Peak fronting in reversed-phase high-performance liquid chromatography: a study of the chromatographic behavior of oxycodone hydrochloride.

Severe peak asymmetry--fronting--was observed for oxycodone during elution at 30 degrees C from a C18 HPLC column using a mobile phase consisting of 14.9% MeOH, 84.5% 0.05 M KH2PO4 (pH 3.0), 0.5% MTBE, and 0.1% TEA. Investigation using deuterium-labeled oxycodone and analysis by LC/MS showed that gem diol and hemiketal adducts of oxycodone formed as a result of the equilibrium addition of water and methanol to the C-6 ketone on oxycodone. As a result of slow equilibrium kinetics at room temperature in aqueous solution, the gem-diol and methyl hemiketal eluted as an unresolved broad band in front of the oxycodone peak. Decreasing the column temperature to 0 degrees C decreased the rates of interconversion and allowed the resolution and separation of these species from each other and from oxycodone. Increasing the column temperature to 60 degrees C increased the rates of interconversion with the result that the three species eluted as a single, homogenous peak with greatly improved peak symmetry.