Microanalysis Of Trace Elements In Silver Jewelry By Or Thogonal Dual-wavelength Dual-pulse LA-LIBS

As an in-situ, fast, accurate and quantitative chemical analysis technique, Laser-Ablation Laser-Induced Breakdown Spectroscopy(LA-LIBS) was proposed to resolve the contradiction between spatial resolution and analytical sensitivity in single pulse LIBS. In this thesis, orthogonal dual-wavelength dual-pulse LA-LIBS was used to analyze trace elements in silver jewelry with enhanced sensitivity and minimal sample ablation.Firstly, in order to get better ablation, the relation between the ablation threshold and the spot size of focused laser beams on focal plane should be measured accurately. The function of laser pulse energy to the radius of the crater produced by focused laser beams on sample’s surface is derived theoretically under the condition that the fluency distribution of the focused laser beam on the focal plane is Gaussian shape. The radii of the craters produced by laser beams from LA-LIBS system with different pulse energies on sample’s surfaces were measured with a microscope. The spot size of focused laser beams on the focal plane and the laser-ablation threshold of the sample are measured simultaneously by curve fitting. Measurement errors for both spot size and laser-ablation threshold are about ±10%.Secondly, LA-LIBS system with 532+1064 nm wavelength combination and 266+1064 nm wavelength combination was established based on one single laser system. The spatial resolution and analytical sensitivity on silver jewelry analysis under these two wavelength combinations were compared. The calibration curve was built and the limit of detection of copper in silver matrix was determined to be 44 ppm when the crater diameter was controlled at 17 μm in LA-LIBS system with 532+1064 nm wavelength combination, while in LA-LIBS system with 266+1064 nm wavelength combination, the result was 37.4 ppm and 6.5μm. Besides, the crater diameter created by 532 nm laser and 266 nm laser under the same laser energy was 25 μm and 17 μm, respectively. The comparison between the two wavelength combinations indicated that UV laser can get better ablation in LA-LIBS microanalysis, and 266+1064 nm wavelength combination could reach high spatial resolution and better analytical sensitivity in comparison with 532+1064 nm wavelength combination.In addition, internal standard method was used to eliminate the influence of different experimental parameters, such as geometrical arrangement. It is demonstrated that the intensity of atomic emission of copper at 324.75 nm and that of silver at 328.07 nm was linearly correlated with high correlation coefficient at different distance from the focus center of 1064 nm laser to sample surface, as well as at different pulse energies for 266 nm or 532 nm lasers. Therefore, it is possible to eliminate the influence of experimental parameters on analysis results by choosing silver atomic emission at 328.07 nm as internal standard to build the calibration curve of copper.In conclusion, orthogonal dual-wavelength dual-pulse LA-LIBS can be used to analyze samples with high spatial resolution and analytical sensitivity, which is suitable for microanalysis of precious samples.