Column efficiency or number of theoretical plates

Column Efficiency

Figure (mentioned below) visually summarizes the fundamental concepts, calculations, and impact of the efficiency term on HPLC to UPLC evolution.

Section A

In HPLC chromatograms, peaks typically manifest in a Gaussian shape, a result of band-broadening effects within the column. This phenomenon is fundamental to all chromatographic separation processes. The term "plate number" (N) serves as a quantitative gauge of the column's efficiency. Put simply, more plates indicate better chromatography, while fewer plates signify poorer chromatographic performance.

Section B

The concept of a plate originates from distillation processes employing a distillation column composed of individual plates. In this setup, the condensing liquid reaches equilibrium with the ascending vapor. A longer distillation column correlates with more "plates" or equilibration stages. Similarly, in chromatography, the height equivalent of a theoretical plate (HETP or H) equates to the total length of the analytical column (L) divided by the number of theoretical plates (N), despite the absence of discrete plates within the HPLC column.

Section C

Column efficiency, quantified as the rate of zone spreading, is calculated by deriving the ratio of retention time to peak width, squared, typically expressed as N. When measuring peak width at half height, the constant is 5.54. If measured at the base, a constant of 16 must be applied to obtain a comparable N value. For a valid determination of column efficiency, retention time and peak width should be measured in the same units.

Section D

The highlighted equations establish the correlation among the three equations, illustrating how Resolution links to plate number, and how plate number correlates with the particle size of the column.

Section E

Reducing particle size results in a greater number of theoretical plates and enhanced separation capabilities, albeit at the cost of increased pressure. Consequently, HPLC manufacturers developed a new iteration of HPLC capable of operating within significantly higher-pressure ranges. Waters Company branded this new design as UPLC, while other manufacturers have their own names for similarly advanced forms of HPLC.