Analysis of Cymenes in Essential Oils: the Case of Lepechinia meyeni (Walp.) Epling.

The similarity of the mass spectra of the o-, m- and p-cymenes together with the small differences of their retention indices make it difficult to unambiguously identify the three cymene isomers. Moreover, there are discrepancies in the various data banks used as a reference in the analysis of essential oils. The authors have taken advantage of the presence of these three compounds in the oil of Lepechinia meyeni to identify the right order of elution of these compounds on both non-polar and polar columns. The order of elution on both DB-5 and S-wax 10 capillary columns is: m-, followed closely by p- and finally at some distance, o-cymene. A fourth compound having a similar mass spectrum was observed. From the literature, it was tentatively identified as 3,7,7-trimethyl-1,3,5-cycloheptatriene.