Selectivity and the HPLC Mobile Phase

Stationary phase (i.e. column type) is a key contributor to selectivity in LC. But what about the mobile phase? A significant advantage, and sometimes challenge, in LC is that both the stationary and mobile phase chemistries can be modified to optimize our method. This contrasts with GC, where the main selectivity option is the stationary phase. LC mobile phase chemistry can be altered by changing the pH, adding ion pair reagents or other modifiers, or changing the type of organic solvent used. For example, phenyl-based stationary phases can offer significantly different selectivity for aromatic, nitro-, and sulfone-containing analytes when using methanol vs. acetonitrile. This is demonstrated for a series of NSAIDs – with acetonitrile, the selectivity of a Biphenyl phase is very similar to a C18. With methanol, however, the Biphenyl vs. C18 selectivity is radically different (see figure). Mobile phase chemistry – another useful tool in LC method development!