Electrospray as an ionisation method for mass spectrometry

One of the methods by which large and non-volatile molecules, especially th ose of biochemical interest, can be made amenable to mass spectrometry (MS) is electrospray (ES) ionisation. In ES MS the technique of electrohydrodyn amic spraying of liquids is used to produce gas-phase ions from sample mole cules present in a diluted solution. which are subsequently transferred int o a mass analyser. The special conditions required for effective ion format ion (voltages, flow rates, solution parameters such as conductivity, sample concentration, solvent composition) are described, as well as the instrume ntation used to transfer and mass-analyse the gas-phase ions. Several varia nts of the ES MS technique are presented which have been developed to meet certain analytical requirements (such as spraying of highly conductive or a queous solutions, coupling to separation techniques as liquid chromatograph y or capillary zone electrophoresis, or the analysis of very small sample q uantities). The mechanisms of liquid charging, aerosol formation and ion re lease from the charged droplets (as far as they are understood) are briefly discussed, and the question is addressed in how Far the ions observed in t he ES mass spectrum reflect the state of the sample molecules in solution. Some examples of applications From the field of peptide/protein analysis ar e given, covering molecular weight determination, sequence and spatial stru cture determination,and studies on non-covalent molecular interactions. Fin ally, a short comment is made on the limitations of ES MS and, from a pract itoner's point of view, which problems need still to be solved. (C) 1999 El sevier Science Ltd. All rights reserved.