Principal Component Spectrum Analysis of Licorice Based on Terahertz Radiation

In this study, a transmission terahertz time-domain spectrum system was used to test the terahertz spectra of glycyrrhizic acid, glycyrrhetic acid, and glycyrrhizin as the main components of glycyrrhiza (licorice). The characteristics of these licorice constituents and their terahertz absorption peaks were found to be close to each other and their absorption spectra were similar. A quantum chemistry method was used to simulate the terahertz absorption spectrum of glycyrrhizic acid; this spectrum was then compared with the experimental spectrum to perform a qualitative analysis of the three elements. In this study, based on the density functional theory (DFT) and PM3 models, a single molecular configuration of glycyrrhizic acid was introduced for structural optimization and frequency calculation. The results showed that the terahertz simulated absorption peaks obtained by the two methods coincided with the experimental absorption peaks, and the terahertz simulated absorption spectrum waveforms obtained based on the DFT model were closer to those of the experimental spectrum. Finally, the characteristic absorption peak of glycyrrhizic acid at 1.655THz and the terahertz absorption coefficients of six nearby numerical points were selected, and the average value was used to perform a one-dimensional linear regression fitting with the concentration. The fitting results verified the licorice theoretically, and the acid terahertz absorption spectrum conformed to Lambert's law.