limonene a-thujene myrcene MNM octanal a-sinensal -,-terpinene /j-pinene/ terpinolene linalool sabinene
Figure 12.7 Authentic cold-pressed mandarin oils from Italy (shaded ranges) compared with commercial samples of different provenance (Greece Co 4, 5; Brazil Co 6; and Argentina Co 7): (A) authenticity range (shaded range, including minimum and maximum fil:iCpDB values of samples Au 1-10); (B) authenticity profile (shaded range, calculated for myrcene as I-IST). MNM, methyl A7-methylanthranilate.
Often, when dealing with complex matrices, the time required for complete gas chromatographic separation of the components of interest can be very long. The use of fast GC techniques can drastically reduce analysis times, maintaining the same or even improving resolution.
Fast GC (David etal., 1999; Mondello etal., 2000) and Fast GC-MS (Mondello etal., 2000) have been applied to the analysis of citrus essential oils. Narrow bore columns have been used for fast GC determination of volatile and also semi-volatile components of some citrus oils.
Narrow bore columns, characterized by a reduced internal diameter and reduced film thickness, allow fast separation as indicated by the Golay equation (Sandra etal., 1987). In fact, as the optimum carrier gas velocity is higher and the H-u plots are flatter for narrow-bore columns, it is possible to work with higher average linear velocities without loss of efficiency. Using narrow-bore columns and hydrogen as the carrier gas, high linear velocity can be applied allowing fast separation with excellent resolution. In order to perform fast GC, several requirements are necessary:
• the instrumentation must allow high inlet pressure and highly controlled split flows (because narrow-bore columns have limited sample capacity, split injections are usually used)
• fast linear heating rates
• fast electronics for detection and data collection.
Fast GC techniques also requires that sample-preparation time is short enough to justify the technique itself. For citrus essential oils this requirement is largely satisfied, since the oils were analysed after only a dilution.
Figure 12.8 shows a comparison between a conventional GC analysis and a fast GC analysis of a lime essential oil. As can be seen, in the fast analysis the same components have been separated with a speed gain of 4.6.
Simply by varying the experimental conditions slightly it is possible to obtain information on the volatile and non-volatile fractions during the same fast GC run, as shown in Figure 12.9- Here, a fast GC chromatogram of a cold-pressed lemon oil is shown. The peaks of herniarin, isopimpinellin and fi-3-carene are separated; the possible contaminations with sweet orange (¿-3-carene) and lime oils (herniarin and isopimpinellin) are easily detectable.
An FID detector is not an information-rich detector in GC analysis. Sometimes, the information required for peak identification is not enough and GC-MS analysis is require. As for the GC-FID analysis of citrus essential oils with conventional columns, GC-MS is often very long. The coupling of a fast mass spectrometer (6750 amu/sec) has been performed. Figure 12.10 shows a TIC fast chromatogram obtained for a lime essential oil. Fifty-five components have been identified. Table 12.5 reports the list of peaks identified and comparison between quantitative results obtained by conventional and fast GC analyses. As can be seen, results are in good agreement. This analysis permits very reliable mass spectra to be obtained. An example is shown in Figure 12.11, which reports the mass spectrum of a-pinene, together with library MS spectrum for comparison. As can be seen, spectrum obtained with the fast GC-MS analysis was similar to that from the MS library.
Coumarins and psoralens
Was this article helpful?