Research On The Influence Of Soil PH On The Accuracy Of LIBS Analysis

Soil is an important material basis for people’s economic development.With the acceleration of urbanization,industrialization in various regions has also caused many problems,such as excessive heavy metals in soil.Heavy metals in the soil will be absorbed by plants,and heavy metals will gradually accumulate in the food chain system,which will eventually seriously affect human health.Soil p H will affect the activity of metal ions,such as toxic ions cadmium,mercury and lead.The activation of these ions will cause direct damage to crop roots and affect vegetation growth.Therefore,how to eliminate the influence of soil p H on the quantitative analysis accuracy of laser induced breakdown spectroscopy(LIBS),find a method that can improve the accuracy of LIBS spectral analysis,and improve the stability and accuracy of quantitative analysis of heavy metals in soil is very important.In recent years,LIBS has developed rapidly at home and abroad,but the stability of LIBS technology is low,and the accuracy of quantitative analysis still needs to be improved.Aiming at the problem of poor stability of LIBS technology,this paper compares and analyzes the electron temperature,electron density and spectral intensity of plasma by analyzing the influence of p H on soil matrix effect,and studies the effect of soil p H on the analysis accuracy of LIBS.At the same time,in view of the problem of poor accuracy of Least Squares Support Vector Machine in quantitative analysis,particle swarm optimization is selected to optimize the model parameters and improve the stability of quantitative analysis of LIBS technology.The specific research work and results are as follows:1.The influence factors of soil p H on spectral analysis were studied.Aiming at the problems of low accuracy in soil quantitative analysis and great influence of soil p H on LIBS analysis accuracy in matrix effect.The ratio of atomic line spectral intensity to ionic line spectral intensity of Cu will increase with the increase of soil alkalinity.The variation trend of plasma temperature and plasma density of Cu element with p H value is analyzed.When the soil changes from neutral to acidic or alkaline,the plasma temperature and plasma density of Cu element will decrease rapidly.When the soil is strongly acidic or alkaline,the variation of plasma temperature will tend to be gentle,the plasma temperature will not increase or decrease greatly with the change of p H value,but the variation trend of plasma density is more obvious,It will still change greatly with the change of p H value.The internal standard method is used to quantitatively analyze the spectral data under different p H values.It is found that the accuracy of LIBS quantitative analysis of soil is the highest when p H=7.The accuracy of LIBS quantitative analysis will be reduced when the soil is acidic or alkaline,which will affect the analysis results.2.The effect of soil p H on the accuracy of spectral analysis was studied.Firstly,the partial least squares method is used to quantitatively analyze a variety of different p H value combination spectral data.Although the analysis accuracy is improved compared with the internal standard method,it still can not eliminate the impact of soil p H.Using the least squares support vector machine model to process the data,the accuracy has been improved compared with PLS.LSSVM is still room for optimization because of the problem of parameter selection in the algorithm itself.Penalty coefficient for LSSVM modelAnd kernel function parameter2)selection is blind.Particle swarm optimization is selected to optimize the model parameters,which avoids the problems of some traditional optimization algorithms in selecting parameters and obtains the best resultsCombined with2),Cu element is(8437.2,154.9284)and Cr element is(4908.6,393.5635).The calibration model of LSSVM is constructed with the best parameter combination.Compared with LSSVM,PSO-LSSVM calibration model has higher accuracy.The R~2 of Cu and Cr elements is increased to0.9836 and 0.9850,and the fitting effect is significantly improved.The root mean square error of the training set of Cu and Cr elements is 0.0254 wt.%And 0.0272wt.%Down to 0.0212wt.%And 0.0191wt.%,The root mean square error of prediction set is0.1006wt.%And 0.0788wt.%Down to 0.0437wt.%And 0.0420wt.%,The stability of the model is further improved.It shows that PSO-LSSVM algorithm can better reduce the impact of sample p H value change,improve the accuracy and stability of analysis results,and find a method to improve the accuracy of spectral analysis by soil p H.The accuracy of quantitative analysis of soil acidity and alkalinity is improved by using svm-pso algorithm,which improves the stability of quantitative analysis of soil acidity and alkalinity.The advantages of laser-induced breakdown spectroscopy in soil quantitative analysis are proved,which provides an effective driving force for the development of LIBS technology.