Plots of k’ vs. pH on the HC-COOH and HC-C8 phases. Chromatographic conditions: 24/76 ACN/water with 10mM NH4Ac at pH 4.0, 4.5, 5.0, 5.5 or 6.0 buffered by acetic acid. 5.0 cm × 0.46 cm column, T = 40 °C, F = 1.0 ml/min. Solutes: (◇) Acetophenone; (▲) Methcathinone; (*) Ephedrine; (•) Cathinone; (■) 3-methoxytyramine. The solid lines are the retentions measured on the HC-COOH phase; The dotted lines are the retentions measured on the HC-C8 phase.