# Plate Theory and Extensions - The Retention Volume of a Solute > Page 13

In practice, the retention volume of an unretained peak eluted at the dead volume (Vo), will not be simply the total volume of mobile phase in the column but will be made up of the volume of mobile phase (Vm) plus extra-column volumes that arise from sample valves, connecting tubes, unions, etc. (VE).

Thus                       Vo = Vr(o) + VE

= Vm + VE                                   (14)

If (VE)<<(Vm), (VE) can be ignored, but for accurate measurements, (VE) should be measured and taken into account.

Thus,          Vr(measured) = Vm + KVS + VE                         (15)

Including second order effects causes the relationship to become more complicated. The dead volume is made up of many components, and will be examined in more detail later. At this time the retention volume equation will be examined in more detail.

Returning to equation (13), the adjusted retention volume, (V'r) as defined will be given by,

V'r = Vr - Vo

Thus, from equations (14) and (15),

V'r = Vm + KVS  + VE -(Vm + VE)

and,                              V'r =  KVS                                          (16)