Preliminary study on the effect of miniaturisation and use of volatile mobile phases in LC for the on-line LC–MS analysis of basic pharmaceuticals

To enhance to compatibility of the on-line coupling of liquid chromatography (LC) with mass spectrometry (MS) for the analysis of basic pharmaceuticals, the use of volatile mobile phase systems in combination with miniaturised LC was investigated. Multifactor analysis of variance (MANOVA) was used to evaluate the data obtained for the various variables (modifier, stationary phase, buffer, buffer pH and buffer concentration) on the resolution, peak symmetry and retention of four basic compounds analysed using LC columns with internal diameters (I.D.) of 0.3, 1.0 and 4.6 mm (conventional). Preliminary results obtained with the investigated micro and conventional columns showed similar behaviour with respect to ruggedness. The various investigated variables showed that miniaturisation by simply downscaling dimensions can result in varying selectivity and peak shapes for basic compounds. When comparing volatile mobile phases (containing ammonium acetate or ammonium citrate) and a conventional non-volatile mobile phase (containing sodium phosphate) under pH 3 conditions, similar separation performances were observed. In the present study, ammonium citrate as the buffering salt, a high buffer concentration and methanol as the modifier showed the best peak symmetry.