Analysis Of Aluminum Alloy Samples With High Repetition Rate Laser-ablation Spark-induction Breakdown Spectroscopy

The ablation laser sources of traditional laser-induced breakdown spectroscopy(LIBS)are lasers with low repetition rate(usually at 5-10 Hz).However,using high repetition rate laser as the laser source in LIBS technology has many outstanding advantages,such as: the analysis speed of LIBS technology can be improved,which promotes the development of LIBS technology in 2D and 3D elemental distribution imaging of solid sample;it is very helpful to build portable LIBS system due to the simple structure of high repetition rate laser;the continuous background of the plasma is usually very weak due to the low pulse energy of high repetition laser,and so on.This paper first discusses that the high repetition rate laser lays the technical foundation for fast scanning mapping of two-dimensional elemental distribution of solid samples and then explains that,with the increase of the repetition rate of the laser pulse,the single pulse energy of the laser becomes lower,leading to low spectral sensitivity of the single-pulse LIBS technology if a high repetition rate laser as a LIBS laser source.Finally,we explore the feasibility of using high repetition rate lasers with low pulse energy to ablate solid samples and then use high-voltage spark discharge to further breakdown the sample to form plasma and enhance atomic emissions of the plasma.Based on the previous research results of the laboratory,an acousto-optic Q-switched Nd:YAG laser with an adjustable repetition rate of 1-10 kHz was used as the ablation laser source for the sample,and the plasma emission was enhanced with high-voltage spark discharge to improve spectral analysis sensitivity.Experimentally,the emission was first analyzed by a monochromator,then realizing photoelectric conversion with a photomultiplier tube,displaying and recording data through a digital storage oscilloscope.The experimental results show that under the effect of spark discharge,the peak intensity and relaxation time of plasma emission of LIBS technology based on high repetition rate laser source have been enhanced and extended respectively.The time-integrated intensity enhancement factor of the signal can reach 1-2 orders of magnitude,and it is easier to apply time-resolved signal detection technique.The limit of detection of trace chromium in aluminum alloys is ~132 ppm.At the same time,the craters formed by ablation laser in theexperiment were observed and it was found that the shape of the ablated crater was almost the same in both cases of laser ablation only and the laser-ablation as well as spark-discharge.This indicates,under the current discharge conditions the signal enhancement of the spark discharge is mainly due to the enhanced atomic emission intensity of the plasma caused by the spark discharge,and the effect on the signal enhancement due to more sample ablation is negligible.This study verified the feasibility of high repetition rate laser-ablation spark-induced breakdown spectroscopy for the analysis of solid sample elements.This technique is expected to play a significant role in the rapid analysis of solid sample elements.