Post-column modification of the mobile phase in microcolumn liquid chromatography–mass spectrometry has been studied. The modification involved addition of sodium, potassium, rubidium and cesium ions to form charged adduct ions. The target molecules studied included modified cyclodextrins and oligosaccarides, bafilomycins and 18-crown-6. Relative complex constants were determined and the concentration and type of cation was evaluated in order to obtain a good sensitivity. An optimum in sensitivity was found with respect to the cation concentration. The optimum cation concentration for singly charged cations was found to be close to 5×10 −5 M. This concentration was independent of the alkali metal ion used. In most cases the relative sensitivity increases slightly with the size of the cation. The technique was applied to complex samples for structure elucidation of unknown compounds.