Modeling of a linear ion trap with driving rectangular waveforms

Abstract

We consider the operation of a digital linear ion trap with resonant radial ejection. A sequence of rectangular voltage pulses with a dipole resonance signal is applied to the trap electrodes. The periodic waveform is piecewise constant, has zero mean, and is determined by an asymmetry parameter $d$: one value is taken on interval $\left(0,\mathrm{dT}\right)$ and another on $\left(\mathrm{dT},T\right)$, where $T$ is the RF period. Ion mass scanning is performed by varying the asymmetry parameter $d$ and amplitude of the negative pulse part with time. The ion oscillation frequencies and acceptance of the linear trap are calculated. The dependence of the ion mass to charge ratio $m/z$ on the parameter $d$ is $m/z~d^2$. The maximum value is about $m/z=30$ kDa for typical parameters of the linear trap: frequency 0.5 MHz, rod radius 4 mm, and negative pulse amplitude 1 kV. The dipolar excitation frequency is 0.125 MHz at which the LIT acceptance is maximal.