Flow Velocity indicators

Indicates the linear velocity of mobile phase through the column. The flow velocity is represented four different ways: the open tube (or superficial) flow velocity, the interstitial flow velocity, the chromatographic flow velocity, and the reduced flow velocity.

Open tube (superficial) flow velocity

The open tube flow velocity is the linear velocity the mobile phase would have if the column were completely empty. It is calculated by:

where
us = open tube (superficial) flow velocity
F = flow rate
O = cross-sectional area of column

Interstitial flow velocity

The interstitial flow velocity is the linear velocity the mobile phase would have if it could only move between the particles (through the interstitial space). It is calculated by:

where
ue = interstitial flow velocity
εe = interstitial porosity

Chromatographic flow velocity

The chromatographic flow velocity is the linear velocity the mobile phase would have if it could move between the particles (through the interstitial space) and through the space within the particles. It is calculated by:

where
u = chromatographic flow velocity
εT = total porosity

Reduced flow velocity

The reduced flow velocity is the interstitial flow velocity normalized to the particle diameter and the molecular diffusivity of the solute in the eluent. It is useful for comparing different chromatographic conditions. In physical terms, it represents the relative contributions of convection and diffusion to axial transport of the analyte through the column. So, when ν = 1, convection and diffusion contribute equally to movement of the analyte through the column. When ν << 1, axial transport is dominated by diffusion, and when ν >> 1, axial transport is dominated by convection.It is calculated by:

where
ν = reduced flow velocity
dp = stationary phase particle diameter
De = molecular diffusivity of the solute in the eluent

Remarks

Different representations of the flow velocity are useful for calculating different chromatographic properties. For example, open tube flow velocity is used to calculate the pressure drop across the column while the reduced flow velocity is used to calculated the reduced plate height.