Chiroptical fingerprints to characterize lavender and lavandin essential oils
Introduction
The first polarimetric chromatogram obtained in liquid chromatography was reported in 1976 by Hesse and Hagel, who used a bench polarimeter to characterize the enantiomers of 2-phenyl-cyclohexanone separated on microcrystalline cellulose triacetate [1]. Since then, chiroptical detectors, i.e. polarimetric and circular dichroism (CD) detectors [2], [3], [4], were developed [5] and are nowadays routinely used in HPLC on chiral support to identify and differentiate chiral molecules [6]. Moreover, chiroptical detectors can be applied to the determination of enantiomeric ratios on chiral stationary phases (CSPs) with baseline [7] or partial separation [8], [9], but also on achiral support [10], [11], [12], [13], to preparative purposes [14], [15], and to the assignment of absolute configurations [16], [17], [18]. They have sometimes been used to characterize complex mixtures focusing on chiral components [19], because the analysis is simplified by hiding achiral molecules, which are not sensed by chiroptical detection. For instance, sugars in human urine were quantified by polarimetric detection without chiral separation [20], enantiomeric ratios of lorazepam in human plasma were determined by CD detection [21], and the absolute configuration of camphor was assigned in different Artemisia essential oils [22].
The chemical composition of an essential oil (EO) depends on the species, the origin, the cultivar, the harvest season, the extraction method and the nature of the vegetal material. The quality and the authenticity of EOs are assessed by the quantification of several compounds, according to the monographs produced by International Organization for Standardization (ISO) or European Pharmacopeia [23]. These chemical components are often chiral, so the use of chiral markers [24] and the determination of enantiomeric excesses are crucial for the characterization of EO [25], [26]. The aim of this study was to use polarimetric chromatograms to characterize French lavender EOs. Indeed, lavender (Lavandula angustifolia) and lavandin (sterile hybrid of Lavandula angustifolia P. Mill.×Lavandula latifolia (L.f.) Medikus) are widely grown for their essential oils (EOs) in Mediterranean Basin [27]. The 39 Lavandula species and their numerous hybrids are of great interest in fragrance and flavor industries [28]: the lavender EOs containing larger amount of linalool and linalyl acetate are raw materials in perfumes and cosmetics, those with higher percentage of camphor are used for their anti-inflammatory and antibacterial properties, whereas lavandin EOs are widespread in household cleaners. These useful properties of lavender EOs are related to the properties of each enantiomer of each chiral component, so that they may be biased by the stereochemical composition of the EO. However, the chiral compounds are not necessarily enantiopure in natural products [29], but they may occur as scalemic or racemic mixtures, thus indicating their formation by several biochemical pathways.
Lavender EOs are sometimes characterized by non-enantioselective techniques, such as gas chromatography [30], vibrational spectroscopies [31], [32] or both [33]. Their chiral components can be studied, on one hand, by enantioselective GC, that is available since the early 90s [34], thanks to the enantioselectivity of cyclodextrin derivatives. Indeed, the separation by GC for the enantiomers of the main markers of lavender EOs has already been published [35], [36], [37] and recently optimized by flow modulated comprehensive two-dimensional GC to be used for quality control [38]. On the other hand, chiroptical vibrational spectroscopy has been used by our group to analyze the enantiomeric composition of the markers in Artemisia herba-alba EOs [39]. In the case of lavender EOs, preliminary tests proved that the intensity of the signal for vibrational and electronic circular dichroism spectra was too weak to be used.
To introduce an original way to characterize EOs, we focused on HPLC on chiral support with polarimetric detection. The principle of a polarimetric detector is to measure the angle of rotation of a linearly polarized light caused by the enantio-enriched molecule(s) in the cell, using one or several wavelength depending on the manufacturer. So, for a racemic mixture, two enantiomers separated on a chiral stationary phase (CSP), give signals of same area but opposite signs, positive and negative. The intensity of the signal is related to the value of the rotatory power at the wavelength(s) used by the polarimetric detector, to the amount and to the enantiomeric excess (ee) of the chiral sample. Liquid chromatography with polarimetric detection has already allowed the determination of enantiomeric excesses for a single EOs component, firstly isolated from the other molecules: for menthyl acetate in peppermint oils [40] and carvone in caraway or spearmint oils [41]. The injection of a crude EO on a HPLC column able to separate even partially several chiral molecules, leads to a polarimetric chromatogram with multiple positive and negative peaks, which can be considered as a chiroptical fingerprint of the mixture. Chemometrics analysis by Partial Least Squares-Discriminant Analysis regression (PLS-DA) of the polarimetric chromatograms was used to discriminate between the species of essential oils studied (lavender and lavandin) and their varieties (Fine, Maillette, Matherone, Abrial, Grosso, Sumian, Super). Lavender/lavandin EOs are good candidates because they contain mainly chiral compounds (80–90% of the total composition); (−)-(R)-linalyl acetate, (−)-(R)-linalool, (+)-(R)-camphor, (+)-(R)-borneol, (+)-(S)-terpinen-4-ol and (−)-(R)-lavandulol have been detected with high enantiomeric purities in lavender/lavandin EOs [36], [42].
Section snippets
Essential oil samples
EOs obtained by steam distillation were provided by local French producers: 158 samples were analyzed including 94 lavandin oil samples and 64 lavender oil samples over 3 years of harvests (2012, 2013 and 2014), various varieties and various French collect areas (``Alpes-de-Haute-Provence'' (04), ``Ardèche'' (07), ``Drôme'' (26) and ``Vaucluse'' (84)). The lavender/lavandin EOs studied were [Lavandula angustifolia Miller] (lavender) and its hybrid [Lavandula angustifolia Miller × Lavandula
Results and discussion
After a classical determination of the EOs chemical composition by gas chromatography, the choice of chromatographic conditions compatible with the use of polarimetric detector is detailed. The chiral markers thus highlighted on a polarimetric chromatogram were used to discriminate between the varieties of lavender / lavandin EOs thanks to chemometrics.
Conclusion
Polarimetric detection combined with chemometric analysis enabled to differentiate lavender and lavandin essential oils and their varieties (Abrial, Grosso, Sumian, Super, Fine, Maillette and Matherone), which were very well predicted with a percentage of correct classification of 100% for lavender/lavandin and between 93 and 100% for their varieties except for FI variety (86%), considering each lavender plant as a variety. According to chemometric analysis, the varieties are mainly
Declaration of Competing Interest
None.
Acknowledgment
The authors are grateful to the ``Société Coopérative Parfums Provence Ventoux'', ``Société Coopérative des Plantes à Parfums de Provence'' and ``France Lavande'', for providing samples of lavender/lavandin essential oils. Authors are thankful to AMU and CNRS for their financial support.
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