Qualitative And Quantitative Detection Of Pesticides By Terahertz Time-domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is a newly developed spectroscopic method, and is gaining more interests from various research fields due to its uniqueness. Its application in food safety control, especially in pesticide residual detection, however, is still at the preliminary stage, and needs further exploration. This paper deals with the qualitative and quantitative detection of pesticides in both solution and solid forms, including the concentration prediction of pesticide solutions based on their terahertz-range absorbance spectra, the spectroscopic analyses of pure pesticide, food and polyethylene powders, and mixtures of pesticide and polyethylene and of pesiticide and food powders, and the quantitative analyses of pesiticide powder mixtures.Its main content and innovations are listed as follows:1) With terahertz time-domain spectroscopy (THz-TDS), the 0.5-1.5 THz absorbance spectra of pesticide solutions with the concentration in the level of severalμg/mls are analyzed, as well as their relationship to the solution concentrations. Both linear and nonlinear regression models are built between the absorbance and the concentration. Influences of signal-to-noise ratio, modeling methods, the number of features selected and the modeling frequency range on the model performance are analyzed experimentally. And a new method based on the THz-TDS technique and the combination of partial least squares (PLS) and least-squares support vector machine (LS-SVM) modeling methods is proposed for the detecton of pesticide solutions. According to the model building and prediction processes, sources influencing the measurement precision are analyzed theoretically as well.2) Using THz-TDS, the absorption coefficient and refractive index spectra of four pure pesticide powders, three pure food powders and polyethylene are qualitatively analysed, with the pesiticide powders discriminated between each other and from those food powders. Moreover, the absorption coefficient spectra of one-component and two-component pesticide mixtures with polyethylene or sticky rice powder as the mixing medium are analyzed, with further analyses of the influences of the pesticide and medium components on the the mixture spectra, and of the relationship between the absorption coefficient spectra and the weight percentage of the pesticide components.3) According to the relationship between the absorption coefficient spectra and the weight percentage of the above mentioned one- and two-component pesticide mixtures, a novel method based on THz-TDS is proposed for the quantitative analysis of solid-form pesticide mixtures. PLS models are built between the absorption coefficient spectra and the concentration. Comparison is made between two commonly used frequency selection methods:the correlation coefficient method and the spectral signal-to-noise ratio method, according to their influences on the model performance. Differences between the detection results of the mixtures with different pesticide and medium components are compared and analyzed. Results show that it is quite promising to use THz-TDS for further pesticide residual detection in food.