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The injected volume of a gas sample is typically less than 0.25 mL.

The number of molecules in a fixed gas volume increases with pressure, so it's necessary to maintain constant sample pressure for each injection. Therefore, most PGCs block and bleed the sample line to allow the sample gas to come to atmospheric pressure, a technique known as atmospheric referencing. Chapter 7 discusses some valve systems to achieve this.

But that still leaves the normal variation of atmospheric pressure, which is quite small, as you can see by inspecting any barometer. Discounting stormy weather, the jobsite pressure variation should not be more than about ±2 %.

In practice, atmospheric referencing works well enough for most applications. If greater precision is desired, it's best to measure local barometric pressure and adjust the measurement values to compensate for any variation found.

Some PGCs have a sensor to measure the absolute pressure of the gas sample and use an algorithm to correct for detected changes.