Reduction of Low Mass Discrimination Effect in a Miniaturized Ion Funnel by Simulation

In order to reduce the low mass discrimination effect of the miniaturized ion funnel, a method of gradient descent radio frequency (RF) signal application was proposed. The RF signals with decreasing amplitude were applied on the outlet electrodes of the ion funnel, which would improve the transmission efficiency of low mass-to-charge ratio ions. The ion transmission performance of the miniature ion funnel with the gradient descent RF signal applying was studied by simulation software SIMION and ion trajectory simulation software AXSIM. When the gradient descent RF voltage was applied, the potential line around the outlet electrodes of miniaturized ion funnel was sparse, and the effective potential was shallower, then it could significantly reduce the axial effective potential of the terminal electrode. The simulation results showed that the gradient descent RF application method could effectively reduce the low mass discrimination, while the transmission efficiency of high mass ions was not affected. As revealed by the simulation, when the initial RF voltage amplitude V1 was 120 V and 80 V, respectively, the corresponding optimal descent gradient σ was 6 V and 3 V, and the ion transmission efficiency could achieve 60% and 62% for the ions at m/z 20, while it still maintained nearly 100% efficiency in high mass range (m/z≥200). Not only that, at lower mass segments (m/z≤20), a lower mass-to-charge ratio cutoff was obtained in the gradient descent RF signal application mode, which demonstrated that this new RF signal application method could significantly extend the low mass range. The influence of RF frequency and direct current (DC) field gradient parameter with the gradient descent RF application method on the ion transmission efficiency was studied in detail, indicating that a RF frequency of 1 MHz and a DC field gradient parameter of 10 V/cm were the optimal value of obtaining a high ion transmission efficiency. Further, the gradient descent RF signal application method was simplified. The ion transfer efficiency achieved by the simplified method decreased slightly, but it was still better than that achieved by the traditional RF application method. An ion transmission efficiency (at m/z 20) of 56.6% was achieved in the simplified gradient descent RF signal application mode, which was decreased by 5.7% compared with that before simplification and improved by 57.2% compared to conventional RF application mode. The simulation result illustrated that the simplified gradient descent RF application mode can reduce low mass discrimination effect to adequate extent. The simplification method can reduce the difficulty of operation and improve the feasibility of the scheme implementation.