Uniliner
®
sleeve
withpress-tight seal
inlet seal
Figure 1
TheUniliner
®
sleeve forms a leak-free connection,
minimizes backflash, and helps focus the sample.
environmental
Applications
note
Restek Corporation • (800) 356-1688 • (814) 353-1300 •
#59187
Techniques toOptimizingGCAnalysisof EthyleneGlycol inWater
environmental
Direct injection
usingaUniliner
®
sleeve—Analternative
tosplitless!
The analysis of ethylene glycol inwater is a very common test
in environmental laboratories.Many of these samples originate
fromwater runoff at airportswhere ethylene glycol is used as a
de-icing agent for airplanes duringwintermonths. Because
ethylene glycol is highly soluble inwater, it is not easily
concentrated by purge and trap. Therefore, themost frequently
used sample introduction technique is direct aqueous injection.
The direct aqueous injection of ethylene glycol can be chal-
lenging because, if not done properly, it can be difficult to
attain reproducibility and good peak shape. The large expan-
sion volume of water can cause backflash, carryover can
cause inconsistent results, and excesswater can extinguish the
FID flame. These problems can prevent achieving the
detection limit for ethylene glycol, whichmay vary in the
1-10ppm range.
Poor Peak Shape
With a column head pressure of 10psig and an injection port
temperature of 250°C, a 1µL injection of water will expand to
1420µLof vapor. This large vapor cloud exceeds the volume of
most inlet liners, causing backflash. If backflash occurs, the
vapor cloud can expand out of the liner and injection port and
result in poor sample transfer onto the column.Also, the glycol
compounds are not focused in a narrow band but, instead, are
focused in the condensedwater that beads onto the column
walls, so the compounds of interest can elute as split peaks.
This peak splitting effect ismost apparent when performing a
splitless injection because of the solvent focusing required.
Split peaks and backflash compromise the analysis by causing
irreproducible peak shapes.
One technique to reduce the effect of vapor expansion and poor
solvent focusing is the use of aUniliner
®
injection port sleeve.
This sleeve forms a leak-free connectionwith the column end
(
Figure 1
), thereby ensuring a complete sample transfer.
Additionally, theUniliner
®
sleeve requires operation at a higher
pressure than traditional splitless liners, which forces the large
vapor cloud to be focused into a narrow bandwhen entering
the column. Thisminimizes sample backflash and eliminates
the need for solvent focusing. By using aUniliner
®
sleeve, the
aqueous ethylene glycol sample is completely vaporized and
properly transferred to the column in a focused, narrow band,
thereby achieving reproducible peak areas. Uniliner
®
sleeves
are available for conversion of packed column injection sys-
tems and for split/splitless injection systems.
SampleResidueCarryover
Carryover is another problem associatedwith ethylene glycol
analysis.When analyzing glycols, carryover can be caused by
sample residue in the syringe being carried over from one
injection to another. If the syringe is not properly cleaned
between analyses, carryover will cause inconsistent results.
Rinsing the syringewith either water or water/methanol (50:50)
three to six times between each injectionwill eliminate sample
residue andminimize the possibility of carryover.
FID Flameout
Column stationary phase choice is a critical considerationwhen
analyzing glycols inwater via direct injection.Water analyzed
on a non-polar stationary phase, such as theRtx
®
-1 column, or
on amoderately polar stationary phase, such as theRtx
®
-200
column, will cause the flame on the FID to be extinguished.
This is because thewater will not partition properly andwill
“bead up” on the phase, producing a large plug of water that
passes through the detector and extinguishes the flame. The
more commonly-usedGCswill experience flameout under
these circumstanceswhile otherswill not.
Tominimize the possibility of extinguishing the flame, select a
polar stationary phase that ismore compatiblewithwater. The
Stabilwax
®
stationary phase is one of themore polar phases,
making it a good choice for water injections. It allowswater to
partition properly, which prevents it from beading up on the
stationary phase and quenching the FID flame.
analytical column
