Migration rate
of solutes in a chromatographic separation:
Average
linear velocity of the solute
where tR
is the retention time
Average
linear velocity of the mobile phase
where
tm is the dead
volume
or retention time of unretained
peak
Distribution
constant and retention time
|
Solute velocity = |
mobile phase velocity x |
fraction
of time spent in stationary phase |
and
|
fraction
of time spent in stationary phase |
=
u x |
moles
of solute in mobile phase total
moles of solute |
substitute
into expression for K
but
these parameters are hard to measure
The Capacity
Factor, k'
defined
as:
so
now
substituting
equations for v and u
and
rearranging:
all
of which can be measured experimentally.
Separation: The Selectivity
Factor, a
easier
to measure:
substituting
so
the difference in retention time leads to separation, and it is related to the
k' of each compound