What is the function of the “O ring” associated with the liner in the GC inlet?

The O ring forms a seal between the glass inlet liner and the wall of the GC inlet, effectively sealing the bottom half of the injection port from the top half. This seal ensures that gas flows down through the liner (rather than around the outside) before either entering the GC column or existing through the split vent of the injection port.

O rings with different temperature ratings can be used, and one that is rated for the temperature your injection port is to be used at should be selected.