Enantio cGC

A real breakthrough in this field occurred when enantio-cGC became more and more available. In particular, since 1988 selectively modified cyclodextrins have been synthesised, serving as chiral stationary phases in enantio-cGC, reported by Schurig and Novotny [I], Konig et al. [2, 3], Armstrong et al. [4], Dietrich et al. [5,6 ], Saturin et al. [7], and Bicchi et al. [8]. 6-O-silylated modified /I-cyclo dextrin and y-cyclodextrin derivatives of well-defined structure and purity were synthesised and have proved to be chiral stationary phases of unique selectivity and versatility and, therefore, are successfully used in simultaneous enantio-cGC analysis [5,6]. Further derivatives were recently reported by Taka-hisa and Engel [9, 10], dealing with 2,3-di-0-methoxymethyl-6-0-tert-butyldi-methylsilyl modified cyclodextrins as chiral stationary phases in enantio-cGG. 

Using a chiral column, coated with a definite modified cyclodextrin as the chiral stationary phase, the elution orders of furanoid and pyranoid linalool oxides are not comparable [11, 12]. Consistently, the chromatographic behaviour of diastereomers and/or enantiomers on modified cyclodextrins is not predictable (Fig. 17.1, Table 17.1). Even by changing the non-chiral polysiloxane part of the chiral stationary phase used, the order of elution may significantly be changed [13]. The reliable assignment of the elution order in enantio-cGC implies the coinjection of structurally well defined references [11-13]. 

Three types of limitations have to be accepted in enantio-cGC ... 

There are only few references on odd-numbered lactones in the literature. The first reports on the natural occurrence of y-nonalactone and y-undecalactone are known from the early flavour literature [28-30], long before sophisticated analytical techniques, such as enantio-cGC-MS, became available. These data have to be reevaluated, should the situation arise. Worner et al. [31] provided the first report on y-nonalactone among the volatile constituents of Artemisia vulgaris L. herb, revealing an amount between 1 andlO pg/kg and an enantiomeric distribution of (i )-y-nonalactone to (S)-y-nonalactone of 34 66 using en-antio-MDGC, coupled online with MS. 

Enantio-cGC, however, fails in the case of non-chiral compounds, such as 1,8-cineol. In this special case 1,8-cineol may be attributed to high-level Melaleuca varieties or to the fraudulent addition of Eucalyptus oil. In order to get reliable results, enantio-MDGC-MS analysis and/or IRMS measurements (as far as possible) are necessary. 

As outlined in Table 6.15,100 % separation occurs, if chiral resolution cRs = 2.50 in practice cRs > 1.50 (99.73% separation) is defined as a baseline resolution ]10]. Thus, optimal chiral resolution (cRs > 1.50) should be achieved using a suitable chiral stationary phase in enantioselective capillary gaschromatography (enantio-cGC). 

Chirality evaluation of linalyl acetate and linalool have been introduced as new and substantial criteria in the authenticity control of lavender oils. In particular, linalyl acetate from genuine lavender oils has high enantiomeric purity favouring the R-configuration, irrespective of the Lavandula species, and storage or work-up conditions [32]. Using 2,3-di-0-acylated-6-0-silylated cyclodextrins as a new generation of chiral stationary phases in enantio-cGC, most of chiral compounds of Lavandula oil, are stereoanalyzed simultaneously (Fig. 6.37 Fig. 6.38). 

Fig, 6,43 Stereodifferentiation of 2,5-dimethyl-4-hydroxy-3[2H]-furanone (1) and its methyl-ether (2) from strawberries (A) and pineapples (B). Al/Bl HPLC-chromatogram of raw extracts. A2/B2 enantio-cGC analysis of fractions I, II from raw extracts (from ref [62])... 

On-line coupling of enantio cGC with IRMS (enantio cGC-IRMS) is one of the latest developments in originspecific analysis of flavouring and fragrance compounds. Enantio-IRMS detects enantiomers of the same source with identical 5 C levels. As outlined in Fig. 6.45, linalool from Coriander oil is detected as an R(20%) S(80%) enantiomeric ratio with identical isotope levels of the enantiomers. 

Fig. 6.46 cGC-lRMS (A), enantioselective (B) and enantio-IRMS measurements (C) from an adulterdated spike oil (from ref [107])... 

Presently, online cGC-IRMS determination of 5 C and 5 H values of flavour compounds are established [107. 118-123] and, further on, 5 0 measurements using online cGC-IRMS procedures will be available. Finally, enantio-MDGC combined with multielement- and multicomponent IRMS methods will open the door to comprehensive authenticity assessment of natural flavour and fragrance compounds in the near future. 

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