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Enantiomeric compounds are very important in the flavors and fragrances field. Chiral capillary GC, with cyclodextrin-based stationary phases, has proven to be a convenient method for characterizing essential oils and differentiating natural flavors from those of synthetic origin.
Chiral compounds from natural origins usually exist as one predominant optical isomer. Also, the inspection of enantiomeric ratios can characterize regional differences between oils. Although sometimes a result of processing, the presence of racemic pairs (one-to-one ratios of each enantiomer) most often indicates adulteration or unnatural origin.
Since essential oils are mixtures of many compounds, coelution of peaks and overlapping of certain optical pairs are sometimes hard to avoid. For this reason, the selectivity of the stationary phase plays a very important role.
Moreover, in modern labs, analysis time is becoming increasingly important in order to obtain data more quickly, but also to lower cost and thus increase the throughput. The Fast-GC technique using narrow-bore capillary columns (typically 100 μm i.d., 5–10 m long) makes this possible, allowing 5–10 times faster analysis (compared to conventional GC) while retaining optimal resolution power.
The most critical step for the Fast-GC technique is the injection. The injection volume should be smaller than with traditional GC because of the limited capacity of narrow-bore capillary columns. Furthermore, it is important not to inject diluted essences so as not to have the interference of a solvent. For this reason, the headspace technique is used for injection.
Methodology
Instrumentation: GC and headspace autosampler
In this work, the procedures were performed with a GC equipped with an HT2000H headspace autosampler (HTA s.r.l., Brescia, Italy). The instrument settings are given in Table 1.
Table 1 – Instrument settings
Materials
Sample
Lavender Essential Oil sample was used.
Standard
Linalool (±) enantiomers standard (Fluka [Sigma-Aldrich Corp., Milan, Italy] cat. #62140):
- Linalyl acetate (±) enantiomers standard (Fluka cat. #45980)
- (–)-Borneol standard (Fluka cat. #15598).
Twenty microliters of pure Lavender Essential Oil was introduced in 20-mL headspace vials for analysis.
Linalool enantiomers, linalyl acetate enantiomers, and (–)-borneol standards were diluted (1 μL/1 mL) in n-hexane and injected as liquid for confirmation. The HT2000H autosampler also allows the injection in manual mode even if the autosampler is installed on the GC. (In fact, it leaves free access to the injector port.)
Figure 1 – Lavender Essential Oil profile chromatogram obtained with headspace injection (performed with HT2000H autosampler) of a pure nondiluted Lavender Essential Oil sample in a Fast-GC Chiral Column (MEGA-DEX DET Beta FAST column,
MEGA S.N.C., Milan, Italy).
Figure 2 – Close-up of Lavender Essential Oil chromatogram (obtained with headspace injection (performed with HT2000H autosampler) of a pure nondiluted Lavender Essential Oil sample in a Fast-GC Chiral Column (MEGA-DEX DET Beta FAST column) with the overlay of the target compounds (linalool, linalyl acetate, and (–)-borneol) for confirmation.
Results
The results are shown in Figures 1 and 2.
Conclusion
These results demonstrate that the headspace technique (performed by the HT2000H autosampler) is very effective for injecting undiluted samples, especially fragrances and essences, directly in Fast-GC columns with narrow diameters.
For more information, please contact HTA s.r.l., via del mella, 77-79, 25131 Brescia, Italy; tel.: +39 030 3582920; fax: +39 030 3582930; e-mail: [email protected]; www.hta-it.com.
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