Determination and comparison of competitive isotherms by frontal analysis and frontal velocity analysis

The competitive isotherms of the two enantiomers of oxazepam on two chiral stationary phases differing by the surface concentration of the chiral sele ctor were derived from adsorption data measured with only the racemic mixtu re. Sample solubility problems limited the range of concentrations within w hich the measurements were possible. Slow racemization of oxazepam at room temperature prevented the use of any other sample composition than that of the racemic mixture. TWO different approaches, the conventional binary fron tal analysis method and the new frontal velocity method were applied for th e determination of these competitive isotherms. Both sets of experimental d ata were found to fit well to the competitive-langmuir isotherm equation. T he best coefficients obtained with the two methods were close. The frontal velocity method has the advantages of a greater experimental simplicity, of being faster, and of requiring a smaller amount of material to acquire the necessary data. It may suffer, however, from the possibility of a systemat ic drift of the isotherm data due to cumulative errors.