Selection of the optimum electrospray voltage for gradient elution LC-MS measurements
Changes in liquid composition during gradient elution liquid chromatography (LC) coupled to mass spectrometry (MS) analyses affect the electrospray operation. To establish methodologies for judicious selection of the electrospray voltage, we monitored in real time the effect of the LC gradient on the spray current. The optimum range of the electrospray voltage decreased as the concentration of organic solvent in the eluent increased during reversed-phase LC analyses. These results and related observations provided the means to rationally select the voltage to ensure effective electrospray operation throughout gradient-elution LC separations. For analyses in which the electrospray was operated at constant voltage, a small run-to-run variation in the spray current was observed, indicating a changing electric field resulting from fouling or degradation of the emitter. Algorithms using feedback from spray current measurements that can maintain the electrospray voltage within the optimum operating range throughout gradient elution LC-MS were evaluated. The electrospray operation with voltage regulation and at a constant, judiciously selected voltage during gradient elution LC-MS measurements produced data with similar reproducibility.