Note

Use of an inexpensive low-temperature liquid crystal stationary phase for the study of volatile oil constituents with short gas chromatographic retention times

All four liquid crystal stationary phases used for the gas chromatography of volatile oil constituents function below their melting points, but the azoxydiphenetole (ADP)-packed column does not exhibit reliable supercooling. Safrole always preceding anethole provides an identifying test for these liquid crystal phases. Various distinctive shifts in solute sequence were noted for each phase. An “MPMS” polysiloxane capillary gave the highest relative retention times of the four phases and is required for satisfactory peaks of monoterpenes with a terminal polar group, like geraniol. It is also good for polar aromatics like thymol, although bis(methoxybenzylidineanil-chloroaniline) in a packed column is an alternative. Linear ether aromatics like estragole can be studied on all four phases, which have this structure themselves. Whilst ADP may be best for terpene hydrocarbons, especially caryophyllene, bis(methoxybenzylidineanil-bi-toluidine) is the best general purpose liquid crystal for packed columns.

Eight cyclic monoterpene volatile oil constituents of short retention time were studied using three liquid crystals as gas chromatographic stationary phases in packed columns. Two phases (below) exhibited a different solute elution sequence after melting and supercooling. The third, bis-(methoxy-benzylidineanil-chloroaniline), did not, but still showed fairly good resolution. However, azoxydiphenetole offered the identification advantage of considerable changes in solute relative retention times, with melted vs. unmelted ratios of × 1.5 to × 3.0. Cineole and terpinolene were distinctive in giving for this ratio only × 0.9 on bis-(methoxy-benzylidineanil-bitoluidine). The results suggest that various parts of the melted or unmelted liquid crystal molecules participate in retaining the various solutes.