Optimizing theRate of Injection:
Figure 14 shows the effect of varying rates of injection for
a 5µL sample.When a rapid injection (5µL/sec.) ismade, the solvent peak tails and the
responses for equal concentrations of each analyte are not reproducible.A 1µL/sec. injection
rate improves the solvent peak shape, but the response for each analyte still is not proportion-
al to the concentration of each analyte. Onlywhen the injection rate is slowed to 0.2µL/sec.
does the response for each analyte become consistent with the amount injected.
Some autosamplers are capable of slowing the injection rate tominimize backflash, but
most autosamplers use a rapid injection sequence. If large-volume injectionsmust bemade
rapidly, adjustments to the carrier gas pressuremust be used to control sample expansion.
Pressure Programming:
Pressure (P) is in the denominator of the equation inTable III
(page 8).Any increase in carrier gas pressurewill help to reduce sample expansion volume.
Most of the latest models of GCs incorporate electronic pressure control (EPC) of the carrier
gas pressure. Pressure can be time-programmed so that the carrier gas pressure initially is
very high, then is reduced after the injection to optimize carrier gas flow rate for best resolu-
tion. Setting the initial carrier gas pressure to a high valuewill reduce the amount of sample
expansion that occurs at the point of injection andwill speed up the transfer of the vaporized
sample from the liner to the head of the column.
Direct Injection as an Alternative to Splitless Injection
Direct injections are an alternative approach
for injecting sampleswith low concentrations
of analytes. Direct injections vaporize the
entire sample in a heated injection port, just
like split and splitless injections. However, in
direct injections, there is only one flow path
through the injection port.All of the carrier
gas is directed into the column and, hence, the
entire vaporized sample is directed into the
column aswell. This can be accomplished by
using a specially designed injection port liner.
Unliner
®
injection port liners have an internal
taper in one end that allows a direct connec-
tion between the liner and the capillary col-
umn.With this connection, the flow path from
the injection port body through the split vent
is blocked and all of the carrier gas flow is
directed into the capillary column. Figure 15
illustrates how aUniliner
®
injection port liner
with a Press-Tight
®
seal forms a leak-free
connection between the liner and the column.
16
5µL/sec.
injection rate
1µL/sec.
injection rate
0.2µL/sec.
injection rate
Figure 14.
The injection ratemust be slower for large volume splitless injections.
Solvent: Isooctane
1.
n
-C12
2.
n
-C14
3.
n
-C16
4.
n
-C18
5.
n
-C20
5µL injection
Chromatograms courtesy of Varian Instrument Co.
Figure 15.
AUniliner
®
liner forms a leak-tight seal
with the column, preventing the sample
from contactingmetal parts at the base
of a splitless injection port.
inlet seal
splitless
liner
Uniliner
®
with a
Press-Tight
®
seal
1
2 3 4 5
✶
For customer service, call
800-356-1688, ext. 3
(814-353-1300, ext. 3)
or call your local
Restek representative.
