Abstract: An improved electrospray ionization (ESI) system source and method is presented including an electrospray ionization source for introducing an ionized molecular sample into a mass spectrometer for analysis. The ESI source is constructed in the configuration of a probe that makes use of a standard 0.5 inch (13 mm) vacuum lock commonly found on conventional mass spectrometers. The ESI probe comprises a desolvation tube, a voltage source for applying a voltage to the desolvation tube, a resistance coil for heating the desolvation tube, a sensor for measuring the temperature of the desolvation tube, a skimmer positioned downstream of the desolvation tube for directing the ions to the lens stack of the mass spectrometer, a voltage source for applying a voltage to the skimmer, a spacer lens positioned upstream of the skimmer for focusing the ions prior to their entering the skimmer, and an evacuable dielectric encasement for housing the components of the probe assembly.