GC-MS method validation and levels of methyl eugenol in a diverse range of tea tree (Melaleuca alternifolia) oils.

Tea tree oil distilled from Melaleuca alternifolia has widespread use in the cosmetic industry as an antimicrobial as well as for other functions in topical products. Concerns were first raised by the European Commission's Scientific Committee on Consumer Products in 2004 about the level of the potentially carcinogenic phenylpropanoid compound methyl eugenol in tea tree oil. Limits on oil content in different types of cosmetic products were set based on a reported upper level of 0.9% methyl eugenol in the oil. A previous publication indicated that these levels were based on oil from a Melaleuca species not used in the commercial production of oil. Even the highest recorded levels in Melaleuca alternifolia, the overwhelmingly most common species used, were ∼15 times less than this, meaning that more oil could be safely used in the products. The current study, including details on methodology and reproducibility, extends that work across a suite of 57 plantation-sourced oils from a range of geographical locations and production years, as well as many Australian and international commercial oils. Lower levels of methyl eugenol in oils of known provenance were confirmed, with a recorded range of 160-552 ppm and a mean of 337 ppm. Analysis of variance showed methyl eugenol levels in Australian plantation oils to be correlated to the geographical region but not to the year of production. Average methyl eugenol levels in commercial oils were significantly lower, and these samples were divided into an authentic group and a group that were suspected of being adulterated based on an independent test. Authentic commercial oils had similar levels of methyl eugenol to Australian provenance material, whilst the oils classed as suspect had significantly lower levels.

Determination of Enantiomeric Distribution of Terpenes for Quality Assessment of Australian Tea Tree Oil.

A number of papers have appeared in recent years proposing the use of enantiomeric ratios of key monoterpenes in Australian tea tree oil (TTO) for detection of adulterated oils. There are however a range of reported values, even from exactly the same suite of authentic oils, and we address here probable reasons for these differences and stress the importance of establishing reference ratios within each laboratory based on oils of known provenance. Any biological variation in the ratio for the key terpene terpinen-4-ol has been demonstrated to be effectively unmeasurable, because the standard deviation on multiple measurements of the same oil is of the same order as that of multiple authentic oils.