# Extra Column Dispersion - Chromatographic Apparatus for Trace Analysis > Page 58

and _{} (18)

where (l) | is the column length, |

(r) | is the column radius, |

and (e) | is the fraction of the column volume that is considered filled with mobile phase (usually taken as 0.65) |

Consequently,
substituting for (V_{r}) in equation (16) from equation (17) and for (V_{o})
from equation (18),

_{} (19)

Thus (m_{D}), the mass sensitivity of the
chromatographic system depends on the detector
sensitivity, column dimensions, column efficiency and the capacity factor of the eluted solute.

However, irrespective of the column properties, the mass sensitivity is still directly related to the detector sensitivity. It will also be shown (see Book 22 on column optimization) that the column radius will also depend on the magnitude of the extra–column dispersion. It follows that the design of the chromatographic apparatus, the detector and detector electronics (to minimize extra column dispersion) together with the detector sensitivity will determine the mass sensitivity of the overall chromatographic system.