Simulation-based optimization of the electrodynamic ion funnel for high sensitivity electrospray ionization mass spectrometry

Abstract

High sensitivity is one of the most important requirements for applications of electrospray ionization mass spectrometry (ESI MS). Recent work with the electrodynamic ion funnel has demonstrated that it can provide significantly improved ion transmission through the ESI interface. Here we summarize the results of the simulations for several ion funnel configurations and their comparison with experimental measurements. We also report an alternative treatment of the ion funnel operation based on the effective potential approximation. The analytical relationships derived are used to generalize the results of computer simulations and develop an optimized ion funnel design. The new configuration reduces the spacing between ring electrodes to 1 mm and provides an optimized profile of the ring electrodes radii. It can also generate a deeper effective potential well, resulting in transmission of higher input ion currents over an extended mass range and increased operating pressure range (up to ∼20 Torr). Furthermore, light m/z ion transmission is improved due to suppression of the effective potential wells near the ion funnel exit. Simulations for an optimized ion funnel configuration indicate unit transmission efficiency in the m/z range of interest for most biomolecular research using ESI (approximately m/z 100–5000), for 1–5 Torr pressure and ion currents >∼1 nA, typical for low flow rate charge-constrained electrosprays (i.e. nanospray). Experimental results obtained with an improved electrodynamic funnel validate the model and demonstrate its general utility for designing rf-damped focusing elements.