Gas chromatography - specifically gas-liquid chromatography - involves a sample being vapourised and injected onto the head of the chromatographic column.The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used.Only a small amount of sample is injected into the column.Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.