# Plate Theory and Extensions - Effective Plate Number > Page 66

Another
parameter that may be of interest would be the minimum value of (a) for a solute pair that can be resolved by
a specific column. The minimum value of (a)
has also been suggested (17) as an alternative parameter to compare the
performance of different columns. There is, however, a disadvantage to this
type of criteria, as the value of (a)
becomes *less* as the resolving power of the column becomes *greater*.
Nevertheless, a knowledge of the minimum value of (a_{A/B})
can be valuable in practice, and it is of interest to know how the minimum
value of (a_{A/B}) is related to the effective plate
number.

The minimum
value of (a_{A/B}) of a pair of solutes can be
assumed to be equal to the ratio of the retention distance of the first peak
plus its width at the base, to its normal retention distance. This assumes that
resolution is satisfactory when the peak maxima are separated by 4s.

Thus,

Again, bearing in mind that

Therefore, (59)

Employing
equation (59) the minimum (a_{A/B}) can be
calculated, and compared with either the resolution, as defined by Giddings, or
the number of effective plates. However, it must be emphasized that all the
parameters will also depend on the (k') of the eluted solute.