Research On Laser Induced Breakdown Spectroscopy In Low Pressure And Calibration-Free Quantitative Method

Laser induced breakdown spectroscopy?LIBS?is a spectral analytical technique suitable for various application.LIBS can realize remote,in-situ,fast and all-element analysis,therefore it is called as“future star of chemical analysis”.In 2012,USA scientists successfully applied LIBS to the exploration of Mars and together with other devices,proved the existence of H2O in the Mars.In recent years,aerospace business of our nation also stepped to the deep space,and the Mars becomes another important destination of exploration after the Moon.Therefore,it is very significant to apply LIBS to the exploration of Mars.This research focused on the problems of realizing precise quantitative measurement of LIBS on Mars.Reliable spectral data is the basis of the quantitative measurement.This article has investigated the improvement of repeatability from LIBS signal generation and signal collection,based on the previous remote LIBS experimental system.Firstly,the time-resolved images were collected to research the evolution mechanism of laser induced air plasma.Then the time-integrated images and spectrum at various experimental setups were measured to compare the repeatability of images and signals.The results showed the exist of rebounding and squeezing process during the evolution of laser induced air plasma,which effected the stability of plasma.At the most suitable setup,laser energy around 250 mJ and focus length around 100 mm,a large and stable excited plasma was formed,due to the massive ablation of air by the large laser energy and the mixing process induced by the redounding process.The signal collection was optimized using The spherical concave mirror,and the final pulse-to-pulse RSD was reduced to 3%.The repeatability has been improved greatly.The Mars atmosphere mainly consists of CO₂ with pressure around 1 kPa,which is different from earth atmosphere.The vacuum chamber was used to simulate the Mars environment,in which a series of soil sample were measured.A partial least squares multi-type calibration model based on primary factor was setup through analyzing spectrum.The results were less than 5%relative difference for the main and minor element,which fulfilled the precise requirement of Mars exploration.Besides,to analyze more sample types the quantitative determination without using known samples was investigated.The principle of calibration free method and the departure from ideal assumption were investigated.CSigma-LIBS based on the theory of curve of growth?COG?was found to be feasible.The method has eliminated the Lorentz broadening of all elements,integrating calibration curves of all elements into only one calibration curve.This method performed well in quantitative analysis.When LIBS in-situ works on Mars,parameters of equipment will be optimized to improve the repeatability of signal.Various samples can be roughly quantitative determined through optimized calibration free model.Besides,the precise quantitative analysis will be done by the multi-type calibration model for certain type sample.