Technique Of The Heavy Metal Determine In Soil Based On The Polarization-resolved Laser-induced Breakdown Spectroscopy

Polarization-resolved laser-induced breakdown spectroscopy(PRLIBS)is a low-cost improved laser-induced breakdown spectroscopy(LIBS)technique,mainly differing discrete spectrum from the background by polarization characteristics.PRLIBS has the advantage of improving the signal-to-background ratio(SBR)and reducing the detection limit of the spectrum.Currently,with many researches on polarization characteristics of plasma,the application of PRLIBS is still undeveloped.The main reason lies in two points:firstly,the theory for the polarization of spectrum is controversial;secondly,the improvement of PRLIBS in researches is seldom repeatable.Some studies have not achieved significant improvement in the experiment.In order to solve these two problems,this study used nanosecond pulsed laser and fiber spectrometer on aluminum alloy samples and soil samples.In theory,some of the mechanisms with low probabilities are excluded by experiments,and most of the laws in polarization spectrum are explained.In the application,by establishing the calibration curve of the relative spectral intensity with respect to the concentration,the prediction model of Cd in the soil under PRLIBS is obtained,which has an improvement on the spectral SBR,relative standard deviation(RSD)and detection limit,compared with non-polarization LIBS.Specific researches are as follows:1.In this paper,the theory of PRLIBS is studied.This paper summarizes the representative explanations of PRLIBS mechanisms at home and abroad and discusses them.The related theory is studied by experiments.The conclusion is that in the normal LIBS experiment,the recombination radiation takes the most part of the background radiation in the visible and ultraviolet range.The polarization characteristics are mainly due to the anisotropic recombining process generated during the plasma recombination stage.In this process,the number of magnetic sub-levels of the excited atoms is not balanced.2.The effects of various parameters on the enhancement of the SBR in PRLIBS are investigated.By varying the energy density,angle of detection,wavelength,analyzer angle,delays,integration time and other factors,data include spectral intensity,degree of polarization and the SBR was collected.With the energy density of 20 J/cm~2,the SBR at Fe?407.123 nm was increased from 4.86 to 12.97.It's found that the effect of the PRLIBS to improve the SBR depends on some experimental parameters,including energy density,delay time,analyzer angle,and wavelength.The result is as follows:SBR decreases and then becomes flat when the energy density increases,which is similar to the trend of normal LIBS;the analyzer angle influences the SBR,which is related to the polarization orientation and polarization degree of the spectrum;the formula for improvement of SBR about the polarization degree,analyzer angle and polarization orientation is derived;the polarization degree of continuum is flat at all wavelength and the polarization degree of the discrete spectrum decreases as the spectral intensity increases;the polarization degree changes with the delay increasing inapparently,whose reason is that the amount of change in the delays is little compared to the integration time.3.The application of PRLIBS in detection of heavy metal is studied based on the experiment of soil samples contained with cadmium by gradient concentration.In this paper,the calibration curve was established based on the relationship between element concentration and spectral intensity.The improvement of PRLIBS on SBR and detection limit was studied.The experimental results show that the RSD of the spectral intensity is reduced from 7.0%without polarization to 5.7%with polarization,and the detection limit of the prediction model is also reduced from 8.32?g/g to 3.2?g/g.The comparison of cadmium detection results shows the application significance and prospect of PRLIBS.