When pumping mobile phase through a particular bed, a certain pressure drop
*δP* over the column will result. The relationship between particle diameter
*d _{p}* and the flow velocity

Accordingly,
*δP* is proportional to the linear velocity
*u*, the column lengths
*L* and the viscosity of the mobile phase
*η*, and inversely proportional to the constant
*k _{0}* and the square of the particle diameter

The equation above shows that the interstitial porosity
*ε _{i}* has a tremendous influence on the permeability of the bed and therewith also on
the pressure drop. For a well packed reversed phase be the interstitial porosity can be assumed to
be 0.33 which leads to

Broad distribution has two negative effects:

- the largest particles control the efficiency and
- the smallest particles control the pressure drop.

Hence, broad particle size distribution should be avoided.

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