Gas Supplies

In early instruments, gases were obtained compressed in gas cylinders or 'tanks'. These gas reservoirs were fitted with reducing valves that supplied gas to the gas controllers at the pressure recommended by the manufacturer (usually about 50 psi). Contemporary instruments, however, are often supplied with gas generators that may provide nitrogen or oxygen from the laboratory compressed air supply. In addition, hydrogen and oxygen can be generated by an electrolytic process that is claimed to be less expensive to operate. Both sources of gas are equally acceptable from a chromatographic point of view. Helium and argon gases are still both supplied in tanks.

The first gas chromatographs were fitted with pressure controllers to ensure that a constant pressure was applied to the column. In practice, however, applying a constant pressure to a column does not ensure a constant flow of mobile phase though it for all operating conditions. For example, in most GC analyses, temperature programming is often employed, where the temperature of the column oven is continuously raised during the development of the separation. Increasing the column temperature continuously reduces the distribution coefficients of the solutes remaining in the column. As shown from the plate theory, this also reduces their retention volumes that cause the solutes to be accelerated and pass more rapidly through the column and, in the process, reduce the retention times.

Unfortunately, as the temperature of a gas is raised, so does its viscosity, and, thus, at a constant inlet pressure, the flow rate will fall. The reduction in flow rate will depend on the temperature program limits and to a certain extent on the temperature gradient. To limit this change in flow rate, contemporary gas chromatographs are fitted with mass-flow controllers that ensures a constant mass of mobile passes through the column per unit time irrespective of the system temperature.