Comparison of the separation characteristics of the organic-inorganic hybrid octadecyl stationary phases XTerra MS C18 and XBridge C18 and shield RP18 in RPLC

Differences in the system constants of the salvation parameter model, discontinuities in retention factor plots (log k against volume fraction of organic solvent) and retention factor correlation plots are used to study the retention mechanism on XTerra MS C-18, XBridge C-18 and XBridge Shield RP18 stationary phases with acetonitrile-water and methanol-water mobile phases containing from 10 to 70% (v/v) organic solvent. Wetting of XBridge C-18 at 10 and 20% (v/v) acetonitrile is incomplete and is responsible for small changes in the retention mechanism. The intermolecular interactions responsible for retention on XTerra MS C-18 and XBridge C-18 are similar with minor differences in cavity formation and hydrogen-bonding interactions responsible for small selectivity differences. On the other hand, for bulky solutes there are large changes in retention at low volume fractions of organic solvent (<40% v/v) associated with steric repulsion on the XTerra MS C-18 stationary phases that is absent for XBridge C-18. Selectivity differences are more apparent for XBridge C-18 and XBridge Shield RP18. For acetonitrile-water mobile phases cavity formation in the solvated XBridge Shield RP18 is slightly more difficult and hydrogen-bond acid and base interactions are more important than for XBridge C-18- With methanol-water mobile phases it becomes slightly easier to form a cavity in the solvated XBridge RP18 compared with XBridge C-18. In addition, the methanol-water solvated XBridge RP18 is a stronger hydrogen-bond base and more dipolar/polarizable than XBridge C-18. These variations in selectivity justify the use of XBridge C-18 and XBridge Shield RP18 as complementary stationary phases for method development.