Characterisation of sandalwood essential oils using high performance thin-layer chromatography and high resolution gas chromatography mass spectrometry

Abstract

Sandalwood essential oil is extensively used in perfumes, cosmetics, and pharmaceuticals. The unique woody aroma of sandalwood essential oil is produced by (Z)-α-santalol and (Z)-β-santalol and these two compounds influence the oil quality. Variation in sesquiterpene composition and oil yield have been observed both within and across Santalum species. Gas Chromatography-Mass Spectrometry (GC-MS) is a technique commonly used to identify and quantify sesquiterpene compounds in sandalwood essential oils. High Performance Thin-Layer Chromatography (HPTLC) has yet to be fully explored as an alternative to GC-MS in authenticating sandalwood essential oils of different species. In this study, sandalwood essential oils were characterised using HPTLC and high resolution GC-MS.

The potential of HPTLC to identify differences in sandalwood essential oils of Santalum album, Santalum spicatum, Santalum austrocaledonicum, Santalum paniculatum, Santalum lanceolatum and a natural substitute for sandalwood, Osyris lanceolata, was explored. Variation was observed in the profile of bands and peak intensity profiles of the essential oils with some bands being unique to the individual species. The potential of HPTLC fingerprinting as a quality control tool in identifying differences in sandalwood essential oils was demonstrated (Study I).

Study I was limited to a maximum of five oils for each species studied. The natural variability was likely not captured for such a small sample size. The potential of HPTLC to generate a sandalwood profile that better represents Santalum album, Santalum spicatum, Santalum austrocaledonicum and Santalum paniculatum was explored. Santalum spicatum could confidently be distinguished from other species with a distinctive pink band at RF 0.71 and unique peak intensities at RF 0.28, 0.45 and 0.47. Santalum album and Santalum paniculatum could easily be distinguished from each other with distinctive peak intensities at RF 0.51 and 0.17 respectively. The intense peak at RF 0.09 displayed by Santalum austrocaledonicum distinguished it from Santalum album, and Santalum paniculatum but not Santalum spicatum (Study II).

An ultrasonication extraction method was optimised for extraction of essential oil from Western Australia sandalwood (Santalum spicatum) using n-hexane, isopropanol, and n-hexane/isopropanol (50:50, v/v) for different extraction times. Oil yield was moderately influenced by solvent, particle size and extraction time. Extracting for 30 minutes with particle size 250µm-500µm using n-hexane gave the highest oil yield and santalol content. The santalol content achieved in Santalum spicatum extract was influenced by particle size and solvent (Study III).

The optimised method in Study III was moderately modified and successively applied to extract oil from 340 milled heartwoods of Santalum spicatum. GC-MS analysis was performed on oil extracts. Variation in sesquiterpene composition of 340 heartwood oil extracts of Santalum spicatum sampled from 19 locations across five regions in Western Australia was explored. Variation was observed in sesquiterpene composition, and oil yield both across and within all five regions. Heartwood oil extracts from trees in the north obtained higher oil yield and santalol content when compared to oil extracts from trees in the south. Interestingly, one oil extract of S. spicatum met the ISO specification of 90% combined α-santalol and β-santalol in Santalum album, obtaining 67.47% α-santalol and 22.92% β-santalol (Study IV)