Food Lipids: Chemistry, Nutrition, and Biotechnology

cation [100]. Similarly, the published literature contains an incorrect report that liquid
canola shortening was contaminated with fatty acids found in animal fat [101].
To help identify peaks, a GC column can be interfaced to another instrument
such as an infrared spectrometer or a mass spectrometer. Hyphenated techniques use
on-line detection to confirm the identity of peaks in a chromatogram. Griffiths et al.
[102] have reviewed interfaces between gas, supercritical fluid, and high performance
liquid chromatography and Fourier transform spectrometery (GC-FTIR, SFC-FTIR,
and HPLC-FTIR, respectively). Some of these hyphenated techniques were first used
to elucidate the structure of unusual fatty acid isomers, including cyclic fatty acid
monomers (CFAM) that often contain trans double bonds in the hydrocarbon chain
and cis double bonds in five- or six-membered rings. The formation and the biological
effects of these cyclic compounds have been reviewed [103].
A. GC-FTIR
For many GC-FTIR instruments, the effluent from the GC column flows continuously
through a light pipe (LP) gas cell [102]. LP instruments generally have detection
limits of 10–50 ng for unknown complex mixtures of fatty acid methyl esters. Online
infrared spectra of CFAM peaks eluting from a gas chromatogram were obtained
with an LP GC-FTIR [104]. The results were used to show which of these CFAM
contained cis and/or trans double bonds. Several minor peaks in this mixture could
not be identified because of the limited sensitivity of the method.
Bourne et al. [105] developed an improved GC-FTIR technique using a matrix
isolation (MI) interface that increases the sensitivity of the GC-FTIR determination
by an order of magnitude. GC–matrix isolation–FTIR is extremely useful for quantitating
peak area, determining peak homogeneity, and obtaining structural information
on a compound. Although the LP GC-FTIR technique can confirm the identity
of intact molecules, its detection limits are higher than those of other GC detectors
(e.g., flame ionization). Matrix isolation (MI) is a technique in which analytes and
an inert gas (argon) are rapidly frozen at cryogenic temperatures (12 K) and are
trapped as a solid matrix on the outer rim of a moving gold-plated disk. The IR
spectra of these molecules are free from indications of intermolecular hydrogen bonding
and other band broadening effects. These combined benefits yield greater sensitivity
that equals, for many applications, that of gas chromatograpy–mass spectrometry
(GC-MS) [106].
GC-MI-FTIR was used to quantitate low levels of saturated trans monoene
[85], diene [107,108], triene, and conjugated diene fatty acids and their isomers [108]
in hydrogenated vegetable and/or fish oils. Figure 7 shows the IR spectra (out-ofplane
deformation absorption bands) for two trans-conjugated diene geometric isomers.
The characteristic absorption bands shown were used for identification. The
conjugated trans,trans diene isomer had an out-of-plane deformation absorption band
at 990 cm �1 , whereas, the cis,trans isomer had absorption bands at 950 and 986
cm �1 . The absorption band for the corresponding methylene-interrupted trans diene
and the trans monoene was at 971 cm �1 . There were also unique absorption bands
for trans and cis diene isomers in the area of the spectrum typical of the carbon–
hydrogen stretch vibrations (3035/3005 cm �1 and 3018 cm �1 , respectively) and in
the area typical of the carbon–hydrogen out-of-plane deformation absorption bands
(972 and 730 cm �1 , respectively).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.