# 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 (V_{o}),
will *not* be simply the total volume of
mobile phase in the column but will be made up of the volume of mobile phase (V_{m})
plus extra-column volumes that arise from sample valves, connecting tubes,
unions, etc. (V_{E}).

Thus
V_{o = }V_{r(o) + }V_{E }

_{ }

_{
= }V_{m + }V_{E (14)}

_{ }

If (V_{E)}<<(V_{m}), (V_{E}) can be ignored, but for accurate
measurements, (V_{E}) should be
measured and taken into account.

Thus, V_{r(measured)} =_{ }V_{m }+ KV_{S} +_{
}V_{E (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} = V_{r} - V_{o }_{ }

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

V'_{r }= V_{m} + KV_{S} + V_{E }-(V_{m + }V_{E})

and,
V'_{r} = KV_{S} (16)

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