Selection of columns for GCxGC analysis of essential oils

Multidimensional separations are performed by combining single analytical separation columns in such a way to greatly enhance peak capacity for the separation of complex multi-component samples. Comprehensive multidimensional gas chromatography (GC¡ÁGC) employs largely independent first- and second-dimension separation mechanisms and typically generates peak capacity of the order of several thousand, making it a highly appropriate technology for the separation and analysis of complex multicomponent samples such as essential oils. A substantial majority of GC¡ÁGC publications for essential oil analysis in the periodical literature have used a "non-polar" column in the first dimension and employed a "polar" column in the second dimension.1 In practice this almost always translates to use of a 100% polydimethylsiloxane or 5% diphenyl 95% dimethyl polysiloxane stationary phase in the first-dimension combined with a 50% diphenyl 50% dimethyl polysiloxane or a polyethylene glycol (wax) second-dimension column, although there are notable departures from this convention, including applications reversing the order of "polarity",2 providing class-type separation of citrus oil components and those using cyclodextrin derivative stationary phases for enantioselective analysis.3¨C6 Even in the case where a conventional non-polar/polar column ensemble is used, the mechanism of retention for solutes in the second dimension depends on volatility and polarity, but using an appropriate temperature programme cancels out the influence of volatility on retention in the second dimension column.7 Thus, GC¡ÁGC separations are temperature-programmed to maximize differences in separation mechanisms in the two dimensions. However, because both columns are commonly installed in the same oven, the temperature stability of one of the separation columns typically imparts an upper temperature limit on the separation system. Applications that use 50% diphenyl 50% dimethyl polysiloxane second dimension columns are benefited in terms of high-temperature stability compared with those applications employing wax columns, but more suitable selectivity of wax columns for polar essential oil components was shown many years ago and wax stationary phases are almost universally preferred.8 Recently, we received a 5 m ¡Á 100 ¦Ìm i.d. MEGA-WAX HT high-temperature wax column with a stationary phase film thickness of 0.10 ¦Ìm (Mega, Legnano, Italy) and have been using this column for our GC¡ÁGC work involving essential oils analysis. The wax second dimension column used in the current investigation has more than 9000 N/m tested in isothermal mode and an upper column oven limit of 300 ¡ãC. Results from the GC¡ÁGC analysis of kunzea essential oil obtained by steam distillation of aerial parts of Kunzea ambigua are reported here.