# Extra Column Dispersion - Dispersion in Tubular Conduits > Page 22

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_{tu}^{2})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)