This technique uses a seamless injection directly from the vial into the carrier gas stream without additional moving parts other than a valve and a needle. It is illustrated in the figure below.
The balanced pressure system, like other techniques, uses an incubation oven to thermostat the vial so the sample reaches equilibrium (step 1). During these initial steps, a needle is inserted into the vial and is then pressurised with a carried gas (step 2). After the vial is pressurised 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 (step 3).
Sample is extracted
Due to the fact that this technique uses a theoretical amount of time to inject the sample, the absolute volume of the sample is unknown.
This technique can be quite accurate but can also suffer from disadvantages, usually arising from the necessary sample transfer line. These negative aspects include sample carry-over and the fact that the injection port is always occupied and therefore not available for manual use.