A number of novel elution strategies are available when using a liquid stationary phase. This is because you are now operating with two liquid phases, rather than a liquid/solid system in the column. When compared with solid-liquid systems where the roles of the two phases are fixed, these novel strategies yield significant performance advantages in terms of separation time, loading and throughput constraints as a consequence of sample solubility) and throughput per se. These new possibilities are outlined below:
'Standard' or 'Classical' elution mode
In counter current chromatography, one of the liquid phases is retained within the column whilst the other is pumped in one direction only through the column where it interacts with the retained phase. The retained liquid is the stationary phase (SP) and the pumped liquid is the mobile phase (MP). Both phases can be mobile or stationary and you can therefore perform a separation in either normal phase (NP) mode where the less polar, always organic, liquid is used as the MP or reverse phase (RP) where the more polar, usually aqueous, liquid is the MP.Elution-Extrusion (EECCC)
This strategy makes use of the fact that when using standard elution mode as described above, compounds may be fully separated inside the column long before elution. Since you are using a liquid stationary phase, it is possible to obtain separated compounds without completing the full standard elution cycle.
In EECCC mode, you start the separation in the same manner as in standard mode counter current chromatography. However, when your run reaches a certain point you stop the mobile phase flow and pump in stationary phase to extrude the column contents. This enables your purification cycle time and solvent usage to be significantly reduced. After extrusion, the column has been completely refilled with fresh SP and is ready for your next sample injection.
Dual-mode
When operating a dual-mode elution strategy, you first pump one of the phases as the mobile phase (e.g. normal phase operation) and after a set period of time you pump the other as mobile (e.g. reverse phase operation). You can repeat this switching procedure a number of times until your desired resolution is achieved. The advantage of this method is that compounds with a strong affinity for the original stationary phase i.e. would be strongly retained with a long classical mode elution time, can be separated and eluted quickly.pH Zone Refining
This elution strategy depends upon the phenomenon that more polar, charged entities (ions) distribute preferentially into aqueous phases and uncharged, less polar species into organic phases. The technique and utilises basic organic phases and acidic aqueous phases (or vice versa). The analytes dissolved in the stationary phase are eluted by mobile phase according to their pKa values and solubility. This strategy enables you to achieve very high loading capacity and high resolution separations for those molecules that have the necessary characteristics appropriately ionisable functionalities.
