# 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 (aA/B) can be valuable in practice, and it is of interest to know how the minimum value of (aA/B) is related to the effective plate number.

The minimum value of (aA/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 (aA/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.