Fig. 1. Structural diagram of terahertz time domain spectrometer

Fig. 2. Partial absorption coefficients of benzoic acid in arrowroot

Fig. 3. Absorption coefficients of benzoic acid in arrowroot with different mass fractions

Fig. 4. Scatter plot of benzoic acid content predicted by PLS

Fig. 5. RMSE versus principal component factor

Fig. 6. Spectra with or without plastic packaging when mass fraction of benzoic acid is 20%

Fig. 7. Scatter plot of benzoic acid content with plastic packaging predicted by PLS

Fig. 8. RMSE of sample with plastic packaging versus principal component factor