Figure 8:
Balanced-pressure system
Because this technique uses a theoretical amount of time to inject the sample, the
absolute volume of the sample is unknown. However, this technique is highly repro-
ducible because the number ofmovingparts areminimized, whichdecreases the
chance for compound adsorption and loss via leaks.An example of a balanced-
pressure system is theHS40XLmanufacturedbyPerkin-Elmer (
Figure 9
).
Pressure-Loop System
The last common injection technique discussed in this guide is the pressure-loop
system. Unlike balanced-pressure, the pressure-loop system uses a known amount of
sample. This technique typically uses a six-port valve, and initially thermostats and
pressurizes the vial as in the previously described techniques (
Figure 10,
Step 1
).After pressurization, the valve is turned and the loop is filledwith the
sample (
Figure 10, Step 2
).After the loophas been filled, the valve is turned again to
redirect the gas flow and flush the sample into the transfer line leading to the analyti-
cal column (
Figure 10, Step3
).
Figure 9
Balanced-pressure autosampler
Perkin-Elmer HS 40XL
The pressure-loop system has several advantages and disadvantages. One of the
advantages of this system is that the loop can be thermostatted to high temperatures,
which helps to lessen adsorption of highermolecular weight and sensitive com-
pounds. The fixed volume of the sample loop also helps to improve run-to-run
reproducibility.A disadvantage of a pressure-loop system is that it may cause ghost
peaks because of sample carryover from a previous analysis.
1
Severalmakes and
models of pressure-loop systems include theOIModel 4632 (
Figure 11
),Varian
Genesis, Tekmar 7000HT, and theHP 7694E.
Step 1
Sample reaches
equilibrium
Step 3
Sample is extracted and
injected
Step 2
Pressurization of injection
Figure 10:
Pressure-loop system
Step 1
Sample reaches
equilibrium/pressurization
Step 3
Sample is injected
Step 2
Sample is extracted from
headspace
7
Figure 11
Pressure-loop system
OIModel 4632
outlet
inlet/outlet
