John Raymond Gibson

Miniature mass spectrometer for chemical and biological sensing

An experimental study has been carried out using a Miniature Quadrupole Mass Spectrometer (MicroQuad) for gas analysis. Conventional quadrupole rods have been replaced with a micromachined mass filter made from silicon with Au metallized specially drawn glass fibers of length 30 mm and diameter 0.5 mm. A standard hot filament ion source and both Faraday detection and a channel electron multiplier have been used. The effect of ion focus voltage has also been modeled by SIMION simulation. Conventional electronics were adapted to run at 6 to 8 MHz and mass spectra in the range 0 - 50 a.m.u. The results indicate a good valley separation between O, OH, H2O and Ar2+ and a best resolution at 10% peak height of 0.9 a.m.u. at mass 40 with the multiplier. Application of a static magnetic field transversely to the body of the mass filter is shown to improve resolution howbeit at the expense of ion transmission through the filter.

Mapping the Stability Diagram of a Quadrupole Mass Spectrometer with a Static Transverse Magnetic Field Applied

AbstractPrevious experimental and theoretical work identified that the application of a static magnetic (B) field can improve the resolution of a quadrupole mass spectrometer (QMS) and this simple method of performance enhancement offers advantages for field deployment. Presented here are further data showing the effect of the transverse magnetic field upon the QMS performance. For the first time, the asymmetry in QMS operation with Bx and By is considered and explained in terms of operation in the fourth quadrant of the stability diagram. The results may be explained by considering the additional Lorentz force (v x B) experienced by the ion trajectories in each case. Using our numerical approach, we model not only the individual ion trajectories for a transverse B field applied in x and y but also the mass spectra and the effect of the magnetic field upon the stability diagram. Our theoretical findings, confirmed by experiment, show an improvement in resolution and ion transmission by application of magnetic field for certain operating conditions. Figureᅟ

A Method of Computing Accurate 3D Fields of a Quadrupole Mass Filter and Their Use for Prediction of Filter Behavior

A method is described that enables the three-dimensional fields of a simple quadrupole mass filter (QMF) to be determined to a high accuracy. The technique produces accurate field values in the fringe field region as well as in the center of the filter. Using fields obtained typical filter performance is determined and shown to differ from that predicted when fringe fields are ignored. The computed performance shows features obtained experimentally and displays more complex variation with ion mass and other parameters than when fringe fields are ignored.

Effect of an Axial Magnetic Field on the Performance of a Quadrupole Mass Spectrometer

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 > 3000 for a QMS with a 200 mm long mass filter via application of an axial magnetic field.

Effect of ion entry acceptance conditions on the performance of a quadrupole mass spectrometer operated in upper and lower stability regions

Computer simulation of ion motion in a quadrupole mass spectrometer has been used to examine the effect of initial ion conditions on performance when operated in the first and third zones of the Mathieu stability diagram. Commercial instruments frequently use round electrodes instead of the better-performing hyperbolic electrodes because the cost of manufacturing is lower. However, adverse features are seen when using round electrodes. Here further insight is provided and a possible method of correction is suggested. For the first time, ion origin for the first stability region for a round electrode quadrupole has been reported.

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.