Why must carrier gas in GC be pure?

[Sandra7]

What eaxctly is the reason that carrier gas in GC must be pure?

[Don_Hilton]

A couple of reasons come to mind right away:

If contaminants can be detected, they add to the detector signal For example, a small quantity of methane (a possible contaminant in helium, which is obtained as a byproduct of natural gas production) would increase the background on and FID.

If conaminants can condense in cool spots in the GC, they will form peaks. So, if you have hydrocarbons from a pump oil present in the carrier gas they will collect on the column when it is cool. When you do a temperature programmed run, the column heats up and the various compunds begin to partition into the gas phase as the temperature rises - resulting in the presence of peaks, even with no injection being made.