GCAnalysis of Volatile Free FattyAcids on the Stabilwax
fi
-DAColumn
Figure 1
1. acetic acid
2. propionic acid
3. isobutyric acid
4.
n
-butyric acid
5. isovaleric acid
6.
n
-valeric acid
7. isocaproic acid
8. caproic acid
9. heptanoic acid
15m, 0.53mm ID, 1.0m Stabilwax
fi
-DA
(cat.# 11052)
0.2 L injection of a 10-20ng/ L free fatty acid
standard inwater. Direct injection using a
Uniliner
fi
liner.
OvenTemp.:
145C isothermal
Inj./Det. Temp.:
250C
Carrier gas:
hydrogen
LinearVelocity:
80cm/sec. (flow rate: 10cc/min.)
FIDSensitivity:
4 x 10
-11
AFS
ID
df
15-meter
0.53mm
1.0 m
cat.# 11052
ID
df
30-meter
0.53mm 0.25 m
cat.# 11025
food& flavor
Restek Corporation • (800) 356-1688 • (814) 353-1300 •
food& flavor
#59155B
InjectingFree FattyAcids
Although free fatty acids can be injected by split or splitless
techniques (especiallywhen using smaller bore columns), a
direct injection technique is recommended. Direct injectionwill
reduce the risk of losing volatile lowmolecular weight fatty
acids through the split vent, which improves quantitative repro-
ducibility. Because free acids can be adsorbed, the analyst must
make every effort to ensure an inert sample pathway by using
properly deactivated direct injection liners and inert capillary
columns. Regular preventivemaintenance of theGC injection
port is strongly recommended to prevent surfaces from becom-
ing active over time.
Selecting aColumn
Although the official methods listed previously indicate the use
of packed column chromatography, capillary chromatography
can providemuch greater resolving power (i.e., column efficien-
cy).We suggest using a Stabilwax
fi
-DAcapillary column for the
analysis of volatile free fatty acids. The Stabilwax
fi
-DAfeatures
a bondedCarbowax
fi
phase that has beenmodified specifically
for the analysis of acidic compounds. To prove its effectiveness,
we analyzed a direct injection of straight chain and branched
fatty acids ranging from acetic acid to heptanoic acid. Because
the Stabilwax
fi
-DAcolumn has a strong affinity for free acids,
excellent separationwas achievedwithin 10minutes (
Figure 1
).
Also notice how the unique deactivation of the Stabilwax
fi
-DA
column produced sharp symmetrical peakswithminimal tailing.
Summary
An optimizedGC system for the analysis of volatile free fatty
acids requires direct injection forminimal discrimination and
maximum inertness in the injection port, and requires an analyt-
ical column like the Stabilwax
fi
-DAto provide selectivity and
good peak shape.
References:
1. AOAC International, OfficialMethods of theAOAC, 15
th
edition, 1990.
Reference not available fromRestek.
Volatile free fatty acids are present inmany foods, including
eggs (AOACOfficialMethod 971.11), seafood (AOACOfficial
Method 973.26), and dairy products (AOCSOfficialMethodCa
5c-87). Gas chromatographic (GC) analysis is used to quantitate
these fatty acids. Unlike fatty acidswith longer hydrocarbon
chain lengths, these volatile free fatty acids generally do not
requiremethylation to obtain peak symmetry and reproducibili-
ty byGC. However, like all free fatty acids, they can be difficult
to analyze because of their adsorptive nature. Therefore, a selec-
tive stationary phase that is specifically designed to analyze
acidic compoundswill provide the best results. TheRestek
Stabilwax
fi
-DAcolumn fits these criteria and provides good
peak shapewithout methylation.
PreparingFree FattyAcids
One can analyze existing free fatty acids in amatrix or saponify
fats to obtain them. Saponification procedures are outlined in
AOACMethods 971.11D and 938.09D
1
. Samples are extracted
with solvent and saponified by heating under a refluxwith an
excess of dilute aqueous ethanolic alkali.After saponification,
the sample is neutralizedwith dilute hydrochloric acid or sulfu-
ric acid. Inmany cases, an aqueous solution of phosphotungstic
acid is added aftermixing or shaking. The samplemay be cen-
trifuged and/or filtered, and then finally diluted to an aqueous
solution.
min. 4 6 8 10
1
2
3 4
5
6
7 8
9
Order a FREE Foods, Flavors, and Fragrances
Catalog!
This 52-page document includes
important analysis tips and chromatograms for
the analysis of fats and oils, carbohydrates, vita-
mins, amino acids, organic acids, preservatives,
flavors and fragrances, essential oils, and chiral
compounds. Retention time indices and com-
plete product listings for all of the relevant GC
andHPLC products also are included (lit. cat.#
59260). Also, request Applications Note detail-
ing food packaging testing (lit. cat.# 59348).
Applications
note
Stabilwax
fi
-DAachieved excellent separation
within 10minutes.
Stabilwax
fi
-DAColumns
