Classical Modes of Chromatographic Operation

Key Insight

The three classical modes of column chromatography, defined in 1943 by Tiselius, are elution chromatography, frontal analysis, and displacement development. Though yielding different results—elution dilutes products, frontal chromatography yields a single pure component, and displacement can concentrate products—they share common operational features. All three are performed in columns using similar equipment, can frequently utilize the same stationary phase, are affected by the same equilibrium and dispersive factors (though relative importance varies), and can be described by the same engineering models and mathematical analyses. The initial step in all is supplying the feed, with subsequent separation determined by the mobile phase modifier's properties.

Elution chromatography involves a mobile phase modifier with lower affinity than any feed component, causing it to travel ahead of the feed. Components then migrate at rates dependent on their affinity for the adsorbent at the modifier's concentration in the column. Isocratic elution uses a constant modifier concentration throughout, often chosen for analysis with small feed injections where adsorption equilibrium is essentially linear (Henry's law limit), allowing components to travel independently and be associated with unique retention times. However, it is inefficient at higher loadings where isotherms become non-linear, leading to concentration-dependent retention, skewed peaks, and poor separation for mixtures with widely differing adsorptivities. Gradient elution varies modifier concentration over time, making it suitable for separating components with widely different affinities or high sensitivity to mobile phase composition. This reduces elution time for strongly retained species, increases productivity by reducing band spreading, and is often the only practical method for biopolymers due to their sharp adsorption transitions. Both isocratic and gradient elution generally increase separation with column length but also result in greater product dilution.

Frontal analysis continuously supplies the feed mixture under conditions where components are strongly and often competitively adsorbed. The least strongly adsorbed species forms a pure component band, followed by the advancing feed front which eventually saturates the column. This technique is best for removing strongly adsorbed impurities from an unretained product, allowing large feed volumes to be processed before impurity breakthrough. It yields only a single pure component in pure form and almost completely avoids product dilution, sometimes concentrating the weakly adsorbed component due to competitive binding, though this can pose solubility challenges. Displacement chromatography loads the column partially with feed, then introduces a 'displacer' species adsorbed more strongly than any feed component. The displacer front desorbs and concentrates feed components downstream, which then competitively readsorb. If the bed is sufficiently long, feed components distribute into an 'isotachic train' of adjacent pure component bands, where each upstream component displaces the one downstream, ideally emerging as rectangular bands in order of increasing affinity. This achieves multi-component separation and often simultaneous concentration (e.g., 12 times for cytochrome c, 20 times for lysozyme in one example), allows high feed loadings, and efficiently utilizes adsorbent capacity. However, separation does not improve by increasing column length once the isotachic pattern is attained, and its application to biopolymers is challenging due to difficulties in finding suitable, non-toxic, cost-effective, and recyclable displacers.