Food& Flavor
Applications
note
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Food&Flavor
Grape FlavorAnalysisUsing anRt-
g
DEXsa
™
GCColumn
The enantiomeric ratios of certain chiral compounds found in
flavorings can sometimes reveal adulteration.Twochiral
indicators ingrape flavor aremethyl-3-hydroxybutyrateand
ethyl-3-hydroxybutyrate.Methyl-3-hydroxybutyrate is racemic
and (R)-ethyl-3-hydroxybutyrate is at least 77%predominant in
natural grape flavor.
1
Extreme alterations of these ratios can
indicate that the flavor is not completely authentic.
Although theexactmechanismsof compound-cyclodextrin
interaction arenot known, it is evident that the sizeof the
cyclodextrincavity is important. Perhaps somecompoundsmay
be too large to effectively interactwith the cavity sizeof beta-
cyclodextrin,which is composedof sevenglucopyranoseunits.
Gamma-cyclodextrins arecomposedof eight glucopyranose
units and sopossess a larger cavity,whichmaybemore interac-
tivewith larger chiralmolecules (see
Figure1
).
Beta-cyclodextrin stationaryphasesprovideenantioselectivity
for avarietyof chiral compounds, but not formethyl-3-
hydroxybutyrateandethyl-3-hydroxybutyrate.
Figure2
(onnext
page) illustrates the analysis of these compounds on threebeta-
cyclodextrin stationaryphases: theRt-
b
DEXm
™
column (2,3,6-
tri-O-methyl-
b
-cyclodextrin), theRt-
b
DEXsm
™
column (2,3-di-
O-methyl-6-O-
tert
-butyldimethylsilyl-
b
-cyclodextrin), and the
Rt-
b
DEXsa
™
column (2,3-di-O-acetyl-6-O-
tert
-
butyldimethylsilyl-
b
-cyclodextrin).Thepeaks tail andexhibit no
chiral separationon theRt-
b
DEXm
™
column.Different deriva-
tives on the cyclodextrinmolecule canhelp increase selectivity
of enantiomers.Although theRt-
b
DEXsm
™
andRt-
b
DEXsa
™
columns show improvedenantioselectivity formethyl-3-
hydroxybutyrateandethyl-3-hydroxybutyrate, thepeaks still tail
and separationof theoptical isomers is not baseline-resolved.
Analysisofmethyl-3-hydroxybutyrateandethyl-3-
hydroxybutyrateon thenewRt-
g
DEXsa
™
column reveals
excellent chiral selectivity for both, as shown in two types of
grape juice in
Figure 3
(onnext page). TheRt-
g
DEXsa
™
contains2,3-di-O-acetyl-6-O-
tert
-butyldimethylsilyl-
g
-
cyclodextrins that aredissolved intocyanopropyl phenyl
stationaryphase (Rtx
®
-1701). This compositionpromotes
thermal stability to amaximum temperatureof 230°C and
longevity that is comparable toother capillary columns.This
column is available in0.32mm ID and0.25mm ID for direct
interfacing into amass spectrometer.
Figure1
Structures of beta- andgamma-2,3-di-O-acetyl-6-O-
tert
-butyldimethylsilyl-cyclodextrins.
a
-Cyclodextrin
b
-Cyclodextrin
g
-Cyclodextrin
Previousarticleshavedemonstrated theutility
ofbeta-cyclodextrincolumns for flavoranalysis
usinggaschromatography (GC), but sometimes
the largergamma-cyclodextrincavitiesprovide
betterenantioselectivity forchiral indicating
compounds.Restek’sRt-
g
DEXsa
™
column
providesbetter separationof specificchiral
compounds ingrape flavor thanmost beta-
cyclodextrinphases.
