Gradient elution in HPLC

The general elution problem. A Normal elution. B Gradient elution. From L. Ft. Snyder, J. Chromatogr. Sci. 8, 692 (1970) Reproduced by permission of Preston Publications. 

The choice of solvents for gradient elution is still somewhat empirical however, using the data from Table 3.5 narrows the choices. Modern HPLC instruments are equipped with solvent programming units that control gradient elution in a stepwise or continuous manner. 

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Gradient elution in HPLC is achieved using two pumps, two different solvent reservoirs, and a solvent mixer. In capillary electrophoresis (CE), electro-osmotic flow controls the flow of the mobile phase, which is, in most cases, an aqueous buffer and is used in place of a mechanical pump. 

The analogue of temperature programming in GC or gradient elution in HPLC is density programming [25] the pressure is changed so that a linear or asymptotic density profile is achieved according to the above polynomial. From eqn (9.3) it can be shown that there is a threshold density which all members of the homologous series co-elute. 

Pulsed amperometric detection (described below) delivers selective and sensitive measurements, also allowing gradient elution in HPLC. This detection technique has been a great breakthrough in carbohydrate analysis. 

In GPC instrumentation the pumping system delivers a single eluent throughout the analysis. It may be a single solvent, a blend of solvents, or a solvent plus an additive. See. II.C, but the composition remains constant. Solvent programming, also termed gradient elution, in HPLC requires more eapability from the solvent delivery system to ensure precise flow and repro-dueible solvent eomposition profiles. 

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