Gema Flores Maria Luisa Ruiz del Castillo Marta Herraiz Instituto de Fermentaciones Industriales, Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain Original Paper Enantiomeric analysis of b-pinene and limonene by direct coupling of reversed phase liquid chromatography and gas chromatography using absorbents as packing materials A method based on the use of absorbents as packing materials inside the interface of the online coupling between RPLC and GC is proposed for the enantiomeric anal- ysis of b-pinene and limonene in essential oils. For that purpose, a comparison of the RSD, detection limit and recovery provided by two absorbents and one adsorb- ent is included in this study. The results found in this work proved the validity of absorbents as packing materials in online RPLC – GC to determine minor com- pounds in complex matrices. In particular, PDMS seemed to be specially useful to analyse nonpolar compounds, such as b-pinene and limonene, since it provided higher sensitivity for this kind of compounds. The developed method was applied to the evaluation of the natural and non-natural character of commercial essential oils by means of the determination of the enantiomeric composition of b-pinene and limonene. Keywords: Absorbent / Adsorbent / Enantiomers / RPLC-GC / Received: June 8, 2007; revised: July 4, 2007; accepted: July 5, 2007 DOI 10.1002/jssc.200700248 1 Introduction Over years the consideration of stereochemical aspects in analytical chemistry has been neglected in most labora- tories mainly due to the difficulty that involves the devel- opment of an analytical methodology capable of deter- mining chiral compounds without triggering their race- mization. This means that both elevated temperatures and extreme pHs must be avoided at all times during the analysis with the aim of not altering the enantiomeric composition. Moreover, many chiral compounds occur as minor components in complex mixtures in such a way that their overlapping with other matrix components may make the determination of their enantiomeric com- position more difficult. For this reason, when chiral com- pounds in real-life samples are intended to be deter- mined, time-consuming and laborious analytical proce- dures are usually required. As an alternative, the use of multidimensional systems can be particularly interest- ing since they enable the compounds of interest to be selectively isolated from the matrix replacing to a great extent the pretreatment step. As a consequence, the sam- ple handling is minimized, which results in a lesser loss of analytes as well as in a higher reliability of the anal- ysis. Among the multidimensional systems, the online cou- pling of both normal phase (NP) and RP, HPLC and GC has demonstrated to be particularly valuable for the analysis of important constituents in complex mixtures [1 – 6]. Specifically, some reports on the analysis of minor compounds in different matrices by using RP in the LC- preseparation have already been published by other authors [7 – 11]. However, as far as the analysis of chiral compounds by LC-GC is concerned, the studies existing in the literature are focussed on the employment of NP in the preseparation step [12, 13]. In this context, we have earlier reported the use of a programmed tempera- ture vaporizer (PTV) as the interface of the RPLC – GC sys- tem to determine chiral compounds in real-life samples [14 – 16]. By applying this approach, the selectivity required to isolate compounds occurring in very com- plex matrices is attained through the RPLC presepara- tion while the internal GC concentration in the system interface allows to achieve the sensitivity necessary to analyse minor compounds. In earlier works we have mainly centred on the study of the performance of the RPLC – GC interface. To that Correspondence: Dr. M. L. Ruiz del Castillo, Instituto de Fer- mentaciones Industriales, CSIC, c/Juan de la Cierva 3, Madrid 28006, Spain E-mail: ifir312@ifi.csic.es Fax: +91-564-48-53 Abbreviations: PTV, programmed temperature vaporizer i 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com 2780 G. Flores et al. J. Sep. Sci. 2007, 30, 2780 – 2785