Chiral Separations 7: Essential oils on dual column
23 Feb 2022Since essential oils are mixtures of many compounds, coelution of peaks and overlapping of specific optical pairs are sometimes hard to avoid. Not all of the chiral compounds found in an essential oil or flavor extract may be resolved on the same column. Connecting two different columns together is possible, but the elution order of some enantiomers may be reversed or coelute. Dual column analysis is a logical alternative to obtain a more complete enantiomeric profile and to provide confirmatory identification of individual constituents. To reduce analysis time, both columns can be installed into the same injection port for simultaneous confirmation.
Lavender oil on Rt-βDEXsm and Rt-βDEXse is a perfect example of using this technique. The Rt-βDEXsm column separates the enantiomers of the primary chiral compounds found in lavender oil, including linalool. Both the cis and trans enantiomeric pairs of the furanoid linalool oxides, which contribute characteristic odors to lavender oils and Clary sage oil, are separated on this column. (R)-Linalool (peak 4a, Fig 1; Peak 5a, Fig 2) is present in at least 85% enantiomeric excess.
| Peaks | tR 1 | tR 2 | tR 3 | tR 4 | |
|---|---|---|---|---|---|
| 1. | Limonene (L,D) | 14.866 | 15.618 | - | - |
| 2. | Eucalyptol | 16.009 | - | - | - |
| 3. | Linalool oxides | 21.865 | 22.765 | 23.272 | 23.601 |
| 4. | Linalool (±) | 25.837 | 26.445 | - | - |
| 5. | Camphor | 27.716 | - | - | - |
| 6. | Linalyl acetate | 29.183 | - | - | -- |
Figure 1: Lavender oil on Rt-βDEXsm. GC_FF1354
Linalyl acetate is another primary constituent in lavender oils. The (R)- (-) enantiomer is predominant in authentic lavender oils. A dual column system comprised of both the Rt-βDEXsm and Rt-βDEXse columns can be used to resolve the enantiomers of linalyl acetate as well (peak 6 in Fig 2). Note that the (R)- enantiomer constitutes >92% of linalyl acetate in this lavender oil (Peak 6a, Fig 2).
| Peaks | tR 1 | tR 2 | |
|---|---|---|---|
| 1. | Eucalyptol | 13.825 | - |
| 2. | Limonene (L,D) | 17.898 | 18.886 |
| 3. | Linalool oxides | 21.210 | 21.638 |
| 4. | Camphor | 24.357 | - |
| 5. | Linalool (±) | 25.688 | 26.651 |
| 6. | Linalyl acetate | 29.166 | 29.494 |
Figure 2: Lavender oil on Rt-βDEXse. GC_FF1357