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.
Davies, N. W., Larkman, T., Marriott, P., & Khan, I. A. (2016). Determination of enantiomeric distribution of terpenes for quality assessment of Australian tea tree oil. Journal of Agricultural and Food Chemistry, 64(23), 4817-4819. https://doi.org/10.1021/acs.jafc.6b01803