Rapid Detection Of Pesticide Residue In Crops Base On Combined Terahertz Spectroscopy With Chemometrics

With the widespread use of pesticides,the problem of pesticide residues in crops is increasingly prominent.This problem causes consumers unprecedented attention.To ensure consumer safety,it is an important issue that how to effectively detect pesticide residues in crops.Existing methods of pesticide residue detection are limited in the detection operation,detection time,detection object and detection precision.Because of easy operation,fast and non-destructive detection,spectral analysis gets more and more attention in recent years.Compared with ultraviolet and infrared spectrum,terahertz(THz)spectrum has penetrability,security and fingerprint.It makes the THz spectrum have a very a broad application on qualitative and quantitative analysis.In this study,four kinds of pesticides(imidacloprid,carbendazim,thiram and dichlobenil)and three kinds of crops(rice,tea and maize)were selected as detection of objects.Terahertz time-domain spectroscopy(THz-TDS)was used as a main line.Terahertz spectrum simulation,qualitative and quantitative analysis of pesticides solid,the detection methods of pesticide solution and the resolution method of mixture spectrum were studied.It hopes that it provides a new way to detect pesticide residues in crops.The main study contents and result are as follows:1.The study of the spectrum simulation.Aiming at the problem of a single molecule structure ignore molecular inter-atomic forces.The four molecular cluster of imidacloprid was built to consider intermolecular forces.And then the molecular cluster was calculated by utilizing PM7 and B3 LYP,respectively.At last,the normal vibration mode of the four molecular cluster of imidacloprid was decomposed by potential energy distribution(PED).The experimental results show that,compared with single molecule,the simulation calculation results of molecular cluster are more close to the actual measurement results.2.The study of pesticide solid qualitative identification.Datasets Data1(including four kinds of pesticide and three kinds of food powders)and Data2(including six kinds of carbendazim and polyethylene mixture with different concentrations)were built.Combining multiple population genetic algorithm(MPGA)and support vector machine(SVM),a supervised classification model MPGA-SVM was proposed for identifying two datasets.In order to avoid hand-classified,an unsupervised classification model PCA-CFSFDP,combining principal component analysis(PCA)and clustering method clustering by fast search and find of density peaks(CFSFDP),was put forward for identifying two datasets.The experimental results show that good recognition results can obtain in two datasets by using MPGA-SVM and PCA-CFSFDP.3.The study of pesticide solid quantitative analysis.Five datasets were established by mixing four pesticides and three food powders with different proportion.From the THz absorption spectrum in five datasets,the relation between sample spectrum and concentration is nonlinear.Thus,the nonlinear model support vector regression(SVR)was selected to predict content of the pesticide in the mixture.The experimental results demonstrate that the performance of SVR is better than partial least squares regression method(PLSR).4.The study of the detection method of pesticide solution.Selecting carbendazim as detection of object,a new detection method,combing THz-TDS with a metamaterial sensor,was proposed to detect a pesticide in solution.The metamaterial sensor composed of metal ohm ring array,was simulated.The simulation results show that ohm ring array is sensitive to the change of the relative dielectric constant on its surface.In the range of 0.4THz to 1.4THz,when the relative permittivity on the surface of Ohm ring arrays decreases,the position of resonant peaks will shift to high frequency.And then,ohm ring array was used for detecting carbendazim in anhydrous ethanol and tea,respectively.The detection results show that as the concentration of carbendazim decreases,the position of resonant peaks will shift to high frequency.The 0.5mg/L carbendazim in tea was detected.It is below to the maximum limit residue of carbendazim in tea by law.The experimental results demonstrate that the proposed method can detect pesticides in solution and effectively improve detection sensitivity.5.The study of resolution mixture spectrum.Because of nonnegative characteristics of terahertz absorption spectrum,nonnegative matrix decomposition(Nonnegative matrix decomposition,NMF)is introduced into to the mixture of terahertz absorption.Before NMF is performed,there are two problems: one is how to estimate the number of components in a mixture.Another is how to initialize the spectra matrix and the concentration matrix.To solve these two problems,the characteristic value of mixture spectra was used to estimate the number of components,and pure variable was used to initialize spectra matrix and the concentration matrix.The experimental results show that NMF base on pure variables initialization can resolve the THz spectra of mixture into the spectra of components.Moreover,the initialization method base on pure variables needs fewer iterations than the initialization method base on random non-negative values.