# Capillary Chromatography - Capillary Column Theory 4

It is seen that for the column 100 mm in diameter and 30 m long, the resistance to mass transfer in the gas phase is at least 20 times that in the stationary phase and this *smallest *value only occurs at low capacity ratio values (*ca*. one or less) and very low mobile phase exit velocities (*ca* 0.5 cm/s.). Capillary columns are rarely operated at exit velocities of less than 20 or 30 cm/s. and at such velocities, the resistance to mass transfer in the stationary phase is less than 2% of the total resistance to mass transfer and this is still at a (k') value of unity. It follows that for this column, the dominant resistance to mass transfer dispersion effect occurs in the mobile phase, except possibly at (k') values close to zero (when the solute is eluted close to the dead volume of the column). Consequently, under most conditions used in practice, the resistance to mass transfer in the mobile phase will be the controlling effect on peak dispersion.

Column Length 30 m Column Diameter 100 mm

**Figure 15. Graph of Resistance to Mass Transfer Ratio against Mobile Phase Exit Velocity.**