Enhancement Approaches Of Guide-deposition Sampling And Fs-ns Double Pulsed Excitation In LIBS Detection

Laser Induced Breakdown Spectroscopy (LIBS) is a qualitative and quantitativediagnostics with advantages of minor damage, real time, stand off, in-situ, multi-elementsdetection and no sample preparation etc. It has been successrully applied to many areaswith different degrees of success. However, it is still a challenge to use LIBS as aneffective method for chemical analysis since the low sensitivity. As for the LIBSapplication, in this thesis the detection enhancement approaches were developed to thecations underwater and the micro-ablation for improving the sensitivity.In order to enhance the detection, two methods were developed, one is “Guide-depositionsampling” for LIBS detection of metal-ions in water, the other one is “fs-ns double-pulsedexcitation” for micro-LIBS. In”Guided-deposition sampling”, the copper cations weredeposited and concentrated as simple substance on the cathode; by ablating the depositionlayer, the copper can be obviously detected than nothing by directly detecting the solution,and the minimum detectable concentration could be as low as500ppt. Also, it wasfound the minimum detectable concentration of Cu2+could be sucessfully lowered (5ppm/5ppb/500ppt) by increasing the voltage (0V/30V/60V). The results suggested thatthe detection sensitivity of LIBS can be altered by changing the voltage.In “fs-ns double-pulsed enhancement” for the micro-LIBS, the sensitivity improvementwas achieved by ns pulse reheating the plasma generated by the fs pulse. It was shownthat the fs-ns double pulse not only extended the plasma persistence from20ns offs-LIBS to3000ns in double pulse mode, but also increased the plasma emission. Itwas interesting that the optimum Signal-to-Noise Ratio was reached with a big pulseinterval as ns pulse fired more than300ns after fs pulse. Compared to the fs-LIBS,fs-ns double-pulsed LIBS can enhance the signal by300folds, and the detection limit could be lowered by30folds when the crater size was1.7microns and the mass was2.9pg. Also, the preliminary research for the ehancement mechanism was carried out.These two methods both could improve the LIBS detection significantly, so for furtherapplication, the “Guide-sampling enhancement” may be helpful for the seawater qualityassessment, and the “fs-ns double pulsed enhancement” might be applied for the sedimentanalysis. It is hoped this thesis be useful for the LIBS application.