Page 244 - Multidimensional Chromatography
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238                                     Multidimensional Chromatography

                           and the amount of information yielded. Sometimes, GC/FTIR has a limited
                           diffusion, compared to GC–MS coupling, due to an overall lower sensitivity. These
                           problems may be overcome if the HRGC/FTIR system is coupled on-line to an LC
                           instrument. An on-line LC–GC/FTIR system has been used for the analysis of cit-
                           ropten and bergapten in bergamot oil, using an on-column interface and partially
                           concurrent eluent evaporation with early vapour exit (55). Figure 10.12 shows the
                           LC chromatogram of bergamot oil (a), plus the corresponding GC/FID (b) and
                           GC/FTIR (c) chromatograms after separation of the transferred fraction. The FTIR
                           chromatogram has been constructed by using a selected wavelength chromatogram
                           (SWC) from 1776 to 1779 cm   1 , which falls into the range 1760–1780 cm   1 , cor-
                           responding to the characteristic C–O stretching band of the lactone ring of citropten
                           and bergapten. In this way, any interaction with interfering peaks is avoided.
                             On-line LC–GC has frequently been used as a clean-up technique for the analysis
                           of trace levels of contaminants (pesticides, plasticizers, dyestuffs and toxic organic
                           chemicals) in water and food products. Several different approaches have been pro-
                           posed for the analysis of contaminants by on-line LC–GC. Since pesticide residues
                           occur at low concentration in water, soil or food, extraction and concentration is
                           needed before GC analysis is carried out.
                             Organophosphorus pesticides (OPPs) are extensively used for the protection of
                           olives against several insects. The class of OPPs contains numerous compounds
                           which vary widely in polarity and molecular weight. Capillary GC analysis is the
                           preferred method for the analysis of OPPs, using selective detectors such as the
                           nitrogen–phosphorus (NPD) and flame photometric (FPD) detectors, as well as
                           mass spectrometry (MS). GC analysis has to be performed after removal of the
                           high-molecular-mass fats from the matrix. This pre-separation can be carried out
                           by off-line methods, such as LC or gel permeation chromatography (GPC),
                           although on-line GPC–GC methods have also been developed (56, 57). In the
                           original version of this technique (56), a solvent system of  n-decane and the
                           azeotropic mixture of ethyl acetate and cyclohexane was found to give an adequate
                           separation between the fat and the organophosphorus pesticides. The pesticide-
                           containing fraction was transferred to the GC unit by using a loop-type interface
                           with an early vapour exit and co-solvent trapping. The system presented some dif-
                           ficulties during the transfer of the large eluent fraction (3 ml), and optimization of
                           the transfer temperature was critical.  An improved version of the on-line
                           GPC–GC (FPD) method for the determination of OPPs in olive oil has been devel-
                           oped (57) which uses an on-column interface with flow regulation. The miniatur-
                           ization of the GPC column (the fraction transferred was now reduced to 1.3 ml)
                           and the selection of another combination of main and co-solvent (methyl
                           acetate/cyclopentane, with  n-nonane as co-solvent), both with lower boiling
                           points, thus produced a very robust method for the analysis of 28 OPPs in olive oil
                           with an overall detection limit of 0.002 mg/kg.
                             The application of automated GPC for the clean-up of various matrices has been
                           demonstrated by other authors (58, 59). As well as organophosphorus pesticides,
                           conventional methods for the analysis of organochlorine pesticides (OPCs) in fatty
                           samples may involve various clean-up methods, such as LC or GPC.  The main
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