| It’s an important development strategy for Chinese agriculture to realize agricultural modernization.However,the development of modern agriculture depends on precision management and scientific guidance.The accurate perception of soil elements can provide reliable data for scientific management and control,and it’s very important to guide the production of precision agriculture.Although conventional methods for soil element analysis have high accuracy,they can’t meet the requirements of rapid and real-time determination in digital agriculture because they are time-consuming and complex.Therefore,it’s an urgent need to propose a rapid,efficient and accurate method for soil element analysis.Laser-induced breakdown spectroscopy(LIBS)has been considered as a“superstar”in the analytical chemistry field in the future,with the advantages of rapid and in situ analysis,no or minimal sample preparation,and remote sensing.LIBS has been successfully applied as a multi-element analysis technique for many applications including space exploration,food safety,industrial detection,and medical diagnosis,etc.At present,LIBS technology has also been greatly concerned and recognized in the direction of soil elements analysis.However,LIBS is facing a new challenge on sensitivity and accuracy because of the complex soil matrix,great differences in the morphological properties of different elements,trace content of toxic elements,and interference of soil moisture and dust.It’s the key to the application of LIBS in the rapid detection of soil elements to explore rapid and high-precision analysis methods and eliminate the influencing factors.Three kinds of LIBS analysis methods were proposed based on the different adsorption strategies combined with LIBS,which could rapidly and accurately detect available potassium,different valence chromium,and trace cadmium in soil.Meanwhile,aiming at the soil moisture interference in the process of soil elements detection by LIBS on-site,the influence law of soil moisture was studied systematically and different stoichiometric algorithms were used to eliminate the moisture interference to improve quantitative accuracy.Finally,the soil sample holding cell of the LIBS portable instrument was redesigned to eliminate the influencing factors and improve the accuracy of sample measurement.The key research contents are as follows.(1)Study on LIBS analysis method for soil available potassium.To solve the problem that it is difficult to distinguish soil potassium forms by LIBS technology currently,a method of cation exchange membrane adsorption combined with LIBS was proposed.Soil available potassium was extracted by using the extraction characteristics of resin membrane,and then the content of soil available potassium was obtained by LIBS ablation of resin membrane.After using resin membrane to extract soil available potassium for 10 min,the determination coefficient of calibration curve was 0.99,and the limit of detection and limit of quantitative of available potassium was 2.2 mg/kg and 7.3 mg/kg,respectively.Real soil samples were analyzed,and the average relative error between the predicted and reference values was 2.58%.This method solved the problem that LIBS technology cannot measure soil available potassium.This method also didn’t require chemical extractants and was markedly faster and less complex than common methods.(2)Study on LIBS analysis method of different valence chromium in soil.The toxicity of hexavalent chromium(Cr,VI)in soil is more than 100 times that of trivalent chromium(Cr,III),so it is necessary to distinguish and detect different valence chromium.Because the LIBS technology cannot directly distinguish and detect the valence state of chromium currently,a method of resin selective adsorption-assisted LIBS technique was proposed.Cr(III)and Cr(VI)in soil were separated by specific resin adsorption,and then the total Cr solution before adsorption and the resin after adsorbed Cr(VI)was detected using LIBS to obtain the content of total Cr and Cr(VI).Then the content of Cr(III)was equal to total Cr content minus Cr(VI)content.The results showed that this method could separate Cr(VI)within 5 min.The determination coefficient of the calibration curve of total Cr,Cr(III)and Cr(VI)were all greater than0.99 with this method.The detection of average relative errors between predicted and real values about the three kinds of chromium content were less than 13.11%.This method not only solved the difficulty that LIBS technology cannot realize the differential detection of different valence chromium in soil but also avoided the chemical extraction of the common detection methods and shortened the analysis time.(3)Study on LIBS analysis method of trace cadmium in soil.To solve the problem of poor detection sensitivity of LIBS technology,which cannot meet the needs of highly sensitive detection of trace Cd in soil,a new LIBS enhancement method based on resin adsorption combined with spatial constraint device was proposed.The Cd in soil was adsorbed using cation exchange resin.Then the resin was detected by spatial constraint device-assisted LIBS to further enhance the spectral signal,and the highly sensitive detection of trace Cd in soil was realized.Compared with the traditional pellet method,the detection concentration was reduced by 10 times using resin adsorption method,and the spectral signal was enhanced by 2-5 times using spatial constraint device.With this method,the determination coefficient of the calibration curve reached 0.993,the limit of detection was 0.13 mg/kg,reaching the requirements of soil pollution risk management and control standards in China(0.3mg/kg).This method was used to detect cadmium in real soil samples from different regions,and the average relative error between predicted and reference values was8.44%.(4)Study on the influence of soil moisture for LIBS and the elimination method.To realize the rapid detection of soil elements in the field by LIBS,it is impossible to dry the soil moisture.However,the actual soil moisture changes greatly,which will affect the measurement results.Therefore,the effect of soil moisture on the detection of soil elements using LIBS was studied systematically and the interference of soil moisture was eliminated.Firstly,the effects of soil moisture were analyzed taking the elements of Cu,Pb,Cr,Al,Ca,Fe,Na,Si,Mg,K,N,O and H in soil as example.The results showed that with the increase of soil moisture,the signal intensity and signal-to-background ratio of elements decreased excepting for H element,however the relative standard deviation of all elements increased.Then taking Cu,Pb,Cr and K elements in soil as examples for quantitative analysis,combined with different chemometrics methods,the spectral data were preprocessed to effectively decrease the interference of moisture.The results showed that the maximum normalization algorithm could effectively eliminate the interference of soil moisture.The R_V~2 of each element calibration set increased from 0.861-0.888 to 0.973-0.988,the R_P~2 of the cross-validation set increased from 0.875-0.883 to 0.920-0.953,and the RMSEC and RMSEP decreased by 54%-67%and 22%-43%,respectively.(5)Study on the error analysis and the improvement of sample holding cell of rapid measurement of soil elements by LIBS portable instrument.There are some problems in the rapid measurement of soil elements by LIBS portable instrument,such as dust spatter and difficult to measure the same sample many times,resulting in poor quantitative accuracy of soil elements.To solve the above problems,the sample holding cell was redesigned.Taking the Cu,Cr,Pb and Cd elements in soil as examples,the quantitative analysis was carried out using the improved sample holding cell.The determination coefficient of calibration curve of the four elements was increased from 0.521-0.675 to 0.993-0.998.The relative standard deviation of cross-validation of Cu,Cr,Pb and Cd elements was less than 15.5%,17.3%,13.3%and 19.7%,respectively.The relative error was less than 12.5%,13.4%,13.3%and15.3%,respectively.Compared with the original sample holding cell,the improved sample holding cell not only reduced dust spatter but also realized multiple measurements of samples to reduce sample loading times and improved the accuracy of sample measurement.In conclusion,focusing on the technical problems in the application of LIBS in soil element analysis,three rapid and accurate analysis methods were proposed for the detection of soil available potassium,different valence chromium,and trace cadmium.To realize the on-site rapid measurement of soil elements by LIBS,the influence law and physical mechanism of soil moisture on the analysis of soil element by LIBS were analyzed,and the different chemometrics algorithms were used to effectively reduce the interference of soil moisture.Furthermore,the soil sample holding cell of LIBS portable instrument was redesigned to reduce dust pollution,which realized multiple measurements of the same sample and improved the accuracy of sample measurement.The above research established a foundation for applying LIBS technology to the on-site rapid detection of soil elements. |
