Terahertz/far infrared spectroscopy as an alternative method for the characterization of major components in Laggera decurrens essential oil

Abstract

The Fourier Transform Infrared technique was used to determine the major components of Laggera decurrens essential oil. The experiment revealed that there are signatures of thymol, carvacrol and thymoquinone in Laggera decurrens essential oil. This was suggested by the existence of shared absorption bands between these pure compounds and Laggera decurrens essential oil. The PBE0 DFT functional was employed to calculate the theoretical spectra of thymol, carvacrol and thymoquinone. A good correspondence was obtained between the experimental and theoretical spectra of these materials. Peak assignments showed that the Laggera decurrens absorption bands at 5.68 THz, 9.47 THz, 14.20 THz, 15.71 THz and 18.10 THz had corresponding absorption bands in either of the three materials with the same vibration modes. These further confirms the existence of thymol, carvacrol and thymoquinone in Laggera decurrens essential oil. This work presents a novel method for the characterization of major components in essential oils which has otherwise been commonly performed using Gas Chromatography-Mass Spectrometry/ Gas Chromatography-Flame Ionization Detector. This presents the use of terahertz or far infrared spectroscopy for the quality control of essential oils. This study further highlights the effectiveness of the Fourier Transform Infrared method in determining constituents of complex samples.