Prefocusing inside a linear ion beam guide—A SIMION study

Abstract

A set of 14 stacked ring electrodes was studied by the SIMION ion trajectory simulation program under a pressure of 1Pa with different applied RF frequencies and RF voltage amplitudes when a classical DC gradient of 38V/cm is applied. Ion–neutral collision effects were considered by adopting a successful and integrated hard sphere, elastic, ion–neutral collision user program provided by SIMION. Mass dependent ion transmission efficiencies could be obtained. In the classical operation mode of the ion beam guide, a RF frequency of 2MHz was crucial to obtain very good ion transmission efficiency along a broad mass range. Lower RF frequencies (<1.5MHz) limited the good transmission efficiencies only to high masses with m/z>500amu. SIMION can reproduce expected efficiency curves which show that low m/z ions below 150amu represent a challenge for such linear ion beam devices even when high RF driving frequency of 2MHz is applied. The simulations also show that a DC gradient of at least 30V/cm is crucial to maintain good transmission efficiencies which are in agreement with previously published results. A new operation mode for running the same device is introduced for earlier ion focusing in between the second and fifth ring electrodes, prior to slight beam divergence after the focus point. This required a significant change of the applied DC voltages to alter the shape of the DC gradient curve along the central z axis. The newly identified operation mode allows enhancing the mass dependent ion transmission efficiency in a wide m/z range between 80 and 1000amu when an RF frequency of 2MHz is applied to the first ten ring electrodes.