Mixed-mode chromatography

Mixed-mode chromatography

Mixed-mode chromatography is a type of chromatography in which several modes of separation are applied to resolve a mixture of different molecules, specially in liquid chromatography.

Background

Ion exchange chromatography, normal phase chromatography, reversed phase chromatography, size exclusion chromatography, and charge exclusion chromatography are some of the typical methods used for separating molecules based on the differences of their physical characteristics (size, dipole moment, presence of charge, solubility etc.).

The classification of chromatography types by interaction is subjective to some extent, since they rarely exist in a pure form. As a result, the design of stationary phases in liquid chromatography is driven to eliminate all but one interaction.

Principle

There is an opposite trend to make stationary phases with more than one interaction type. If more than one mode of separation is used at the same time, then this becomes a mixed-mode separation. The typical example of mixed mode separation is chiral (chirality) separation. To resolve a pair of optical isomers the simultaneous interaction of more than one type is required. In other types of liquid chromatography, mixed-mode separation also is not uncommon.

While in many cases one type of separation provides sufficient selectivity, there are many analytical situations where one separation mode can not completely resolve a mixture. In that case, a properly chosen and more complex interaction (mixed-mode) can provide a desired analytical result.

Examples

Example of a mixed-mode separation is provided by combinational phases with ion-exchange and reversed phase characteristics at the same time.
These stationary phases are available from several column manufacturers.


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