Raman microspectroscopy as a tool for analysis of essential oil constituents in glandular trichomes of Thymus species

Thymus is one of the largest genera in the Lamiaceae family, and all its species are used as medicinal and aromatic plants. In this research, Raman spectroscopy is used as a fast and nondestructive tool for chemotype assessment by measuring spectra directly from glandular trichomes. Samples of two Thymus pulegioides subsp. pannonicus populations were collected from Banat (Serbia), analyzed using a 785 nm laser, and compared with reference spectra of citral and thymol standards. Spectra of samples from the Vršačke planine locality showed the presence of dominant bands related to citral (around 1671 and 1633 cm-1). Spectra of samples from the Devojački bunar locality displayed dominant bands at around 1640 and 1668 cm-1, corresponding to ν(C=C) and ν(C=O) vibrations related to conjugated and oxygenated monoterpene stretching, respectively. The dominant bands of thymol were at 740, 1062, and 1262 cm-1, confirming that samples from Devojački bunar are not of the thymol chemotype. GC-FID/MS analysis revealed that germacrene D is a major compound in the essential oil from this population. Additionally, PCA revealed a clear separation of the two investigated populations, demonstrating that Raman spectroscopy can be used for chemotype differentiation.