基于衰减全反射式太赫兹时域光谱技术的食用油光谱特性研究

食用油是人类营养和能量的重要来源，为人体提供必需的脂肪酸，研究食用油在太赫兹波段光学特性，对食用油成分分析及品质评价具有重要价值。衰减全反射式太赫兹时域光谱技术是一种新型的太赫兹时域光谱技术，通过样品与倏逝波的相互作用，获取样品的太赫兹光谱。与透射式或反射式太赫兹时域光谱技术相比，该技术能有效地避免测量食用油等液体样品时样品池对光学参数的影响，并能获得样品的精确光学参数。分别利用透射式太赫兹时域光谱技术和衰减全反射式太赫兹时域光谱技术测量了大豆油的吸收光谱。结果表明，与透射式太赫兹时域光谱技术相比，衰减全反射式太赫兹时域光谱技术能更有效地提取大豆油的吸收系数、吸收峰分布等光学特性。进一步利用衰减全反射式太赫兹时域光谱技术研究了大豆油、核桃油、葡萄籽油在太赫兹波段的光学特性，获得了三种食用油在1~1.8 THz范围内的折射率谱和吸收光谱。利用密度泛函理论计算了食用油中四种主要成分(软脂酸、硬脂酸、油酸和亚油酸)在太赫兹波段的振动、转动模式，理论计算结果同实验测量结果吻合较好。研究表明，在太赫兹波段食用油的吸收峰与所含脂肪酸分子种类与含量有关，其主要来源为脂肪酸分子的低频振动和转动。研究成果对食用油成分定性定量分析及品质检测等具有指导意义。

Investigation on Characteristics of Edible Oil Spectra with Terahertz Time-Domain Attenuated Total Reflection Spectroscopy

Edible oil is a significant source of human nutrition and energy, providing essential fatty acids for human body. Thus, the investigation on the optical properties of edible oils in THz region for component analysis and quality evaluation is considered to be of great importance. As a newly developed-technique, Terahertz time-domain attenuated total reflection spectroscopy can obtain the THz spectra through the interaction between sample and evanescent wave. Compared with Terahertz time-domain transmission or reflection spectroscopy, this technique can effectively avoid the influence of sample cell when measuring liquid samples such as edible oils, thus acquiring the optical parameters more accurately. The absorption spectra of soybean oil were measured via both Terahertz time-domain transmission spectroscopy and Terahertz time-domain attenuated total reflection spectroscopy. Compared with Terahertz time-domain transmission spectroscopy, the Terahertz time-domain attenuated total reflection spectroscopy showed greater advantages in acquiring the absorption coefficients and distribution of absorption peaks. Furthermore, the optical properties of soybean oil, walnut oil and grape seed oil were investigated by using Terahertz time-domain attenuated total reflection spectroscopy. In addition, the refractive index and absorption spectra of the three edible oils in the range from 1 to 1.8 THz were obtained. Using density functional theory, we also calculated the vibration and rotation modes of four kinds of principal components (hexadecanoic acid, octadecanoic acid, octadecenoic acid, and octadecadienoic acid) in THz region which were consistent well with the experimental results. In conclusion, the absorption peaks of edible oils in terahertz region are related to the components and contents of fatty acidsas well as result from their vibration and rotation modes. Moreover, this research may contribute to qualitative and quantitative components analysis as well as quality inspection of edible oils.