Thomas J. Hogan

Performance Simulation of a Quadrupole Mass Filter Operating in the First and Third Stability Zones

A method of computation that accurately simulates the performance of quadrupole mass filters (QMFs) is described. Behavior is described by determining the individual trajectories of a large number of ions (108) as they are injected into the QMF. The effects of the ratio of circular electrode radius r to electric field radius r0 on the performance characteristics have been investigated for zone 1 (a ap 0.237 and q ap 0.706) and zone 3 (a ap 3.16 and q ap 3.23) operation. We demonstrate that performance sensitivity to the r/r0 ratio is different for zone 3 than those previously reported for zone 1. The magnitude and variation of the ldquotailrdquo in the mass spectral peak shapes that are apparent for zone 1 is much decreased for zone 3 and does not influence QMF resolution. Variation in ion trajectories and associated power-spatial frequency spectra when operated in zones 1 and 3 with varying r/r0 geometrical ratios are also presented. We demonstrate that these provide an alternative method in determining an ideal value for r/r0.

A Quadrupole Mass Spectrometer for Resolution of Low Mass Isotopes

The qualitative and quantitative identification of low mass isotopes in the mass range 1–6 u poses certain difficulties when attempting to achieve the required resolution with an instrument suitable for deployment within a process environment. Certain adjacent species present in the process sample (HT and D2) require a resolution greater than 930 to achieve an accurate measurement. We demonstrate here through simulation techniques that this level of performance required is unachievable using commercially available instruments. Using previously reported simulation techniques, this article demonstrates how the required performance for resolving the low mass isotopes can be achieved by a quadrupole mass spectrometer (QMS), which incorporates a quadrupole mass filter (QMF) constructed from hyperbolic electrodes and operated in zone 3 of the Mathieu stability diagram.

Performance Characteristics of a MEMS Quadrupole Mass Filter With Square Electrodes: Experimental and Simulated Results

Size reduction in quadrupole mass spectrometers (QMSs) is an ongoing requirement driven by the needs of space exploration, portable, and covert monitoring applications. Microelectromechanical systems (MEMS) technology provides a method of achieving this size reduction. A quadrupole mass filter (QMF) is one component of a QMS and is suitable for microfabrication. MEMS manufacturing techniques are more suitable to the production of rectilinear electrodes, instead of the more widely used circular electrodes. Present understanding of the performance characteristics of rectilinear electrodes and the dependence of these characteristics on electrode geometry are not well documented. In this paper, we report on the performance characteristics of a square-electrode QMF. Both the predicted performances obtained by computer simulation and experimental data are presented for operation in stability zone 1 (0.236, 0.706) and zone 3 (3.16, 3.23). A comparison between these results and the simulated data for equivalent devices constructed using hyperbolic and circular electrodes for operation in zone 1 is also made. This comparison demonstrates that, although the field produced by square electrodes is far from the “ideal,” it is still possible to achieve useful filtering action. Our results also show that, for operation in zone 3, performance comparable with that of hyperbolic and circular electrodes operating in zone 1 is achievable.

Factors Influencing the QMF Resolution for Operation in Stability Zones 1 and 3

AbstractThis study uses a computer model to simulate a quadrupole mass filter (QMF) instrument under different operating conditions for Mathieu stability zones 1 and 3. The investigation considers the factors that limit the maximum resolution (Rmax), which can be obtained for a given QMF for a particular value of scan line. Previously, QMF resolution (R) has been found to be dependent on number (N) of radio frequency (rf) cycles experienced by the ions in the mass filter, according to R = Nn/K, where n and K are the constants. However, this expression does not predict the limit to QMF resolution observed in practice and is true only for the linear regions of the performance curve for QMF operation in zone 1 and zone 3 of the stability diagram. Here we model the saturated regions of the performance curve for QMF operation in zone 1 according to R = q(1 – 2cN)/∆q, where c is a constant and ∆q is the width of the intersection of the operating scan line with the stability zone 1, measured at q-axis of the Mathieu stability diagram. Also by careful calculations of the detail of the stability tip of zone 1, the following relationship was established between Rmax and percentage U/V ratio: Rmax  = q/(0.9330-0.00933U/V). For QMF operation in zone 3 the expression R = a – bcN simulates well the linear and saturated regions of the performance curve for a range of operational conditions, where a, b, and c are constants.

Effects of Mechanical Tolerances on QMF Performance for Operation in the Third Stability Zone

Our previously reported method of accurately simulating the performance of a quadrupole mass filter (QMF) has been applied to the investigation of the effects of electrode positional tolerance on the performance of a QMF when operated in stability zone 3 (a ≈ 3.16 and q ≈ 3.23). Simulations for single- and dual-electrode positional errors have been undertaken. Single-axis errors produced changes in mass peak shape that are similar to those previously reported for zone 1. Compound errors produce changes in mass peak shape that are approximately a summation of the effects obtained from individual single-axis errors. Our results show that the direction of the electrode displacement, not the electrode, is the important factor in determining the effect on QMF performance. We also show that the effects of an individual electrode radius tolerance result in changes to the mass peak shape that are similar to those produced by individual electrode positional errors. Simulations also show the suitability of unbalanced excitation voltages as a method of compensating for mechanical tolerance when operating in zone 3. From these results, we are able to provide suitable limits for the voltage accuracy and stability when employing this method of compensation.

Quadrupole Mass Filter: Design and Performance for Operation in Stability Zone 3

AbstractThe predicted performance of a quadrupole mass filter (QMF) operating in Mathieu stability zone 3 is described in detail using computer simulations. The investigation considers the factors that limit the ultimate maximum resolution (Rmax) and percentage transmission (%Tx), which can be obtained for a given QMF for a particular scan line of operation. The performance curve (i.e., the resolution (R) versus number (N) of radio frequency (rf) cycles experienced by the ions in the mass filter) has been modeled for the upper and lower tip of stability zone 3. The saturation behavior of the performance curve observed in practice for zone 3 is explained. Furthermore, new design equations are presented by examining the intersection of the scan line with stability zone 3. Resolution versus transmission characteristics of stability zones 1 and 3 are compared and the dependence of performance for zones 1 and 3 is related to particular instrument operating parameters. Figureᅟ

Simulation of a QMS Including the Effects of Pressure in the Electron-Impact Ion Source

This paper is concerned with the computer modeling of a quadrupole mass spectrometer (QMS) to include the effect of pressure in the ion source. The paper simulates the spectra over the pressure range from 10-6 to 10-4 mbar. An important contribution is the development of a novel procedure to include pressure dependence of the ion source to allow better prediction of instrument performance. Electron-impact total ionization cross sections in the ionic current expression are calculated using the binary-encounter-Bethe theory for argon gas. The predicted results show good agreement with the experimental results obtained from a commercial QMS used for residual gas analysis.