Quadrapole Mass Spectrometer (QMS)

Quadrapole Mass Spectrometer at a glance

A new operating mode for the quadrupole mass spectrometer (QMS) is being developed with direct applications for future planetary science and cometary missions. Substantially improved performance is achieved through auxiliary excitation of the QMS RF waveform, which results in increased mass resolutions, enabling measurements over a significantly larger mass range.  To demonstrate this new mode, a multi-resonant tank circuit has been designed which enables the operation of auxiliary excitation without increasing power consumption. This circuit will interface with an existing Pfeiffer QMG422 circular rod quadrupole system to demonstrate improved peak shape and mass resolution.

Instrument

The quadrupole mass spectrometer (QMS) was invented by Paul and Steinwedel in 1953, [1] enabling unprecedented measurements of neutral gas composition and isotopic ratios. Quadrupole mass spectrometers have extensive terrestrial commercial applications in trace gas analysis, determining isotopic ratios, and neutral gas composition measurements. In addition, QMSs have been flown on almost every mission to a planetary body in our solar system, providing a set of ion and neutral composition data which has led to significant discoveries in atmospheric physics, chemistry, and composition.

An ideal QMS is composed of four parallel conducting rods of hyperbolic cross sections. DC (amplitude U) and RF (amplitude V and frequency O) voltages are applied to opposing pairs of rods with opposite polarities to create an electric field inside the rod system. Low energy ions/ionized neutrals are injected through the rods and deflected by the electric field such that only ions with a particular mass to charge ratio have stable (periodic and bounded) trajectories. Circular rods, (which function similarly) are sometimes used but produce a slightly distorted quadrupole electric field, and consequently, reduced performance results. (Figure 1) [2]

Figure 1. Overall principle of the QMS adapted from March, 1997 [3].

Stable ion trajectories are periodic and bounded. These trajectories have several frequency components associated with them. Driving the quadrupole rods at one of these frequencies (?) excites 'parametric resonances,' which create bands of instability in the previously solid stable region. The original stability region breaks into smaller islands, including an 'upper stability island' which has been the source of significant study. (Figure 2b) It has been shown [4,5] that operating a QMS in the upper stability island can result in improved peak shapes and mass resolution.

stabtot.png

Figure 2. (a) Without excitation, for a given mass per charge, a voltage scan line passing through the ion stability region will yield a mass peak. (b) When excitation is added, the previously solid stability region splits into smaller islands of stability, which affects the shape of mass peaks. In this mode of operation, the voltage scan line is typically adjusted so it passes through the upper stability island, yielding a single mass peak.

Engineering Feats of Note

We have introduced a novel operational mode and system design that substantially improves the resolution of QMS instruments (Figure 3), enabling new breakthrough measurements with the most proven space-borne mass spectrometry technology. This design supports auxiliary excitation of the quadrupole RF near ?/O = 2, and is applicable for hyperbolic and circular rods. This technology is implemented through the use of a multiple resonant frequency tank circuit and has been applied to an existing Pfeiffer QMG422 quadrupole sensor.

Figure 3. (a) Measured stability island(s) for normal QMS operation and operation with 15% excitation at ?/O = 1.9167. (b) Corresponding transmission curves (mass peaks) for normal QMS operation (blue) and parametric excitation modes (red). Operating with parametric excitation results in a higher attainable mass resolution, and improved peak shape.

Team

Thomas Zurbuchen (UM) — Principal Investigator
Bruce Block (UM) — Deputy Principal Investigator
Martin Rubin (UM)
Paul Mahaffy (NASA/GSFC)
Medhi Benna (NASA/GSFC)
Dan Gershman (UM)

References
[1] Paul W., Steinwedel, H. Z Naturforch. 8a, 448 (1953).
[2] Dawson, P. H. ed., AIP Press, Melville, N.Y., (1995).
[3] March, R. E. J. Mass Spectr. 32, 351 (1997).
[4] Zhao, XZ., et al. Anal. Chem. 81,14 (2009).
[5] Konenkov, N. V., et al. Int. J. Mass Spectrom. 208, 17 (2001).