Table

Different Approaches of Enantioselective GC

1. Chiral diamide stationary phases (Gil-Av, 1965)

Hydrogen bonding interaction 2. Chiral transition metal complexation (Schurig, 1977)

Complexation gas chromatography 3. Cyclodextrin derivatives (König, Schurig, 1988)

Host-guest interaction, inclusion gas chromatography

König and coworkers reported their first results in 1988 with per-O-pentylated and selectively 3-0-acylated-2,6-di-0-pentylated a-, ß-, and g-cyclodextrins, which are highly stable, soluble in nonpolar solvents, and which possess a high enantioselectivity toward many chiral compounds. In the following years a number of further cyclodextrin derivatives have been synthesized and tested by several groups, allowing the separation of a wide range of chiral compounds, especially due to the improved thermal stability (Table 2.4). With the application of 2,3-pentyl-6-methyl-ß- and -g-cyclodextrin as stationary phases, all monoterpene hydrocarbons commonly occurring in essential oils could be separated (König et al., 1992). The reason for application of two different columns with complementary properties was that on one column not all enantiomers were satisfactorily resolved. Thus, the simultaneous use of these two columns provided a maximum of information and reliability in peak assignment.

After successful application of enantioselective GC to the analysis of enantiomeric composition of monoterpenoids in many essential oils (e.g., Werkhoff et al., 1993; Bicchi et al., 1995; and references cited therein), the studies have been extended to the sesquiterpene fraction. Standard mixtures of known enantiomeric composition were prepared by isolation of individual enantiomers from numerous essential oils by preparative GC and by preparative enantioselective GC. A gas chromatographic separation of a series of isolated or prepared sesquiterpene hydrocarbon enantiomers, showing the separation of 12 commonly occurring sesquiterpene hydrocarbons on a 2,6-methyl-3-pentyl-ß-cyclodextrin capillary column has been presented by König et al. (1995). Further investigations on sesquiterpenes have been published by König et al. (1994). However, due to the complexity of the sesquiterpene pattern in many essential oils, it is often impossible to perform directly an enantio-selective analysis by coinjection with standard samples on a capillary column with a chiral stationary phase alone. Therefore, in many cases two-dimensional GC had to be performed.

2.2.2.2.3 Two-Dimensional Gas Chromatography

After preseparation of the oil on a nonchiral stationary phase, the peaks of interest have to be transferred to a second capillary column coated with a chiral phase, a technique usually referred to as "heart cutting." In the simplest case, two GC capillaries with different selectivities are serially connected and the portion of unresolved components from the effluent of the first column is directed into a second column, for example, a capillary with a chiral coating. The basic arrangement used in two-dimensional gas chromatography (GC-GC) is shown in Figure 2.3. By means of a valve, the individual fractions of interest eluting from the first column are directed to the second, chiral column, while the rest of the sample may be discarded. With this heart-cutting technique many

FIGURE 2.2 a-Glucose unit of a cyclodextrin.

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