Effect of the overall buffer concentration in the mobile phase on the resolution of sample pairs: methionin enkephalin and val-tyr-val (squares) and val-tyr-val and LHRH (circles). Conditions: Poly(hexyl methacrylate-co-1,3-butenediol dimethacrylate-co-2-(acryloyloxy) ethyltrimethyl ammonium chloride) monolithic column, total length 33.5 cm, active length 25 cm, 100 µm i.d. Mobile phase: 40:60 v/v acetonitrile –Tris phosphate buffer pH 2.5; Electrokinetic injection −3 kV, 3 s; Voltage: −15 kV; UV detection at 200 nm.