Research On The Detection Method Of Heavy Metals In Soil Under The Enhancement Of LIBS Signal

In recent years,at the same time as China’s rapid economic growth,environmental problems have become more severe,and heavy metal pollution in the soil is particularly prominent.Laser induced breakdown spectroscopy(LIBS)has the advantages of approximately no sample pretreatment,rapid detection of multiple elements at the same time,and is very suitable for the research of soil heavy metal detection.This article introduces the current status and existing problems of LIBS technology in soil heavy metal detection at home and abroad.The related theory of laser plasma was systematically explained,and an experimental system was set up based on the LIBS knowledge.The following research focuses on the detection and quantitative analysis of heavy metal elements in soil:(1)Selection of spectral lines,optimization of experimental parameters of LIBS,and noise reduction of spectral data.First,the signal-to-back ratio of different characteristic spectral lines of the same element and their sensitivity to changes in element content are compared,and appropriate characteristic spectral lines are selected as the analysis lines of LIBS.Second,the signal-to-background ratio of the spectral signal,the plasma electron temperature,and the LIBS signal intensity were used as evaluation indicators to optimize the acquisition delay of the spectrometer,the distance between the lens and the sample,and the location of the plasma collected by the fiber optic probe.The optimal delay times of Cu and Cr elements are 1.8μs and 2.0μs respectively;the distance between the optimal lens and the sample is 96 mm and 98 mm;the detection angle and distance of the fiber optic probe are 25°and 1.5 cm,respectively.Finally,the Savitzky-Golay filter algorithm is selected as the method of spectral preprocessing to remove noise from the spectral data.(2)Reduce the effect of soil matrix effect on LIBS quantitative analysis.TThe accuracy of LIBS soil plasma quantitative analysis is mainly affected by the matrix effect.Eliminate the interference of matrix effect from adding soil coagulant and using metal matrix to assist LIBS detection.In terms of adding soil coagulant,the addition of 80%KBr coagulant was selected to improve the coupling efficiency of laser and soil,the ablation amount of the sample increased by 0.234g,The spectral line intensity of the Cd element has increased by nearly 2 times and its average RSD has been reduced by 3.11%,and the dispersion of data points with different concentrations in the calibration curve has been improved.The correlation fitting coefficient R~2=0.934 is much higher than the directly measured R~2=0.759,and the average value of relative prediction error is 21.06%lower than that of direct measurement.In terms of metal matrix-assisted LIBS detection,LIBS was applied to the surface of Cu,Al,Zn three kinds of metal substrates by muddying the soil.The high-temperature metal plasma generated by the pulsed laser on the surface of the metal substrate heats the sample to be tested twice,which will further excite the atoms of the target element to generate a stronger LIBS signal.The correlation coefficients R~2 of the target element calibration curve are 0.903,0.962 and 0.956,respectively,and the average relative standard deviation(RSD)is 3.87%,6.20%and 7.39%,respectively,realized the effective reduction of the matrix effect.(3)For the free calibration method(Calibration Free-LIBS,)applied to the optimization of soil heavy metal element quantitative analysis.As the soil composition is unknown and complex,the calibration curve established by conventional quantitative analysis methods has certain limitations and applicability.This paper uses CF-LIBS without the need to establish a calibration curve to analyze the content of soil heavy metal elements.The analysis results were compared with those obtained by inductively coupled plasma mass spectrometry(ICP-MS).The average relative error of the target elements was 10.87%.Then,the internal reference for self-absor-ption correction was used to correct the effect of the self-absorption effect on the quantitative analysis of LIBS,and the target element content was analyzed again in combination with CF-LIBS.The results show that the corrected target element’s The average relative error is 5.21%,and the accuracy of CF-LIBS has been significantly improved.