Food Lipids: Chemistry, Nutrition, and Biotechnology

When GC-MI-FTIR was used to quantitate the trans monoene isomers in margarine,
the results had a high bias relative to the GC FID’s response [107,108]. The
higher GC-MI-FTIR values presumably resulted from its higher specificity. This is
because the MI-FTIR determination is based on a discriminatory feature (971 cm �1
absorbance band) that is observed only for trans species. The intensity of this band
is not affected by cis isomers even when they chromatographically overlap. These
results confirm the conclusion that quantitation by means of GC peak areas can result
in an underestimation of trans monoenes.
GC-FTIR analysis was used to determine the geometric configuration of fatty
acid methyl esters separated by silver ion HPLC [110]. This was achieved with the
more recent direct deposition (DD) interface that condenses GC eluates on a moving
zinc selenide window cooled to near liquid nitrogen temperature. This DD instrumentation
is even more sensitive than the matrix isolation interface [111] because
the analytes are condensed on a track that is about 100 �m wide, and microscope
objectives are used to collect and focus the infrared beam. Unlike GC-MI-FTIR,
during GC-DD-FTIR operations, the analytes are not diluted in an argon or any other
matrix.
B. GC-EIMS
MS can be a very effective tool when used in combination with GC to determine
the location of double bonds in fatty acids and their positional isomers. The major
problem of analyzing mass spectra of fatty acid methyl esters is the tendency of the
double bonds to migrate during electron ionization. The mass spectra exhibit low
mass ions that do not provide structural information. Some of the methods used to
overcome this problem include soft ionization and derivatization [112]. Chemical
ionization (CI) methods for the determination of double- and triple-bond positions
were reviewed [113]. A CI-MS procedure was used to determine the double-bond
positions in fatty acids from marine organisms [114].
The most successful approach has been to derivatize the carboxyl group to a
nitrogen-containing compound. Common derivatizing agents have included pyrrolidide,
picolinyl ester, and 4,4-dimethyloxazoline (DMOX). A recent review of these
derivatives indicates that the most useful ones by far are the picolinyl ester and
DMOX derivatives [115]. With these derivatives, double-bond ionization and migration
are minimized. Simple radical-induced cleavage occurs at each C—C bond
along the chain. Therefore, for unsaturated fatty acids containing up to several double
bonds, there is decreased abundance of low mass ions and an increase in a series of
ions resulting from carbon–carbon bond scission. Diagnostic ions occur wherever
there is a functional group in the chain that interrupts the pattern of cleavage from
C—C bonds. A C—C bond or a five- or six-membered ring might be responsible
for such disruptions.
GC-EIMS analysis of picolinyl ester derivatives can unequivocally identify
polyunsaturated fatty acids [116,117]. In the work just cited, however, the GC resolution
of the picolinyl fatty acid esters was not as good as that of other derivatives.
Reversed phase HPLC fractionation of the picolinyl fatty acid esters prior to identification
by GC-MS was necessary to obtain acceptable results for hydrogenated
samples [118]. A total of 39 fatty acid components in cod liver oil were identified
using this method. In a more recent study, silver ion HPLC was used to fractionate
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