Food Lipids: Chemistry, Nutrition, and Biotechnology

CFAMs before converting them to the picolinyl ester derivatives [119]. Some of the
GC problems associated with picolinyl esters could be overcome by using high
temperature, low bleed, cross-linked polar columns [120].
Mossoba et al. reported that 2-alkenyl-4,4-dimethyloxazoline derivatives of
diunsaturated CFAMs exhibited distinctive mass spectral fragmentation patterns that
could be used to pinpoint the positions of double bonds and of 1,2-disubstituted,
unsaturated, five- and six-membered rings along the hydrocarbon chain [121]. One
of the advantages of using the DMOX derivatives was the good chromatographic
resolution: sometimes higher than that observed for derivatives of fatty acid methyl
esters [122,123]. Most CFAMs in heated flaxseed oil were identified, and the doublebond
configurations (cis or trans) were unequivocally established by using GC-MI-
FTIR (Table 3).
Recently, Mossoba et al. [110] confirmed the identity of individual trans monoene
fatty acid positional isomers in partially hydrogenated soybean oil. Fatty acid
methyl esters were fractionated by silver ion HPLC and then analyzed by GC-DD-
FTIR to determine geometric configuration and by GC-EIMS on DMOX derivatives
to determine double-bond position [110]. GC peak resolution obtained with a 100m
capillary column was higher for the DMOX derivatives than for those of the fatty
acid methyl esters. Figure 8 demonstrates the excellent resolution that was obtained
for DMOX derivatives of trans monoene positional isomers. The bottom GC profile,
with about twice the amount of injected sample, shows evidence of overload in the
early part of the trace but enhanced response for the �13 and �14 positional isomers.
The double-bond positions for nine individual trans monoene positional isomers were
confirmed by their unique DMOX mass spectra. Most significantly, this was the first
report of the capillary GC separation of the �13 and �14 trans monoene positional
isomers in hydrogenated vegetable oil. Figure 9 presents the mass spectral data that
confirmed their bond position. The identity of the trans-�13 isomer was further
confirmed by comparison with a standard. Hence, the double-bond position and configuration
could be readily established for a complex distribution of monounsaturated
fatty acid isomers found in dietary fat by using the two hyphenated techniques, GC-
DD-FTIR and GC-EIMS.
C. SFC-FTIR
Capillary SFC is an effective tool for separating nonpolar to moderately polar complex
mixtures of natural products having molecular weights of 100–1000 Da. SFC
uses a gas compressed above its critical temperature and pressure to carry analytes
through a chromatographic column. The many applications of SFC to the analysis
of lipids have been reviewed [124–128]. SFC can be used to separate some compounds
at lower temperatures than GC, and a simultaneous interface with FID and
FTIR detectors can be readily achieved. Different SFC columns can be used to
separate lipids according to carbon number or degree of unsaturation. However,
capillary GC and silver ion HPLC columns give better peak resolution when separating
fatty acid derivatives from hydrogenated oils.
SFC has been used to obtain structural information, backed up by mass spectrometry
for detection. The main limitation of early applications has been the lack
of suitable commercial interfaces [129].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.