Principles and Applications of Gradient Elution Chromatography

Key Insight

Gradient elution chromatography involves modulating the binding strength of feed components by varying the concentration of a mobile phase modifier at the column entrance. This process is analogous to using temperature or pressure in gas chromatography, though the effects of the modifier are generally more complex as it can compete for adsorption with feed components.

Gradient elution is widely used in protein chromatography due to the complex nature of protein mixtures, which often exhibit a broad range of adsorption strengths. It allows for the separation of weakly retained components early in the gradient and strongly retained species later. A key reason for its utility is the extreme sensitivity of biopolymer retention to mobile phase composition; for example, in reversed phase chromatography (RPC), minute changes in organic modifier can shift proteins from complete to no retention. Gradient operation provides a more robust process, averting the need for precise mobile phase control required by isocratic methods.

The type of gradient implemented depends on the interaction chemistry: RPC uses increasing organic modifier, ion exchange chromatography (IEC) uses increasing salt or pH, and hydrophobic interaction chromatography (HIC) uses decreasing salt. While pH gradients offer powerful separations and elution at low ionic strengths, they are less common due to control difficulties and the risk of inducing protein precipitation, aggregation, or unfolding near their isoelectric point. Gradient shapes vary from common linear gradients to advantageous non-linear or step gradients for proteins, with shallow, smooth gradients often required for difficult separations like resolving protein charge variants.