Mobile Phase (Carrier Gas):

A carrier gas is generally available in a compressed gas cylinder. It is used to transport the vaporized solutes from the injection port through the column to the detector. A carrier gas must be

i) Inert,

ii) Dry, and

iii) Virtually free of impurities.

It must be inert to an extent to which no chemical reactions could take place along with any portion of the instrument, column or solute. It must be dry since water could and commonly does react with a few portion of the column. This results in loss of resolutions and tends to generates asymmetric or "tailing" peaks. Unwanted compounds or ghost peaks might also appear. Another effect is a net loss of sensitivity, which is, the ability to quantify small concentrations of solutes. The dotted lines in Figure display the effect of water within carrier gas on column performance.

Figure:  Effect of water on column efficiency