Methods for Describing Column Efficiency
These parameters are used to measure a column’s resolving power (ability to differentiate between closely migrating peaks  sometimes called efficiency). The two parameters are related by the following equation: where L is the length of the column packing (usually in cm  this is a constant for a given column). The term theoretical plates refers to an early explanation of the resolving power of a column where the column was viewed as a ‘stack’ of closely related but discrete layers (or plates). The more plates in the column the greater the resolving power of the column but as the column has a definite length, more plates equals thinner plates. In other words N and H are inversely proportional. As N increases (for a given column) H decreases. There are great differences in the plate height and numbers of theoretical plates between columns due to differences in the column type as well as the stationary and mobile phases used. Thus variations of the following magnitudes exist:
There are a number of complex methods for determining N but a simple approximation of N is given by: where W/2 = half of the width of the peak at its base, which is also equal to the width of the peak at half its maximum height and t_{g} = retention time (time taken for the peak to come off the column) of the analyte. Sample problem: Solution Practical application of N
