The purpose of a gas standard is to establish accurate reference points for the verification of an analysis.
Static mixtures in gas cylinders are often unreliable and inaccurate at the levels (ppm to ppb) that analytical technology now demands.
In gas cylinders containing low level mixtures, the trace components often adsorb on to, or react with, the cylinder walls causing the standard to be inaccurate and unreliable.
Also, it may be impossible to get a matrix gas that is completely free of the trace component, thus making it impossible to get a true reference point with a cylinder mixture. These problems are solved by using permeation tubes in lieu of gas cylinders.