28.2: Column Efficiency in Liquid Chromatography

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In Chapter 26 we considered three factors that affect band broadening—multiple paths, longitudinal diffusion, and mass transfer—expressing the relationship between the height of a theoretical plate, \(H\), as a function of the mobile phase's velocity, \(u\), using the van Deemter equation

\[H = A + \frac{B}{u} + Cu \nonumber \]

where \(A\) is the contribution from multiple paths, \(B\) is the contribution from longitudinal diffusion, and \(C\) is the contribution from mass transfer. Unlike gas chromatography, where there is little distance between the point of injection and the column, and little distance between the column and the detector, an HPLC instrument must often include additional tubing to connect together the sample injection port and the column, and the column and the detector. Solutes moving through this tubing, which does not include stationary phase, travel with a velocity that is slower at the walls of the tubing and faster at the center of the tubing; the result is additional band broadening. The magnitude of this contribution to band broadening is minimized by keeping the length of connecting tubing as short as possible. by using tubing with a smaller internal diameter, and by using lower flow rates.