mn inseawater solution and calibration curve was plotted. All results indicated that the paperenrichment assisted LIBS was expected as a feasible approach for the detection of heavymetal elements in aqueous solutions.Aluminum substrate assisted indirect ablation LIBS was developed for detection of trace metal elements in viscous liquids, which was achieved in Lyon1University. In this thesis,the physics mechanism of this approach was first investigated, and the enhanced detectionwas then achieved. In experiment, oil samples prepared was smeared with the help of aglass slide as uniformly as possible on the polished surface of an aluminum target to forma uniform thin layer with the thickness of the layer as about15μm. A laser pulse isfocused slightly under the surface of the target which induces a hot aluminum plasma.The interaction between the aluminum plasma and the layer leads to an indirectbreakdown of the oil layer, which was therefore named as aluminum substrate assistedindirect ablation LIBS. After the evolution of the plasma, one gets a mixture of thealuminum plamsa and the layer plamsa with very high temperature (about15000K) forthe sensitive detection of elements contained in the oil layer. As temporal and spatialdynamics of plasma induced by this approach were investiated, we had obtained thesensitive detection of elements from the oil layer with a detection delay of2μs and a fiberposition of2mm above the target surface.In order to evaluate the enhancement effect of this approach, this approach was used forthe detection of wear metals in engine oils and the calibration curves were plotted. Thestandard engine oils were prepared, including12trace wear metallic elements. Weobtained the analytical line of each element and the corresponding calibration curves. Theresults showed that the LODs for wear metals in engine oils were in the several ppmlevels. Especially, the LODs for the3elements (Mg, Cu and Ag) are determined lowerthan μg/g, i.e. in the sub-ppm level. Comparison with the previously published datashowed moreover the efficiency of the introduced ablation configuration as one of themost suitable methods for highly sensitive and precise wear metal analysis in oils.Furthermore, the matrix effect induced in different oils was investigated. It suggested thatthe analytical results were hardly affected by matrix effect among different oils. Allresults indicated that aluminum substrate assisted indirect ablation LIBS was expected asa feasible approach for the detection of trace metallic elements in viscous liquids.Besides, in this thesis, both approaches of paper enrichment assisted LIBS and aluminumsubstrate indirect ablation assisted LIBS are also discussed on how to improve thestability and flexibility for the application. For aluminum substrate indirect ablationassisted LIBS, the indirect ablation plasma produced with aluminum substrate is notimplemented, which is a pity, and will be done in further research work.