Combined analysis of the essential oil of Chenopodium ambrosioides by GC, GC-MS and 13C-NMR spectroscopy: quantitative determination of ascaridole, a heat-sensitive compound.

A commercial sample of the essential oil of Chenopodium ambrosioides L. from Madagascar was analysed by GC, GC-MS and 13C-NMR. By GC analysis, the major constituents were found to be ascaridole (1) (41.8%), isoascaridole (2) (18.1%), p-cymene (16.2%), alpha-terpinene (9.7%) and limonene (3.8%). However, ascaridole undergoes a partial thermal isomerisation to 2 and hence the amount of 1 is under-estimated by GC analysis. The actual contents of 1 and 2 (55.3 and 4.6%, respectively) were obtained following combined analysis of the sample by GC and 13C-NMR. Several hydroxy- and polyhydroxy-menthanes were identified by 13C-NMR.