Column Efficiency: Plate Theory

The chromatographic column efficiency describes how much a solute band broadens during separation. It was originally defined by the plate theory of chromatography, named for the plates in a fractional distillation column.

The theory visualizes a column as consisting of discrete yet continuous narrow layers called theoretical plates, with the solute repeatedly equilibrating between the mobile and stationary phases as it travels down the column.

Although it is now known that no such equilibration steps occur, the primary term measuring column efficiency is still called the plate number or number of theoretical plates, symbolized N.

As chromatographic peaks are nearly Gaussian in shape, N expresses peak broadening in terms of variance—the square of the standard deviation—and it relates to retention time and peak width at the base or half of the height.

Another term originating from plate theory that describes column efficiency is the height equivalent to a theoretical plate, or HETP, which is the column length divided by N.