The dispersion that takes place in an open tube is described by the Golay equation (7), (see Dispersion in Chromatography Columns ) but under conditions where there is no stationary phase present and thus, k'=0.

As a result, the variance due to an open tube (s_tu²)will be

(8)

where (u)	is the linear velocity of the mobile phase,
(r)	is the radius of the tube,
and (Dm)	is the diffusivity of the solute in the mobile phase.

If u >> D_m/r (which will be true for most practical chromatographic systems, then equation (8) reduces to,

(9)