with UV-visible spectra  obtained by using a photodiode array detection (DAD) detector (Figure 6.13). A recent study of the carotenoid fraction of a saponified mandarin oil has been performed by means of comprehensive LC, in which a 1D microbore silica column was applied for the determination of free carotenoids, and a cyanopropyl column for the separation of esters; a monolithic column was used in the 2D . Detection was performed by connecting a DAD system in parallel with an MS detection system operated in the atmospheric pressure chemical ionization (APCI) positive-ion mode. Thus, the identification of free carotenoid and carotenoid esters was carried out by combining the information provided by the DAD and MS systems, and the peak positions in the two-dimensional chromatograms.
6.3.9 On-Line Coupled Liquid Chromatography-Gas Chromatography (LC-GC)
The analysis of very complex mixtures is often troublesome due to the variety of chemical classes to which the samples components belong to, and to their wide range of concentrations. As such, several compounds cannot be resolved by monodimensional GC. In this respect, less complex and more homogeneous mixtures can be attained by the fractionation of the matrix by means of LC prior to GC separation. The multidimensional LC-GC approach combines the selectivity of the LC separation with the high efficiency and sensitivity of GC separation, enabling the separation of compounds with similar physicochemical properties in samples characterized by a great number of chemical classes.
For the highly volatile components, commonly present in essential oils, the most adequate transfer technique is partially concurrent eluent evaporation . In the latter technique, proposed by Grob, a retention gap is installed, followed by a few meters of precolumn and the analytical capillary GC column, both with identical stationary phase, for the separation of the LC fractionated components. A vapor exit is placed between the precolumn and the analytical column, allowing partial evaporation of the solvent. Hence, column and detector overloading are avoided. This transfer technique can be applied to the analysis of GC components with a boiling point of at least 50°C higher than the solvent.
The composition of citrus essential oils has been greatly exploited by means of LC-GC, and the development of new methods for the study of single classes of components has been well reported. The aldehyde composition in sweet orange oil has been investigated , as also industrial citrus oil mono- and sesquiterpene hydrocarbons , and the enantiomeric distribution of monoterpene alcohols in lemon, mandarin, sweet orange, and bitter orange oils [140,141].
The hyphenation of LC-GC systems to mass spectrometric detectors has also been reported for the analyses of neroli , bitter and sweet oranges, lemon, and petitgrain mandarin oils . It has to be highlighted that the preliminary LC separation, which reduces mutual component interference, greatly simplifies MS identification.
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