A new technique for unbiased external ion accumulation in a quadrupole two-dimensional ion trap for electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

External ion accumulation in a two-dimensional (2D) multipole trap has been shown to increase the sensitivity, dynamic range and duty cycle of a Fouri er transform ion cyclotron resonance (FTICR) mass spectrometer. However, it is important that trapped ions be detected without significant bias at lon ger accumulation times in the external 2D multipole trap. With increasing i on accumulation time pronounced m/z discrimination was observed when trappi ng ions in an accumulation quadrupole. In this work we show that superimpos ing lower rf-amplitude dipolar excitation over the main rf-field in the acc umulation quadrupole results in disruption of the m/z discrimination and ca n potentially be used to achieve unbiased external ion accumulation with FT ICR. Copyright (C) 2001 John Wiley & Sons, Ltd.