[Spectral analysis of nitrofurantoin in the terahertz frequency range].

The present article measured the absorption coefficient spectra and refractive index spectra of nitrofurantoin original drug, which is one kind of nitrofuran drugs, in the terahertz frequency range from 0.2 to 1.8 THz using terahertz time-domain spectroscopy. The results showed that there exist a number of characteristic absorption peaks of nitrofurantoin with different intensity in the range and the absorption coefficient spectra can be used to identify nitrofurantoin. The article also simulated absorption coefficient spectra of nitrofurantoin molecule within 0.2 - 1.8 THz using density functional theory by Gaussian software, and vibrational modes of some peaks in the experimental absorption coefficient spectra were analyzed and identified. The results show that the experimental absorption peaks at 1.25 and 1.60 THz correspond with the theoretical peaks at 1.30 and 1.67 THz, and these experimental peaks were caused by intramolecular vibrational modes of nitrofurantoin.

[Terahertz time-domain spectroscopy of alpha endosulfan persistent organic pollutant].

Frequency-dependent absorption coefficient spectrum and refractive index spectrum of alpha endosulfan, a kind of persistent organic pollutants, are presented in the terahertz frequency region by terahertz (THz) time-domain spectroscopy (TDS). The spectral features in the THz region have a number of unique characteristic absorption peaks. The result demonstrates that THz-TDS is a promising method to identify materials. Then we adopted density functional calculation method to analyze theoretic absorption coefficients of single alpha endosulfan molecule within 0.2-2 THz. The results show that absorption peaks at 1.68, 1.91 THz in theoretical calculation correspond to 1.7, 1.88 THz in the experiment. Finally, vibrational modes and approximate assignments were discussed, showing that these matched peaks are caused by intramolecular vibrational modes of alpha endosulfan. Others might be related to intermolecular vibrational modes or combined vibrational modes.

[Terahertz time-domain spectroscopy of ractopamine hydrochloride].

The terahertz spectra of Ractopamine hydrochloride in the range of 0.2 to 2.2 THz was obtained by THz time-domain spectroscopy, and the absorption and refraction spectra of Ractopamine hydrochloride was got meanwhile. The structure and vibrational frequencies of Ractopamine molecule in the THz range were simulated by density functional theory. The difference between experimental and theoretical results was analyzed. And assisted by Gaussian View 3.09, the origin of the vibrational frequencies was recognized. The results show that besides the intramolecular vibrations, THz absorption of Ractopamine hydrochloride originated from the intermolecular hydrogen bond network and Van der Waals force between molecules. This study demonstrated the feasibility of time-domain terahertz spectroscopy for the identification of Ractopamine hydrochloride and provided a new way for the detection of Ractopamine hydrochloride.

[Terahertz time-domain spectroscopy of Clenbuterol hydrochloride].

The terahertz spectra of Clenbuterol hydrochloride in the range of 0.2 to 2.6 THz were obtained by THz time-domain spectroscopy, the absorption and refraction spectra of Clenbuterol hydrochloride was got meanwhile. The structure and vibrational frequencies of Clenbuterol molecule, Clenbuterol hydrochloride molecule and Clenbuterol hydrochloride crystal in the THz range were simulated. Based on the difference between experimental and theoretical results, the origin of the vibrational frequencies was analyzed. This study demonstrated the feasibility of time-domain terahertz spectroscopy for the identification of Clenbuterol hydrochloride and provides a new way for the detection of Clenbuterol hydrochloride.

[Terahertz spectroscopic investigation of elaidic acid].

The optical properties of elaidic acid in the THz band were investigated by using THz time-domain spectroscopy (THZ-TDS). The absorption and refractive index spectra in the frequency range from 0.5 to 2.5 THz were obtained at room temperature under nitrogen atmosphere, and the results show that elaidic acid had some characteristic absorption peaks, and the average refractive index of the sample was 1.43. The structure and vibrational frequencies of elaidic acid molecule in the THz range were simulated by the B3LYP algorithm of density functional theory, and the THz characteristic absorption peaks of elaidic acid molecule were identified by using Gaussian View software. The results show that the experimental and theoretical results agree in general, and the observed absorption peaks in the THz range were mainly caused by the intramolecular and intermolecular vibrations jointly. At the same time, the fingerprint spectra of elaidic acid in the THz band verify that the time-domain terahertz spectroscopy can be used to detect oleic acid, and this study provides a new experimental method for the detection of trans fatty acids in food.

[Terahertz time-domain spectroscopy of L-ascorbic acid].

The present paper reports the theoretical and experimental vibrational frequencies of L-ascorbic acid in the frequency range from 0.2 to 2.4 THz respectively. The vibrational frequencies of L-ascorbic acid in the THz range were simulated by Becke-3-Lee-Yang-Parr (B3LYP) algorithm of density functional theory. Simultaneously, the absorption and refractive index spectra of L-ascorbic acid were obtained using terahertz time-domain spectroscopy (THz-TDS). Based on the results from density functional calculations, the observed vibrational peaks were assigned with the aid of Gaussian View 3.07. Finally, the tera-hertz absorption spectra of tablets of vitamin C were compared with those of L-ascorbic acid. It was found that L-ascorbic acid had some distinct fingerprint spectra in the THz region and the theoretical results were in good agreement with the experimental results. The observed vibrational peaks were induced jointly by the intramolecular and intermolecular vibrations. The absorption peaks of L-ascorbic acid could be observed in the absorption spectra of the tablets of vitamin C. Our research is helpful to understanding the role of L-ascorbic acid in biological reactions. Still, the research results show the potential of the application of THz-TDS in the quantification of L-ascorbic acid.

Inversion of thicknesses of multi-layered structures from eddy current testing measurements.

Luquire et al.'s impedance change model of a rectangular cross section probe coil above a structure with an arbitrary number of parallel layers was used to study the principle of measuring thicknesses of multi-layered structures in terms of eddy current testing voltage measurements. An experimental system for multi-layered thickness measurement was developed and several fitting models to formulate the relationships between detected impedance/voltage measurements and thickness are put forward using least square method. The determination of multi-layered thicknesses was investigated after inversing the voltage outputs of the detecting system. The best fitting and inversion models are presented.