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