Evolution of Mixed-Mode Chromatography

Mixed-mode chromatography is emerging as a powerful tool in separation of various molecules. Mixed-mode is defined as liquid chromatography in which at least two modes of interactions exist simultaneously, both controllable by mobile phase selection. In the last few years, several companies have introduced new mixed-mode columns with different stationary phases (SIELC, Dionex, Imtakt, Sepax, Agela, etc).

Mixed-mode approach emerged due to a need for better retention control for both polar and non-polar molecules. Multiple mechanisms of interactions allow the use of one stationary phase for a much wider range of applications as compared to reversed-phase or ion-exchange approach. Simultaneous analysis of polar and non-polar, ionisable and neutral, and organic and inorganic compounds is possible, adding ability to reduce number of methods for pharmaceutical formulations and other complex mixtures. Modern mixed-mode stationary phases are designed with the purpose of creating controllable multiple interactions, and to provide reproducibility, high efficiency and loadability. Tools are available for efficient method development [1-3]

History of Mixed-Mode Chromatography
Mixed-mode interactions have been known for decades, and for the most part Liquid Chromatography column stationary-phase design has been dominated by the goal of elimination of extra interactions and obtaining a simple and predictable single
retention mechanism. Unfortunately, simplification of the retention process limits the range of compounds for simultaneous analysis, and reduces the ability to control elution order of analytes as well as the scope of available applications the system can be used for. As a response to this limitation, hundreds of different reverse-phase columns have been introduced in the past few years, all with slightly different characteristics. While working mainly by hydrophobic mechanism, those columns provide additional weak poorly controllable interactions.