Mixed-mode chromatography allows to separate various hydrophobic and hydrophilic drugs in one HPLC method. Presence of at least two mechanism of retention allows adjust selectivity of separation by changing three parameters: amount of acetonitrile, buffer concentration and buffer pH. Seven common drugs are separated on an Obelisc R trimodal HPLC column in gradient method. Method provides alternative selectivity to traditional reversed-phase chromatography. Operational range in which basic compounds are retained the most is from pH 3 to 5. At higher pH, longer retention can be achieved. Mixed-mode chromatography provides longer retention and better peak shapes for basic and acidic analytes. Method can be used in analysis of drugs and pharmaceuticals.

Common hydrophobic basic and hydrophilic basic drugs are separated by mixed-mode chromatography with greater selectivity and resolution than traditional reversed-phase column. Drugs are retained by combination of reversed-phase and cation-exchange mechanisms. Retention time is controlled by amount of acetonitrile, buffer concentration and buffer pH. Available detection techniques are based on buffer selection and include UV, Evaporative Light-Scattering Detector (ELSD), Corona (CAD), LC/MS, etc. This HPLC method can be adopted as general approach for analysis of drugs and pharmaceuticals.

The presence of three drugs, dextromethorphan, amitriptyline and trimipramine in saliva were analyzed by mixed-mode chromatography on a Primesep D reversed-phase anion-exchange column. No sample preparation is required as proteins from saliva are not retained at these conditions and elute in the void of the column due to cation-exclusion effect. Both column and proteins are positively charged at current conditions. Mobile phase consists of ACN-water-formic acid and is compatible with LC/MS detection technique. This general HPLC method can be used for analysis of hydrophobic basic drugs in biofluids.