Characteristics of probe electrospray generated from a solid needle.

Probe electrospray ionization (PESI) has recently been developed, in which the electrospray was generated from a solid needle instead of by using a capillary. In this paper, the characteristics of probe electrospray ionization were studied based on the measurement of spray current, optical microscopy, and PESI mass spectrometry. In the experiment, the solid needle was moved up and down a vertical axis, and a small amount of sample was repeatedly loaded to the needle when the tip of the needle touched the surface of the liquid sample at the lowest position. After the application of high voltage, a liquid droplet was formed on the tip of the solid needle probe, with its size was determined by the size of the needle tip. The liquid flow rate to the tip, as indicated by the spray current, depends on the voltage applied to the needle as well as the loaded liquid amount. Stable electrospray can be maintained until the total consumption of liquid sample. The kilohertz current pulsation takes place in the case of overloading the sample to the needle. The influences of the applied voltage and the liquid flow rate on the PESI mass spectra were also examined.