# Plate Theory and Extensions - Effective Plate Number > Page 63

# Effective Plate Number

The *effective
plate number* was introduced by Purnell [13], Desty [14] and others in the
late 1950s. It was evoked as a result of the introduction of the capillary
column or open tubular column. The open tubular column could be constructed to
produce efficiencies of up to a million theoretical plates [15] even in 1960.
However, it was clear that the high efficiencies were only obtained at very low
(k') values, close to the column dead volume. More importantly, the high
efficiencies did not correspond to the greatly increased resolution that would
be expected. The poor performance of columns having high efficiencies at low
(k') values is precisely due to the effect just described. To provide a more
rational measure of column performance that would appear commensurate with the
resolution obtained, the *effective plate number* was introduced.

The effective
plate number is calculated in the same way as column efficiency, but uses the *corrected
retention distance*, as opposed to the total retention distance in conjunction
with the peak width. As a consequence, the effective plate number is
significantly smaller than the number of theoretical plates at low (k') values.
The column efficiency and the effective plate number converge to the same value
at high (k') values. It follows, that the effective plate number more nearly
corresponds to the actual resolving power of the column. Although the
theoretical plate, as defined by the plate theory, has a practical significance
and can be used in column design, the concept of the *effective plate* is
not theoretically unsound and is related directly to the theoretical plate.

The efficiency of a column (n), in number of theoretical plates, has been shown to be given by the following equation,