Authentication of the naturalness of wintergreen (Gaultheria genus) essential oils by gas chromatography, isotope ratio mass spectrometry and radiocarbon assessment

The essential oil of wintergreen, which is extracted by steam distillation from Gaultheria genus leaves, is mainly used in aromatherapy. However, due to its adulteration easiness with synthetic material, it is necessary to control samples naturalness with accuracy. The purpose of this work was to develop a methodology to authenticate the essential oil of wintergreen. Wintergreen essential oil is composed of more than 99% methyl salicylate. This aromatic ester can be easily synthesized and used to adulterate wintergreen essential oil. Authentic wintergreen essential oil can be distinguished from adulterated oils by examining their compositions. The detection of methyl salicylate synthetic marker compounds (methyl 4-hydroxybenzoate, dimethyl 4-hydroxyisophthalate or dimethyl 2-hydroxyisophthalate) or the absence of several naturally occurring minor secondary metabolites (ethyl salicylate and vitispirane) contribute to the authentication. Isotopic values of bulk wintergreen essential oil have also been determined using an isotope ratio mass spectrometer (IRMS): the delta C-13 values of authentic samples range from -36.78 to -33.36 parts per thousand, the delta H-2 values range from -173 to -115 parts per thousand and the delta O-18 values range from -1.3 to 5.7 parts per thousand. However, these analytical methods cannot account for the natural variability in the essential oils. To determine the boundaries of the natural isotopic values, C-14 radioactive isotope activity assessment was undertaken, which allows for the determination of the genuineness of samples that are not assessed using multistable isotope approaches. Additional studies evaluating C-14 activity of a noncompliant sample using IRMS identified a C-14-labeled synthetic methyl salicylate adulteration.