Enantiomer identification in the flavour and fragrance fields by “interactive” combination of linear retention indices from enantioselective gas chromatography and mass spectrometry
This study describes the development of a gas chromatography–mass spectrometry (GC–MS) library to identify optically active compounds in the flavour and fragrance field using enantioselective GC with cyclodextrin derivatives (CDs) as chiral selectors in combination with MS. The library operates on the “interactive” combination of linear retention indices (IT values) in parallel to MS spectra, so as to identify enantiomers reliably and to measure EE and/or ER unequivocally. Since MS is not a selective probe to discriminate optical isomers, mass spectra (or diagnostic ions in SIM mode) are used to locate the enantiomer(s) in the chromatogram, and IT values to identify it(them) safely and reliably in particular in complex mixtures. The library has been built up through the following steps: Selection of CD derivatives able to cover a wide range of racemate separations. Four cyclodextrin derivatives were used: 2,6-di-O-methyl-3-O-pentyl-Î²-CD, 2,3-di-O-methyl-6-O-tert-butyldimethylsilyl-Î²-CD, 2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl-Î²-CD, and 2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl-Î²-CD. Determination of the analytes’ IT values and evaluation of their stability and reliability at both intra- and inter-laboratory level. Determination of the range within which the IT of an enantiomer has to fall to be correctly identified, i.e. determination of a common retention index allowance (RIA). Construction of the library, at the moment comprising the enantiomers of 134 racemates. A record has been attributed to each enantiomer including IT values determined on the four CD coated columns, mass spectrum, IUPAC chemical name, CAS number, molecular weight, and, when separated, racemate enantiomer resolution on the CD investigated. Some applications of the library are also reported.