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These few measurements are enough to evaluate the performance of a single column. By way of example, let's calculate the plate number (N) for the peak in Figure 3.4. The math is not difficult:

(3.3)

Inserting the data from Figure 3.4 gives:

A plate number of 576 is very low and would indicate a very inefficient column, since most columns generate about 2000 plates per meter. Of course, this peak is intentionally drawn wide so the measurements are clearly seen. A real peak would be much narrower than this: a more typical peak might have one third of the width and nine times the plate number.

The alternative equation using the peak width at half height (w_0.5) is:

(3.4)

Chromatographers calculate plate number as a way to evaluate the performance of a column. Any change of operating parameters that increases plate number automatically improves the separating power of a column.

Another performance indicator is the plate height (H), which is the length of a column required to generate one plate. It's usually reported in mm. Knowing the column length (L), you can easily calculate the plate height:

(3.5)

Plate height gives a measure of column performance that is independent of the column length and is the primary variable used for optimizing performance.