Study on the THz spectra of four kinds of Nipagin esters

Abstract

The transmission spectra of methylparaben, ethylparaben, propylparaben, and butylparaben in the range of 0.2–2.5 THz are measured using terahertz time-domain spectroscopy (THz-TDS). Their refractive index and absorption spectra are obtained at room temperature in nitrogen atmosphere. The results show that the four kinds of Nipagin esters have obvious absorption peaks at 1.54 and 1.83 THz for methylparaben, 1.98 THz for ethylparaben, 1.37 and 1.61 THz for propylparaben, and 0.76 and 1.68 THz for butylparaben. Their anomalous dispersion of the refractive index appears at the corresponding position. Based on the density functional theory (DFT), the vibration absorption spectra of their single molecule are simulated. The numerical simulation and experimental results generally agree with each other. Then the reasons for their minor differences are analyzed. Overall, the spectral properties of Nipagin esters are found to have great significance for the detection of preservatives and for food safety. OCIS codes: 300.1030, 300.6495. doi: 10.3788/COL201109.S10503. Food preservation is an enduring topic. With continuous scientific and technological progress, the preservatives in synthetic food have been found to threaten human health. With the improvement of living and consumption levels, people have thus proposed higher requirements on food safety and health. Therefore, food safety testing is very important. Food components can be analyzed by Terahertz Time-Domain Spectroscopy (THz-TDS). It has practical significance for the study of molecular structure and preservative functions. The molecular formula of methylparaben is C8H8O3. It is mainly used in organic synthesis, food, cosmetics, and preservatives. The molecular formula of ethylparaben is C9H10O3. It is mainly utilized in the production of soy sauce and beer and is also used in other aspects of corrosion. The molecular formula of propylparaben is C10H12O3. It is used in food, cosmetics, medicines, and biocides. The molecular formula of butylparaben is C11H14O3. It is used in medicines, food, and cosmetics preservatives. To date, the methods used for the analysis of Nipagin esters are high performance liquid chromatography (HPLC) [1] , capillary column gas chromatography [2] , gas chromatography [3] , infrared spectroscopy [4] , and liquid chromatography [5] , etc. The mechanism involved in Nipagin esters in food has been determined. The Fuzhou Product Quality Monitoring Bureau has examined the application of accelerating solvent extraction (ASE) and chromatograph of gel permeation (GPC) techniques for parabens assay [6] . However, there have been no studies to date which employed the terahertz (THz) spectroscopy of Nipagin esters. THz radiation is a kind of electromagnetic wave located between infrared ray and microwave with a frequency in the range of 0.1–10 THz and a wavelength in the range of 30 µm–3 mm [7] . It belongs to the far-infrared band and the transition region in electronics to photonics. It has important academic value and potential applications. Experimental results are obtained using THz spectrum technology in the domain of preservative food testing. Using fast Fourier transformation (FFT) of time-domain spectroscopy, frequency domain spectra such as amplitude and phase are obtained. A series of optical parameters can then be derived [8] such as refractive index, extinction coefficient, absorption coefficient, and so on. This is followed by the analysis of the THz optical characteristics of the materials. In a previous work, the components of vitamins have been analyzed using THz-TDS measurement [9] . In this letter, the THz transmission spectra of methylparaben, ethylparaben, propylparaben, and butylparaben in the range of 0.2– 2.5 THz were measured by THz-TDS. Their refractive index and absorption spectra were measured. Numerical simulations based on the Density Functional Theory (DFT) were also conducted using Gaussian 03 software package. By comparing the calculated results and the experimental measurements, their differences were analyzed. THz-TDS is expected to be used as a method for non-destructive spectral inspection. The experimental samples are commercially available, with their purity more than 99%. Four samples of methylparaben, ethylparaben, propylparaben, and butylparaben were pressed under the pressure of 4 tons after grinding by a tablet press machine. Their thicknesses are 0.754, 0.800, 0.724, and 0.806 mm, respectively, and their diameter is 13 mm. The surfaces of the samples are smooth and parallel. Using THz-TDS, the THz