Measurement:
Different terms arise because the measurement of σ can be made at different heights in the peak. At the base of the peak wb is 4σ, then the numerical constant is 4 2 or 16. At half height, wh 2.354σ and the constant becomes 5.54. Independent of the symbols used, both the numerator and denominator must be given in the same units and, thus, N is unitless. Classically, both the retention time and the peak width are measured as distance on the chromatographic chart. Instead, both could be in either volume units or time units. No matter which calculation is made, a huge value for N denotes an efficient column that is highly desirable. A related parameter which expresses the efficiency of a column is the plate height, (H)
wH = L/N
while, L is the column length and H has the units of length and is better than N for comparing efficiencies of columns of differing lengths. It is also known as the Height Equivalent to one Theoretical Plate (HETP), a term that carried over from distillation terminology. A good column will have a large N and small H.