5
Compound of Interest
Derivatizing Reagent
Resulting Derivative
fatty acids
methanol
esterification
with boron trifluoride
glycerol
acetic anhydride
acetylation
with sodium carbonate
For more information on derivatization, please refer to the “Handbook of Analytical
DerivatizationReactions” byDaniel R. Knapp or to the text at right.
Table III
Common reagents used to derivatize compounds of interest.
Common derivatization techniques used in reaction headspace/GC are esterification,
acetylation, silylation, and alkylation.Any of these derivatization techniques can be
performed using the sample vial as the reaction vessel (see
Table III
for a list of
commonly used reagents).Although derivatizationmay improve chromatographic
performance and volatility for some compounds, derivatization reactionsmay
introduce other problems into the analytical scheme. Derivatization reagents aswell
as the by-products from derivatization reactionmay be volatile and can partition into
the headspace alongwith derivatized compounds. These extra volatile compounds
may pose problems by elutingwith similar retention times as the compounds of
interest, causing either partial or complete coelutions.
Derivatization reactions also are typically run at elevated temperatures. Pressures
inside the sample vialmay exceed the pressure handling capabilities of the vial or
the septa. Specially designed septa are available that allow excess pressure to be
vented during derivatization reactions.
Headspace Sample Size
In addition toworkingwithK,
β
, and derivatization reactions, sensitivity also can be
improved by simply increasing the size of the headspace sample that iswithdrawn
from the sample vial and transferred to theGC. Increasing the sample size also
means that the amount of time it takes to transfer the sample to the columnwill
increase in proportion to the column volumetric flow rate. Sample size can be
increased only to the point that increases in peakwidth, as a result of longer sample
transfer times, will not affect chromatographic separations. Larger sample sizes and
longer transfer times can be offset by using cryogenic cooling and sample refocus-
ing at the head of the column.
Derivatization/Reaction Headspace
Derivatization is another technique that can be used to increase sensitivity and
chromatographic performance for specific compounds. Compounds such as acids,
alcohols, and amines are difficult to analyze because of the presence of reactive
hydrogens.When attempting to analyze these types of compounds, they can react
with the surface of the injection port or the analytical column and result in tailing
peaks and low response. In addition, theymay be highly soluble in the sample
phase, causing very poor partitioning into the headspace and low response.
Derivatization can improve their volatility, aswell as reduce the potential for surface
adsorption once they enter theGC system.
For more information on headspace
anaysis, check out the textbook,
Static Headspace-Gas
Chromatography, Theory and Practice
by Bruno Kolb and Leslie S. Ettre.
