# Accurate Determination of Geographical Origin of Tea Based on Terahertz Spectroscopy
## 1 Introduction  
In China, and in many other regions of the world, tea is an important economic crop ranking first among other types of well-known beverages, including cocoa and coffee. Tea is also a major part of China's culture and history. It is planted in large and geographically-diverse production areas across the country; there are numerous types of tea with distinct regional and qualitative features to be found in different regions. Their market approval and consumer preference also vary. Thus, tea is considered a typical product with protected geographical indication. Most countries govern products with protected geographical indication under specialized policies and laws. For example, Regulations for Protecting Products of Original Origins are implemented by the General Administration of Quality Supervision, Inspection, and Quarantine of the People's Republic of China to protect the products of original tea sources [1]. Regulations on agricultural products with protected designation of origin and protected geographical indication are also implemented by the European Commission [2]. Imported foods in the U.S., Japan, and other developed countries are strictly tracked. Although the regulations on teas with protected geographical indication are generally upheld, false labeling of products and disguising fake products as high-quality do occur. Consequently, fair market trade, brand protection, and consumer rights and interests can be severely infringed upon. Effective tracing techniques are necessary to track the origin and types of products conveniently, rapidly, and accurately to protect locally-featured products. Sensory evaluation and chemical methods are traditional techniques applied to identify tea. The former may be rendered subjective or unreliable due to human factors and external environments [3; 4]. The latter, such as those using trace minerals, aromatic substances, and thermo-chemical properties, can be employed to accurately identify the type and source of tea; nevertheless, unfortunately, they are complex and expensive and, thus, unable to rapidly identify the origin of various teas [5; 6; 7; 8]. Unlike other detection techniques, terahertz time-domain spectroscopy (THz-TDS) is non-destructive, non-ionizing, highly flexible, and efficient. THz-TDS also provides a high signal-to-noise ratio for an absorption spectrum of samples. THz radiation usually refers to an electromagnetic wave with a frequency between 0.1 and 10 THz or wavelength of 30 \(\upmu\)m to 3 mm, and a wave band between microwave and infrared within the scope of far-infrared radiation. THz-TDS is commonly employed in a variety of fields, such as bio-molecule identification [9], material development research [10], explosive tests [11], drug analysis [12], and research on the reorientation of dipoles in liquids [13]. Four kinds of tea were selected from different geographical origins where cumulative temperature, illumination, weather conditions, soil, altitude, and other natural factors differ. The organic matter inside the respective teas is unique, to this effect, which allows for identifying teas of by origin via THz spectroscopy. THz-TDS is applicable to quantitative, as well as qualitative, analysis. [PERSON] et al. qualitatively researched the THz spectra of acephate based on both first principles calculations and experimental data [14], for example. [PERSON] et al. qualitatively and quantitatively analyzed the THz spectra of pesticides, food powders, and a mixture to obtain the weight ratios of imidaloprid in a mixture using the partial least squares (PLS) method [15]. [PERSON] et al. qualitatively and quantitatively analyzed a mixture of thiabendazole and polyethylene using various algorithms (biPLS, iPLS, PLS, mwPLS) [16]. These reports demonstrated qualitative and quantitative analyses using linear regression methods. This paper proposes a new approach for detecting and identifying green tea samples with different origins, but similar spectral characteristics.