Chromatography is the separation of a mixture into its individual components, achieved by the repeated partitioning of the components between two phases, one stationary, the second moving. In gas chromatography, the mobile phase is an inert gas such as helium or nitrogen; the stationary phase may be a solid such as silica used essentially in its natural state or coated with an inert non-volatile liquid. When analysing natural gas, the amount of each separated component is measured accurately.