Pulsation phenomena during electrospray ionization

Abstract Capillary current pulsations appearing under conditions of electrospray mass spectrometry and their dependence on the disintegration of an H 2 O/MeOH solution at the end of the capillary have been investigated by combining measurements of the capillary current with optical microscopy of the disintegration of the liquid at the capillary tip. With increasing capillary potential two pulsating and one continuous axial spray mode could be distinguished. Pulsations with frequencies in the low kilohertz range are related to a transient emission of liquid via a cone jet and are caused by an imbalance between the emission rate of liquid and its supply rate to the apex of the cone. Low frequency pulsations of a sequence of emission pulses are due to an imbalance between the supply of liquid to the cone volume and its loss by a sequence of emission pulses. A mechanism of the disintegration of the jet into charged droplets is proposed. The dependence of the pulsations on the flow rate, electrolyte concentration, surface tension, and capillary diameter is shown. Mass spectrometry revealed that high ion signal intensities are obtained at higher capillary potentials in the continuous axial spray mode and at lower capillary potentials in the transition range between the two pulsating axial spray modes because of the formation of small highly charged droplets. In experiments with nanoelectrospray capillaries, current pulsations were observed only after the onset of a (corona) discharge.