2.3.1 Carrier Gases
The first important component is the carrier gas or mobile phase. For a basic GC system, extremely pure helium is usually used, and hydrogen is less frequently encountered due to its explosive nature. Helium is used due to its inertness, non-reactive nature, and the shape of its van Deemter curve that allows for a relatively wide range of optimum mobile phase linear velocities. The common grade of helium used is referred to as “five-nine gas”, meaning that it is 99.999% pure. But this level of purity is still not sufficient for most systems when trace (parts per million or parts per billion) analyses are being conducted. Before entering the GC, the 2500 psi (18 000 kPa) pressure in the gas cylinder is reduced to approximately 60 psi (400 kPa) with a two-stage regulator before entry into the GC. But first, the He gas is passed through at least one resin trap to further remove hydrocarbons, oxygen, trace analytes, and/or water vapor that could interfere with analysis, degrade the column or interfere with the detector.
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