Laser photoacoustic detection of the essential oil vapors of thyme, mint, and anise

Abstract

Abstract Photoacoustic studies of the vapours of the essential oils of thyme, mint and anise have been made using a line-tunablewaveguide CO2 laser in conjunction with a heat-pipe type of photoacoustic vapour sample cell operated over thetemperature range 2O18OO C. Identifying spectral fmgerprint features are found in the 9-10 m spectral region for eachof the three essential oils investigated. The principal features of the photoacoustic spectrum of each essential oil areassociated with the dominant chemicals present i.e. thymol in thyme oil, menthol in mint and anethole in anise.Keywords: laser photoacoustics, essential oils, mint oil, thyme oil, anise oil. 1. INTRODUCTION Essential oils constitute the most important oderous substances found in plants. Their volatility makes them steamdistillable from those parts of plants where they are produced. They find wide commercial applications in the productionof perfumes, cosmetics, food and beverage flavourings, aromatherapy, pharmaceuticals, insecticides and pesticides and soare of substantial economic value.The essential oils contain many chemical substances, with as many as 100 or more components. Frequently, thechemical composition of an essential oil is dominated by a single major component, which gives it its characteristic odour.Since plants, from which essential oils are derived, grow under different conditions of climate, rainfall and hours ofsunshine, the percentage composition of a given chemical compound can vary greatly, even from the same plant species.Analysis of the chemical composition usually makes use of chromatographic methods, combined with mass spectroscopyand infrared spectroscopy.With the wider application of laser photoacoustic spectroscopy, the possibility arises of the sensitive monitoring ofessential oil vapour emissions from living plants. This could provide a means whereby the potential essential oil yield couldbe assessed prior to harvesting. The basic requirement is a sufficiently large vapour pressure and an infrared absorptionspectrum distinctive of the particular essential oil, or major component of it, which falls within the range of wavelengthscovered by the laser used. In this paper, a preliminary study is made of the essential oil vapours of thyme, mint and anise,using a line-tunable CO2 laser in conjunction with a temperature-variable heat-pipe type of photoacoustic cell.2. EXPERIMENTAL SYSTEMA line-tunable waveguide CO2 laser is used to excite a heat-pipe type of photoacoustic cell, the temperature of whichcan be varied over the range 5 to 1800 C. Nitrogen was used as the carrier gas of the essential oil vapour.The experimental set-up uses the CO2 laser to provide highly directional and line-tunable radiation (9.2 -