[firzzy87]

what is the effect of column temperature on the retention times??

what is the differences btwn isothermal and temperature programming in GC ??

[JGK]

Increasing temperature will reduce RT (both in HPLC and GC).

A temperature gradient in GC has the same effect as a solvent gradient in HPLC. It is used to optimize resolution of analytes.

[SemiAuto]

Increasing column temp. (oven temp) will result in the components spending less time interacting with the walls of the column - which takes time.  So in a hotter over then the components will be speeding along in the carrier gas stream toward the detector.  In general the longer the components are interacting with the inside column surface - the better chance of separating them before they hit the detector.
Isothermal = one temperature the entire GC run.
Programmed run allow you to speed off peaks that may take too long to come off at the original starting temperature, thus allowing you to get your run over with quicker and not wasting time.