Research On The Optimization Of Soil Heavy Metal Detection Method Based On Laser Induced Breakdown Spectroscopy

In recent years,with the rapid development of economy and the improvement of people's life quality,the problem of soil pollution is more serious.The pollution of heavy metals in soil seriously affects the safety of agriculture and food.Therefore,the efficient and accurate detection of heavy metals pollution is an important issue to be solved.Laser induced breakdown spectroscopy(LIBS)technology is very suitable for the study of heavy metal elements in soil because of its advantages such as high analysis speed,simple sample pretreatment and simultaneous detection of multi elements.In this paper,LIBS technology was used to optimize the detection method of heavy metals in soil.Based on the detailed analysis of the optical system commonly used in the LIBS technology detection system and its application fields,a LIBS soil heavy metal detection system was built by using the light guide arm.The effects of breakdown threshold,plasma characteristic parameters on the optimization of experimental conditions and the interference between spectral lines are analyzed theoretically and experimentally.The specific research contents are as follows:(1)Analysis of energy threshold and spectral characteristics changing with spot size based on LIBS TechnologyIn order to study the threshold value of laser-induced optical breakdown energy and the relationship between spectral signal and spot size,the theoretical model of laser spot radius and threshold laser energy is obtained by using avalanche breakdown threshold theory,light intensity distribution theory and the relationship between pulse energy and peak power.Through the experiment,the mathematical model of laser energy and spot radius at the threshold is obtained,and the theoretical threshold model is modified.The relative error between the modified theoretical threshold model and the experimental threshold model is less than 5.3%.At the same time,the influence of the spot radius on the spectral signal and electron density is analyzed.The maximum values of spectral intensity and plasma density are obtained at the spot radius of 11?m.Therefore,it is necessary to find the best spot size in LIBS experiment.This provides a reliable theoretical basis and experimental reference for further study of the influence of spot size on LIBS measurement.(2)Quantitative analysis of Pb and Ni elements in soil with plasma characteristicsIn LIBS measurement,the plasma characteristic parameters fluctuate greatly with the change of experimental conditions,which will have a great impact on the quantitative measurement.Therefore,the influence of the experimental conditions on the plasma characteristic parameters should be considered in determining the optimal experimental conditions.In this paper,the plasma characteristic parameters and the signal-to-noise ratio of the spectral signal are used to determine the best lens to sample distance(LTSD)and delay time.Under the optimal LTSD and delay time,the calibration curves of the two elements are established by internal standard method,and the correlation coefficient of the calibration curves of the two elements is more than 0.993.The maximum relative standard deviation(RSD)was 4.47%and 4.76%respectively.The maximum relative errors are 12%and 4.8%respectively.The experimental results show that LIBS measurement,combined with the analysis of plasma characteristic parameters,is of great significance in the quantitative analysis of heavy metal elements in soil.(3)Quantitative analysis of eliminating element interference in soil base based on background subtractionWhen LIBS technology was used to analyze the Ni element in soil,it was found that the characteristic peak of Ni element in soil with wavelength of 373.68 nm would be affected by the characteristic peak of Al element at 373.39 nm.In this paper,a background subtraction method is proposed to eliminate the interference of Al element on Ni element.The best experimental conditions were determined.The internal standard method and the external standard method were used for the quantitative analysis of Ni in the two soil samples.It was found that the calibration curve of Ni element in the internal standard method based on pure aluminum and the base background subtraction method had a good correlation.The correlation coefficient R~2 was 0.997,and the maximum standard deviation(RSD)of the sample was 4.34%.In the internal standard method,the maximum relative errors of the two samples are 4%and 6.1%respectively,and in the external standard method,the maximum relative errors of the two samples are 7.5%and 10%respectively.The experimental results show that in the quantitative analysis of heavy metal elements in soil by LIBS technology,under the condition of limited element characteristic spectral lines,the detection accuracy is improved,and the method of background subtraction can effectively eliminate the spectral line interference between elements.