The sample preparation of the alkaline drugs dioxopromethazinehydrochloride and trapidil from urine was studied systematically. The weak acid cation exchanger Wofatit CP was used as stationary phase. Batch mode and column chromatographic sample preparation were compared. The usable capacity of the ion exchanger for the analyte is an important characteristic for the sample preparation. The great influence of pH and ionic strength of the sample on the usable capacity was proved.