Effect of an axial magnetic field on the performance of a quadrupole mass spectrometer

Abstract

We consider the case of a quadrupole mass spectrometer (QMS) in which a static magnetic field is applied axially in the z-direction along the length of the mass filter. The theoretical approach assumed in the model is that the QMS contains hyperbolic rods as electrodes and that the magnetic field acts over the full length of the mass filter assembly. Initial experimental results with argon and helium for a low-resolution instrument confirm the predicted theoretical trends. The analysis also predicts for which values of operating parameters an enhancement of the instrument resolution is achieved when an axial magnetic field is applied. The model predicts instrument resolution R Ͼ3000 for a QMS with a 200 mm long mass filter via application of an axial magnetic field. Attributes of the QMS such as versatility, accuracy, low cost, mass range, and sensitivity have ensured that it has been deployed in a wide range of applications, varying considerably from the modest residual gas analyzer to high-performance mass spectrometer for chemical analysis of simple and complex molecules, e.g., when used in conjunction with gas chromatography [7,8]. Different applications have different requirements for resolution, sensitivity, and stability.There have been many analytical predictions of the behavior of QMS. Dawson used matrix methods to calculate ion transmission for various scan lines and various apertures both with and without fringing fields, based on maximum ion displacements for mass filters [9]. Later research describes the performance of the QMS more comprehensively by solving the Mathieu equation in two dimensions for an infinitely long mass filter [2]. Batey showed that some features of the behavior of the QMS could be predicted by tracing the motion of ion through the mass filter [10]. Muntean used the matrix method to develop a computer simulation program to model ion transmission through the filter by calculating ion trajectories in radio frequency (rf) only quadrupoles [11]. More examples of such types of analytical work include the modeling of ion transmission through the filter by calculating ion trajectories in exactly determined hyperbolic quadrupole fields [12], the effects of rf frequency, phase and magnitude on the performance of QMS [13], and the effects of initial ion energy and quadrupole rod length on transmission percentage of ions through the mass filter along with the effects of aperture parameters [14]. Some workers have developed computational methods to determine the trajectories of large number of ions in QMS; their computer program generates large number of ions (at least 10 5 ions injected into the quadrupole model at each point on the mass scale), thus providing a detailed computer simulation for both hyperbolic and circular rods [15,16]. Douglas and Konenkov have used numerical calculations to investigate the influence of electrode radius r to field radius r 0 , which is referred to as (r/ r 0 ) on the peak shape for a linear QMF constructed with round rods [17]. Other workers have used com...

Quadrupole mass filter operation under the influence of magnetic field

“…An option of introducing magnetic in all the three directions (B x , B y , B z ) , along the whole length of the mass filter, is also incorporated. The original model has been upgraded to provide: improved sensitivity (using a larger number of steps along the mass scale), to include large number of ions at each point on the mass scale and to simulate a QMF with a very large number of rf cycles.…”

“…The effect of axial magnetic field on the mass spectrum is somewhat different compared with that of transverse, as there is reduction in low mass tail as well as high mass tail upon application of the magnetic field. Our previous work described the effect of Bz on performance of a QMF in detail. With axial magnetic field, the mass filter can be operated close to the tip of stability zone 1.…”

“…However, surprisingly little work has been done in relation to the application of magnetic field to the QMF. Previously published experimental and theoretical results for QMF operating in stability zone 1 showed: the effect of a transverse magnetic field on the resolution of QMF, the effect of a static magnetic field on the ion trajectories and the effect of an axial magnetic field on the performance of a QMF . For QMF operating in stability zone 3, previous work had shown the effect of transverse magnetic field on resolution .…”

QMS in the Third Stability Zone with a Transverse Magnetic Field Applied

“…The theory for ion motion in a QMS with an applied magnetic field is as published previously [19], and is reproduced here for completeness. Ions oscillating tangentially to a magnetic field will experience a Lorentz force…”

“…This was a motivation for this investigation. Previously published experimental and theoretical results for stability zone 1 showed the effect of a transverse magnetic field on the resolution of QMS [17]; the effect of a static magnetic field on the ion trajectories [18]; and the effect of an axial magnetic field on the performance of a QMS [19]. Our original model was modified to include mass scanning for stability zone 3; and with an option to include the effect of magnetic field when operating in zone 3.…”