Investigating the application of Raman spectroscopy on the discrimination of Origanum taxa and the quantification of their essential oils constituents

Abstract

The potential of Raman spectroscopy for the identification and quantification of essential oils constituents obtained from different Origanum taxa was investigated, particularly focusing on the reproducible analysis of volatile liquids in very small quantities. Compared to headspace Gas Chromatography Mass Spectrometry (HS-GC/MS), which is routinely applied for the analysis of essential oils [1], Raman spectroscopy offers the advantage of time efficiency, no sample preparation and non-destructiveness. It is also environmentally friendly and cost effective, as it does not require the use of chemical reagents [2]. A series of Raman measurements were conducted both on reference compounds present in essential oils and on samples of essential oils distilled from samples of Origanum taxa, either collected from nature or purchased from the market. To address issues such as the volatility and the volume requirement of the analytes, after testing different experimental arrangements, an in-house assembly based on the use of a capillary tube inserted in a 3D-printed adjustable base was selected as the most appropriate. The quantification method used for the essential oilsʼ constituents was based on Multiple Linear Regression using the spectra of the reference compounds as input. The results were very encouraging, being in good agreement with the respective ones of GC/MS, with a deviation of less than 10%. Additionally, by collecting Raman spectra directly from the sessile glands of aromatic plantsʼ leaves, it was possible to detect the characteristic substances and consequently facilitate the chemotaxonomy of Origanum taxa.