Abstract
This work provides the experimental and theoretical fundamentals for detecting the molecular fingerprints of six kinds of pesticides by using terahertz (THz) time-domain spectroscopy (THz-TDS). The spectra of absorption coefficient and refractive index of the pesticides, chlorpyrifos, fipronil, carbofuran, dimethoate, methomyl, and thidiazuron are obtained in frequencies of 0.1–3.5 THz. To accurately describe the THz spectral characteristics of pesticides, the wavelet threshold de-noising (WTD) method with db 5 wavelet fucntion, 5-layer decomposition, and soft-threshold de-noising was used to eliminate the spectral noise. The spectral baseline correction (SBC) method based on asymmetric least squares smoothing was used to remove the baseline drift. Spectral results show that chlorpyrifo had three characteristic absorption peaks at 1.47, 1.93, and 2.73 THz. Fipronil showed three peaks at 0.76, 1.23, and 2.31 THz. Carbofuran showed two peaks at 2.72 and 3.06 THz. Dimethoate showed three peaks at 1.05, 1.89, and 2.92 THz. Methomyl showed five peaks at 1.01, 1.65, 1.91, 2.72, and 3.20 THz. Thidiazuron showed four peaks at 0.99, 1.57, 2.17, and 2.66 THz. The density functional theory (DFT) of B3LYP/6-31G+(d,p) was applied to simulate the molecular dynamics for peak analyzing of the pesticides based on isolated molecules. The theoretical spectra are in good agreement with the experimental spectra processed by WTD + SBC, which implies the validity of WTD + SBC spectral processing methods and the accuracy of DFT spectral peak analysis. These results support that the combination of THz-TDS and DFT is an effective tool for pesticide fingerprint analysis and the molecular dynamics simulations.
Highlights
As a novel analytical technique, terahertz (THz) spectroscopy has multivariate properties such as fingerprint absorption, penetration, coherence, transient and low ionization damage [1,2]
The THz spectra of polyethylene (PE) and pesticide samples are measured by THz time-domain spectroscopy (THz-TDS) as the reference and signal spectra, respectively
This work presents an analytical strategy for studying fingerprint absorption characteristics of six pesticides using THz-TDS
Summary
As a novel analytical technique, terahertz (THz) spectroscopy has multivariate properties such as fingerprint absorption, penetration, coherence, transient and low ionization damage [1,2]. It has Molecules 2018, 23, 1607; doi:10.3390/molecules23071607 www.mdpi.com/journal/molecules. It can be used to characterize the specific spectral information of the materials [7,8]. It provides useful information for the analysis of materials requiring accurate identification and detection [9]. The inevitable systematic and random errors of THz spectrometer have a negative influence on the optical properties characterization of materials [11]
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