Simulation study of digital waveform‐driven quadrupole mass filter operated in higher stability regions for high‐resolution inductively coupled plasma mass spectrometry

Abstract

RationaleThe use of a frequency‐scanned digital quadrupole mass filter (QMF) with varying duty cycles shows promise for application as a high‐resolution mass analyzer design for inductively coupled plasma mass spectrometry (ICP‐MS). High resolution in ICP‐MS is important to overcome isobaric polyatomic interferences. Here, we explore the possibility and the characteristics of using a digital quadrupole operating in higher stability regions for ICP‐MS.MethodsWe perform computational simulations in SIMION of a digital QMF that is operated by scanning the frequency of the digital waveform at a fixed driving voltage and various duty cycles. For ions in the atomic mass range (7–238 m/z), we investigate the expected resolution, transmission, fringe field effects, and ion trajectories. We compare different characteristics between sine and digital waveform QMF.ResultsWithin the capability of current digital waveform generation technology, a digital QMF can produce variable mass resolution, from several hundred to more than 10 000. This mass resolution covers the low, medium, and high resolutions that are typical for sector‐field ICP‐MS. Additionally, simulations suggest that transmission of the QMF remains high at high resolution. For example, with 87.50/12.50 duty cycle (zone 4,1), resolution at 10% peak width is 10 420 for m/z 80. The transmission through the quadrupole, which is constant for all isoenergetic ions, is ~2.5%, and most ion loss is due to the defocusing effects of the fringe field. Compared to sinusoidal QMFs, ions need many fewer cycles in the digital QMF to obtain high resolution.ConclusionThe results demonstrate that the use of a frequency‐scanned, duty‐cycle‐modulated digital QMF as the mass analyzer for ICP‐MS has the potential to produce high resolution while maintaining considerable transmission, thus overcoming most spectral interferences in elemental MS.

Experimental Investigation of a Digital Quadrupole Inductively Coupled Plasma Mass Spectrometer for Elemental Analysis

“…This makes ions' stability higher x direction, in which ion motion is confined due to the net positive potential, and ion motion extremely unstable in the y direction, where the average potential is negative. 18 This is also reflected in the stability diagrams, as shown in Figure 1b, where the ion stability region for the x direction is very wide and covers a large area, while that for the y direction is a thin strip. This asymmetric stability space could also mean that ion transmission is highly dependent on the shape and energy of the ion beam as it enters the quadrupole, i.e., the acceptance of the QMF.…”

“…For example, with a duty cycle of 83.50/16.50 and 100 V 0‑p , there is an average net potential of +67 V on the x-rods and −67 V on the y-rods. This makes ions’ stability higher x direction, in which ion motion is confined due to the net positive potential, and ion motion extremely unstable in the y direction, where the average potential is negative . This is also reflected in the stability diagrams, as shown in Figure b, where the ion stability region for the x direction is very wide and covers a large area, while that for the y direction is a thin strip.…”