Experimental Evaluation of Higher Order Stability Zones Using a Digitally Operated Quadrupole Mass Filter.

Abstract

Recent improvements to the comparison-based method of digital waveform generation increased the reproducibility of the waveforms so that the higher-order Mathieu stability zones can be accessed reliably. Digitally driven quadrupole mass filters access these zones using a fixed AC voltage and rectangular waveforms that are defined by a duty cycle. In this context, the duty cycle is the fraction of the waveform period where the waveform remains in the high state. Because digitally driven quadrupoles navigate stability using a duty cycle, there is no need to apply a resolving DC offset between electrode pairs. Accessing the higher stability zones using a conventional resonantly tuned RF requires the use of thousands of AC and DC voltages making the mode of operation less accessible with these devices. Stability zones higher than (1,1) and (2,1) have theoretical resolving powers that are on the order 1,140 and 3,447 at fwhm which drives efforts to practically access these operational conditions. Accessing these zones digitally requires the use of extremely precise waveforms. In a previous effort, waveform generation produced waveforms to reliably access the (1,1) and (2,1) zones without impacting performance. However, recent work found more improvement was needed to reliably access neighboring higher stability zones. Derived from that work, it was determined that a waveform resolution of ∼10 ppm or less was needed to reliably access the (3,1) and (3,2) zones. The present work utilized digital waveforms that achieve this level of precision to experimentally access and characterize attributes of the (3,1) and (3,2) zones. This work dives into the investigation of different beam energies to overcome the destabilizing fringing fields, improve transmission, and their overall effect on the experimental resolving power and signal-to-noise. In addition, the AC voltage of the driving RF was varied to understand the effects on the initial ion beam energy that is needed to achieve balanced separation and how the overall signal-to-noise is affected. Lastly, an assessment was made on the effects of the temporal parameters of a digital mass scan on peak sensitivity, peak fidelity, and overall duration for a scan.