Fig. 1. Instrument light path schematic. (a) THz-TDS; (b) FTIR
System performance | Test system |
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THz-TDS | FTIR |
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Radiation source | High radiation source stability | Radiation source covers a wide frequency range and can detect weak signals | Working frequency/ THz | 0.1-4 | Switch the beam splitter,can detect 0.1-100 | Data collection | Direct access to amplitude, phase, power spectrum | Must perform inverse Fourier transform on the interferogram and the information extraction is relatively complicated | Testing sample | Can extract multi-layer sample internal information | Unable to extract internal information for layered samples[6] | Instrument resolution/ GHz | 3.8/7.6 | 13.5 |
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Table 1. Comparison of pesticide detection methods based on terahertz spectroscopy
Researchdirection | Chemometric method |
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Method | Introduction |
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Qualitativeresearch | PCA | Reduce dimensional optical parameters, simplify samples, and maximize sample characterization | CA | Classification using the principle of similarity | SVM | Establish an optimal classification hyperplane to maximize the distance between the samples on both sides and the plane | Quantitativeresearch | PLS | One of the most commonly used quantitative regression methods for situations where there is a correlation between variables | OLS | Find the best matching function by summing the squared sum of the actual value and the minimum value of the fitted value | ANN | It is extremely efficient and has a certain fault tolerance. It is a superior unsupervised learning method | SVR | Find the optimal hyperplane to minimize the total deviation of all sample points from the hyperplane |
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Table 2. Chemometric method
Pesticide | Absorption peak /THz | Accuracy /% |
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Carbendazim | 1.29、1.52[19] | 97.67 | Imidacloprid | 0.89、1.13[19] | 98.20 | Chlorpyrifos | 1.47、1.93、2.73[28] | | Carbofuran | 2.72、3.06[28] | | Dimethoate | 1.05、1.09、2.92[28] | | Methomyl | 1.01、1.65、1.91、2.72、3.20[28] | | Thiabendron | 0.99、1.57、2.17、2.66[28] | 94.41 | 2,4-dichlorophenoxyacetic acid | 1.36、1.60、2.38[30] | | Indole acetic acid | 2.5[30] | 97.60 | Chlorpyrifos | 1.77、2.44[30] | 97.60 | Thiacloprid | 0.65[31] | | Herbicidal ether | 0.48、0.98、1.33[31] | | Nitroguanidine | 1.08、1.48[31] | | Chlorothalonil | 0.75、1.02、1.3、1.72[32] | | Epoxystrobin | 1.49、1.93[32] | | Carbofuran | 1.88[32] | | Thiophanate | 1.28、1.65、2.23[32] | | Procymidone | 0.75、1.7、2.23[32] | | Dicofol | 0.35、0.65、0.96、1.30、1.57、1.84、2.18[33] | | Trichlorfon | 1.18、1.56、2.05[33] | | Imine sulfur | 0.90、1.23、1.68、1.84、1.98、2.11[33] | | Hexachlorobenzene | 0.64、0.87、1.06、1.23、1.54、1.74、1.93[33] | | Deltamethrin | 0.91、1.14、1.34、1.53、1.78、1.92[34] | 92.20 | Lacy-cyhalothrin | 0.55、1.11、1.59、1.72、1.85、1.97、2.15[34] | 97.27 | Beta-cypermethrin | 1.01、1.27、1.58、1.72、1.85、2.02、2.16[34] | 96.90 | Diflubenzuron | 0.65、1.55、1.81、1.98、2.12[34] | 95.23 | 6-benzylaminopurine | 2.07、2.98[35] | 96.00 | Sodium p-chlorophenoxyacetate | 1.37、1.96、2.13[35] | 95.77 | Sodium o-nitrophenolate | 0.42、0.51、0.96、1.18[35] | 95.77 | Methyl thiophanate | 1.05、1.23、1.65[36] | | Benomyl | 0.78、1.14、1.35、1.55[36] | | Maleic hydrazide | 2.34[37] | 100 |
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Table 3. Fingerprint database of 32 pesticides