The first separations of essential oils were carried out on packed columns that provided limited efficiency but, nevertheless represented a tremendous advance on distillation. The introduction of the technique of temperature programming improved the separation even more. However, it was not until the capillary column, with its many thousands of theoretical plates, became commercially available that the true complex nature of many of the essential oils was revealed. The chemical structure of the individual components of many of the oils, elucidated by the GC/MS tandem systems, provided the knowledge necessary to synthesize a number of commercially important synthetic flavors. For example, the synthetic flavors that closely imitate those of the peach, melon and other fruits that are presently available to the contemporary food chemist are a direct result of the separating capabilities of gas chromatography.

An example of the separation of lime oil employing modern GC techniques is shown in figure 39. The separation was carried out on a SB–5 column that contained poly(5%diphenyl-95%–dimethylsiloxane) as the stationary phase. Although the diphenyl group will contribute some induced polar capability to interact with polar solutes, it is largely a dispersive stationary phase, and, thus, substances are eluted roughly in order of their boiling points (excepting very polar solutes). The introduction of the diphenyl groups contributes more to phase temperature stability than it does to solute selectivity. The column was 30 m long, 250 mm I.D. carrying a film 0.25 mm thick of stationary phase. Helium was used as the carrier gas at a linear velocity of 25 cm/sec(set at 155°C).

The column was held isothermally for 8 min. at 75°C and then programmed up to 200°C at 4°C/min. and finally held at 200°C for 4 min. The sample volume was 0.5 ml that was split at 100:1 ratio allowing about 5 mg to be placed on the column. It is seen from figure 5 that a very good separation is obtained that convincingly confirms the complex nature of the essential oil. In practice, however, the net flavor or odor impact can often be achieved by a relatively simple mixture of synthetic compounds.

1. a–Pinene	7. g–Terpinene	13. Geraniol
2. Camphene	8. Terpinolene	14. Neryl Acetate
3. b–Pinene	9. Linalool	15. Geranyl Acetate
4. Myrcene	10. Terpinene–4–ol	16. Caryophyllene
5. p–Cymene	11. a–Terpineol	17.trans–a–Bergamotene
6. Limonene	12. Neral	18. b–Bisabolen

Courtesy of Supelco Inc.

Figure 39 A Chromatogram of Lime Oil