An Ion Funnel Interface for Improved Ion Focusing and Sensitivity Using Electrospray Ionization Mass Spectrometry

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Abstract
To improve upon the already impressive sensitivity achievable with electrospray ionization sources, a novel electrohydrodynamic ion funnel interface has been developed and implemented with a triple-quadrupole mass spectrometer. The ion funnel interface effectively consists of a series of ring electrodes of increasingly small internal diameters to which rf and dc electric potentials are coapplied. In the 1−10-Torr pressure range, the electric fields cause the collisionally damped ions to be more effectively focused and transmitted as a collimated ion beam. This paper describes the ion funnel design and presents an evaluation of its performance using a triple-quadrupole mass spectrometer. Ion transmission and m/z discriminating parameters (resulting in both effective low- and high-m/z cutoffs) are presented based upon both ion current measurements and mass spectra. Electrospray ionization mass spectra of selected protein solutions demonstrated well over 1 order of magnitude increase in signal relative to that of the instrument operated in its standard (inlet capillary−skimmer) configuration under similar conditions. The present results suggest that it will be feasible to realize close to 100% ion transmission efficiency for analytically relevant ions through the electrospray ionization interface and into the mass analyzer.