Injection temperature must be low enough so no decomposition of the volatile sample occurs and must be high enough to vaporize the sample so no discrimination occurs as the sample passes from the injector onto the column. It is not required the temperature be above the boiling point of a compound. That temperature is determined at standard conditions and the injector of a GC is not at standard conditions. What is required is the complete physical phase change from liquid to vapor for the sample amount that is injected.
In general, temperatures below 250°C are adequate for most volatile GC samples.
The detector temperature is usually recommended to be 20°C above the final column temperature so deposition of sample or column phase in the detector does not occur.
Column temperature is limited by the type of liquid phase coating the solid phase, and in columns without liquid phase, the temperature at which the solid phase begins to be affected and change in its physical characteristics.
So there is no simple rule to your query. Read the literature and you will begin to see appropriate temperatures for different compositions of samples.
best wishes,
Rodney George
