Selecting Separation Modes and Selectivities for Multi-Dimensional LC

One of the most important steps in the development of any 2D-LC method involves choosing which modes of separation to use (e.g., reversed-phase (RP), ion-exchange, etc.). The rationale leading to this step involves not only the characteristics of the analytes in the sample at hand, but also the likelihood that the two separations chosen at this stage will work well together in a 2D separation system. In this chapter we first discuss how the concept of “sample dimensionality” can be used as a framework for choosing which two separation modes to use. We then go on to discuss how different combinations of modes do or do not work well together in 2D systems, and briefly discuss strengths and weaknesses of individual separation modes as they relate to multi-dimensional separations. Finally, in the specific case of RP separations we summarize strategies that have been used to select specific stationary phases for use in 2D-LC separations involving RP in at least one dimension. This alone can be a challenge given the commercial availability of more than 1,000 different phases.