Studies On The Quantitative Elemental Analysis Technique Based On High Repetition Rate Laser-ablation Spark-induced Breakdown Spectroscopy

Direct sampling and element analysis for solid samples based on laser-ablation(LA)is a leading edge research field in analytical science.One representative technique is laser-induced breakdown spectroscopy(LIBS).LIBS has the following advantages: without requiring complicated sample preparation,fast analysis speed,real-time analysis,on-line or remote analysis,possibility of analyzing any type sample or any element.Therefore,it has been widely applied in the fields of material science,biomedicine,agriculture,environmental science,archaeology,space exploration and so on.However,there still exist some typical drawbacks in conventional LIBS.Further improving the spectral analysis performance of LIBS,including the analytical sensitivity,analytical speed and quantitative analysis accuracy,has been the key scientific problems concerned by the relevant scientists all around the world.In order to improve the analytical sensitivity and speed of conventional LIBS and realize more convenient spectral analysis,a new spectral analysis technique,high repetition rate laser-ablation spark-induced breakdown spectroscopy(HRR LA-SIBS)is proposed for the first time in this work.Elemental analysis of aluminum alloy and copper alloy samples has been carried out with this technique while using acousto-optically Q-switched Nd:YAG laser,Ti-sapphire femtosecond laser and portable fiber laser as the ablation laser sources,respectively.In order to fully understand the characteristics of HRR LA-SIBS and evaluate its spectral analysis performance,in this work,the influence of laser parameters on sample ablation and signal intensities while using different ablation laser sources has been investigated.The effects of electrical parameters of spark discharge to signal intensity and signal to noise ratio,including discharge voltage,capacitance of the energy storage capacitor,resistance of the current limiting resistor have also been investigated.The temporal profiles of atomic emissions of the laser-induced plasma and spark discharge enhanced plasma were observed and compared to help understand the mechanism of plasma emission enhancement.Under the optimized experimental conditions,the quantitative analysis of trace elements in aluminum alloy and copper alloy samples was carried out.Calibration curves were established based on standard samples analysis by HRR LA-SIBS;and the detection limit of each element was evaluated.It is demonstrated that,limited by the lower laser pulse energy of the laser operated at high repetition rate,the spectral analysis sensitivity is relatively low if carrying out elemental analysis only based on optical emission of the laser-induced plasma without the assistance of the spark discharge.However,under the second excitation by the spark discharge,the volume of the plasma has been increased.The peak intensity and the duration time of atomic emission of the plasma has been increased and obviously prolonged respectively.Thus the spectral analysis sensitivity can be improved significantly.For different trace metal elements in aluminum alloy and copper alloy,about one order enhancement factor on the detection limit can be achieved under the assistance of the spark discharge.As a new spectral analysis technique,HRR LA-SIBS has the following characteristics and advantages: First,the operation at high repetition rate significantly reduces the time required to collect spectral data.This not only improves the analytical speed,but also improves the signal-to-noise ratio through the average of multiple sets spectral data repeatedly recorded in a short time.Thus the sensitivity and reproducibility of the spectral analysis can be improved.Second,the continuous background of plasma emission in HRR LA-SIBS is very weak,thus the spectra can be directly recorded under non-gated signal recording mode and these spectra can be used for quantitative analysis of elements.This is helpful to reduce the dependence on the expensive spectrometer coupled with gated optoelectronic detector,such as intensified charge-coupled device(ICCD).Third,when femtosecond laser is selected as the ablation laser;high spatial resolution can be potentially achieved and the sample can be free from damage caused by thermal effect.Finally,if a compact fiber laser is used as the ablation laser,it is possible to build a portable spectral analysis system of HRR LA-SIBS.HRR LA-SIBS developed in this work is particularly suitable for convenient,rapid and highly sensitive element quantitative analysis of alloy samples.It has important application values and can be widely applied in metallurgical industry and the research field of metal materials.