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,