Comparative Study On The Detection Of Nutrient Elements In Soil By Collinear Double Pulse And Single Pulse Laser Induced Breakdown Spectroscopy

Soil is a loose material on the surface of the earth.It can supply and coordinate the essential air,water,nutrients,heat and other living conditions for crop growth and development.It is an important part of the terrestrial ecosystem.Controlling soil fertility is very important for agricultural cultivation.Soil testing and formula fertilization can save costs,increase crop yields and reduce environmental pollution,so that the rapid and accurate acquisition of nutrients in soil becomes crucial.Laser induced breakdown spectroscopy(LIBS)technology has the advantages of field,rapid and real-time detection,and is used in many fields of material detection.It only needs to sample pretreatment simple or without treatment,but the single pulse LIBS has poor stability,poor reproducibility,easily affected by the environment and the low detection sensitivity.The double pulse can improve the above problems and the signal pulse form of collinear is suitable for online application enhancement.This study explored the feasibility and superiority of qualitative and quantitative analysis of eight nutrient metal elements(potassium,calcium,sodium,magnesium,iron,manganese,zinc and copper)in soil under the condition of collinear double pulse LIBS compared with single pulse.Firstly,experimental parameters were optimized(single pulse optimization parameters including delay time,pulse energy;parameter optimization including double pulse delay time,pulse interval and pulse energy),the relative standard as soon as possible to obtain greater signal-to-noise ratio,greater signal intensity and the smaller RSD.Because of many kinds of elements,comprehensive evaluation index is introduced to assist analysis.Under the optimal experimental parameters,the signal intensity,signal to noise ratio,relative standard deviation,and the change of appearance and internal parameters of the ablative pit were compared.The traditional calibration method and multiple linear regression method were used to quantitatively analyze eight nutrient elements in soil,and the correlation coefficient,detection limit and prediction accuracy were compared.It is found that the collinear double pulse LIBS can improve the fitting linear relationship,reduce the detection limit and improve the prediction accuracy,providing a theoretical basis for the subsequent exploration of different pulse forms and the development of portable instruments.The results of the study are as follows:(1)The experimental parameters of eight kinds of elements(potassium,calcium,sodium,magnesium,iron,manganese,zinc and copper)were optimized under the two pulse forms of single and collinear double pulse.The optimization parameters in the form of single pulse include delay time,pulse energy and collinear double pulse form.The optimization parameters include delay time,pulse energy distribution and pulse interval.Signal to noise ratio(SNR),signal intensity(Intensity)and relative standard deviation(RSD)are used as the main indicators.The signal-to-noise ratios of Mn,Fe,K and Mg with the variation of pulse energy show that the trends of the wavelength variation of different characteristic lines of the same element are roughly the same.Therefore,for each subsequent element,one wavelength(KI766.49 nm,CaI445.48 nm,NaI819.47 nm,MgI517.26 nm,FeI428.8 nm,MnI403.31 nm,ZnI468.01 nm,CuI324.76 nm)was selected for subsequent optimization analysis.In order to make a clearer judgment of the optimal parameters,a comprehensive index(comprehensive SNR,comprehensive SBR and comprehensive RSD)is introduced as an auxiliary reference.The final optimization results for single pulse conditions:the optimal delay time is 6us,the optimal pulse energy is 100 mJ;collinear double pulse conditions:the optimal delay time is 6us,the first laser pulse energy is 25mJ,the second is 75mJ and the optimal pulse interval is 1200ns.(2)In addition to the two wavelengths of K element,the signal intensities under double pulse conditions are greatly enhanced,and the enhancement effect is about 2-3 times.At the same time,the signal-to-noise ratio SNR is correspondingly enhanced,and the enhancement factor is about 2-3 times.RSD is correspondingly lower,indicating that collinear double pulse can enhance the detection ability of the instrument and improve the stability of signal acquisition while improving the signal intensity of the element;Finally,the ablation pit was scanned by shape measurement laser microscope.The 3D morphology of the ablation pit,the longitudinal section of ablation and the depth and volume of the ablation pit were compared.The results show that there are obvious differences in the appearance of the ablation pit between the two,and the removal amount,depth and surface area of the target are relatively larger under the condition of collinear double pulse,which lays a good foundation and pave the way for the study of the enhancement mechanism of the subsequent collinear double pulse.(3)Quantitative analysis of eight elements in soils with collinear double pulse and single pulse LIBS was performed.First,a single variable linear regression method based on spectral line intensity is used for traditional calibration analysis.Each element selects a characteristic wavelength(Ca I445.48nm,Na I819.47nm,Mg I518.36nm,Cu I324.755nm?Mn I403.45nm,Zn I468.01nm,Fe 1404.58nm,KI766.49nm).The result of the calibration curve is as follows:Under single pulse conditions,the linear correlation coefficient R and the root mean square error of prediction(RMSEP)of each element were Ca:0.8680&6.4788mg/g,Na:0.9061&1.7502 mg/g,Mg:0.6701&1.9948 mg/:0.6633&0.0048 mg/g Mn:0.4123&0.0710 mg/g K:0.5916&1.6648 mg/g Zn:0.1356 Fe:0.1356(Zn and Fe do not calculate RMSEP,because there is no linear correlation between concentration and strength under single pulse condition.)Under the condition of double pulse,the linear correlation coefficient R and RMSEP were respectively Ca:0.8994&3.4105mg/g,Na:0.9271&1.1051mg/g,Mg:0.7433&1.4371mg/g,Cu:0.7201&0.0052mg/g,Mn:0.6173&0.0815 mg/g,K:0.6633&12.5465 mg/g,Zn:0.6551&0.0118 mg/g,Fe:0.6566&1.0825 mg/g,It can be seen that under the condition of double pulse LIBS,the calibration fitting effect is better,the correlation coefficient is higher,the root mean square error of prediction is lower,the prediction accuracy is improved,the detection sensitivity is increased and the element detection limit is reduced;Then a PLSR multiple linear regression model was established.Each element selected a wavelength range(about 200 wavelengths)covering the wavelength information to establish a quantitative analysis model.The results show that PLSR model fitting and predicting effect are higher than univariate calibration model under the same pulse conditions,and the two-pulse LIBS of most elements is better than the one-pulse LIBS fitting,the correlation coefficient R is higher,RMSEP is lower.