Preliminary evaluation of the utility of compound specific isotope analysis of γ-terpinene and terpinen-4-ol in Australian tea tree oil for authentication and provenance

A growing international demand for tea tree oil (TTO) has increased the need for methods to verify authenticity and origin. While several approaches exist to detect adulteration in commercial TTO, methods to determine geographic origin have not yet been established. In this study, gas chromatography isotope ratio mass spectrometry (GC-irMS) was used to conduct the first compound-specific isotope analysis of terpinen-4-ol and γ-terpinene in authentic and oxidised Australian TTO from New South Wales (n = 13). Across authentic samples, the mean δ¹³C values for γ-terpinene and terpinen-4-ol were−30.4 ± 0.50 ‰ and −31.3 ± 0.37 ‰ respectvely. Oxidation increased δ¹³C values for γ-terpinene but did not affect terpinen-4-ol. Analysis of locally and internationally sourced commercial samples showed that these isotope values did not distinguish adulterated oils. Although some sample groupings suggested a possible relationship with growing region, broader international sampling is needed to evaluate provenance more reliably.