## 5 Spectroscopy Analysis
### Component Analysis  
Based on the obtained characteristics in the THz region, qualitative and quantitative analysis methods could be used to analyze the components for practical samples. The convenience of the ATR configuration, such as non-invasion to the sample and without complex pre-treatment, makes it possible and suitable for the rapid detection and discrimination or even online monitoring.  
Figure 5: Explanation of dielectric response in the THz region for hydrogen bonds and hydration in aqueous solution. **(a)** The real part (up) and imaginary part (down) of the complex dielectric permittivity of the pure water. Reproduced with permission from [54], Copyright © 2008, Elsevier. **(b)** Model of water molecules and solutes in aqueous, the water molecules are segmented into two stations, bulk water and solvation shell (hydrated water). Reproduced with permission from [56], Copyright © 2017, American Chemical Society. **(c)** Dielectric loss spectra of solute, hydrated water, and bulk water. Reproduced with permission from [57], Copyright © 2008, Elsevier.  
[PERSON] et al. [63] analyzed the correlation between the absorption spectra of raw milk and the milk components by the partial least square regression and full cross validation as a test for the calibration models, the results showing that THz-ATR spectroscopy is suitable to predict several milk contents such as milk fat, total solid, and somatic cell counts. [PERSON] et al. [37] analyzed blood plasma specimens from healthy rats and diabetic rats. The results showed that spectra of blood plasma specimens obtained from diabetic rats had small but significant differences from those of healthy rats. Further, they studied the ATR spectra of palm skin in a consecutive measurement after glucose intake [64]. The results show a correlation between the changes in the blood glucose level and the variations of the ATR spectra, demonstrating the possibility of a non-invasive real-time measurement of blood glucose concentration. [PERSON] et al. [65] obtained a concentration dependence of ATR spectra with NaCl solutions and theophylline solutions. The concentration vibration only changed the amplitude of the ATR spectra other than the refractive index. [PERSON] et al. [66] applied THz-ATR spectroscopy to detect changes in the physical properties of lactose during the lubrication process required for drug formulation. They found that both the concentration ratio of components and blending time determined the final magnitude of spectra at the lactose-specific region. [PERSON] et al. [67] obtained the absorption spectra of three kinds of honey, Medlar, Vitex, and Acacia. The apparent distinction among them could be attributed to the different types and contents of chemical compositions, such as glucose, fructose, and water. The quality of honey could be identified by the partial least squares-discriminant analysis (PLS-DA) model in the range of 0.5-1.5 THz, of which the accuracy of the validation set reached was 88.46%. [PERSON] et al. [68] presented a THz-ATR setup and investigated the mixtures of water and ground calcium carbonate (GCC). Both refractive index and absorption coefficient showed correlation with the GGC concentration in the range between 30 and 40 wt %, which proved the feasibility for the distinction of various concentrations of GCC in inline measurements. [PERSON] et al. [69] determined the complex refractive indices of tetracycline hydrochloride (TCH) in pure water and in pure milk at the 0.3-2.0 THz region. The presence of TCH could lower the complex refractive index at an equivalent frequency in the whole band, which could be related to the change in the relaxation dynamics of hydration water. The quantitative determination of TCH in pure water and in pure milk was further realized by analyzing the concentration dependence of complex refractive indices at 0.5 THz.