Mixed-mode chromatography

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

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Ion-exchange chromatography, normal phase chromatography, reversed phase chromatography, size exclusion chromatography, and charge exclusion 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 types of interaction are 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. 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.

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.