Instrumentation
Gas-Tight Syringe Injection
Use of a gas-tight syringe autosampling system is one of three common techniques
(gas-tight syringe, balanced pressure, and pressure loop) used to transfer a
headspace sample.Most of the autosampling units can retrofit to a standardGCwith
a split/splitless injection port, making them relatively simple to use and understand.
These systems do not require the use of special configurations or special instrumen-
tation other than the autosampler itself. The gas-tight syringe autosampler is
beneficial for usewith diverse samples because of the variety of sampler configura-
tions andmethod options available.
The gas-tight syringe technique operates by initially thermostatting the sample in an
incubation oven at a given temperature and for a given time until it has reached a
state of equilibrium (
Figure 6, Step 1
). Once the sample has reached an equilibrium,
an aliquot is taken from the headspace using the gas-tight syringe (
Figure 6, Step
2
), and the aliquot is injected into theGC as if it were a liquid sample injection
(
Figure 6, Step 3
).
Several concerns exist regarding this technique. Because the sample is being trans-
ferred from a heatedoven, the syringe alsomust be heated to ensure that the samplewill
not recondense in the syringe.Manymanufacturers have taken this into consideration
and their samplers now comewith a heated syringe assembly. There also are reproduc-
ibility issues because of possible sample loss.As the sample is transferred from the vial
to the injectionport, some of itmaybe lost because of the pressure differences between
the vial and atmospheric conditions. Beyond these concerns, the gas-tight syringe
technique is simple touse, can retrofit into a varietyofGC systems, and is best suited
for diverse samples. Examples ofmanufacturers andmodels of the gas-tight syringe
units are: theThermoQuest TRACE
™
HS2000 andHS850 (
Figure 7
)Headspace
Autosamplers and theLeapTechnologiesCTCCOMBI PALSampler.
Balanced-Pressure System
Figure 7
Gas-tight syringe autosampler
TRACEHS850
6
Figure 6:
Gas syringe system
Step 1
Sample reaches
equilibrium
Step 3
Sample is injected
Step 2
Sample is extracted from
headspace
Another common technique is the balanced-pressure system, which is capable of
generating resultswith a high degree of repeatability. It uses a seamless injection
directly from the vial into the carrier gas streamwithout additional moving parts other
than a valve and a needle. The balanced-pressure system, like other techniques, uses
an incubation oven to thermostat the vial so the sample reaches equilibrium (
Figure 8,
Step 1
). During these initial steps, a needle is inserted into the vial and then is
pressurizedwith a carrier gas (
Figure 8, Step 2
).After the vial is pressurized and
equilibrium has been reached, the valve is switched for a specific amount of time to
redirect the sample into the transfer line and onto the column (
Figure 8, Step 3
).
