2009 Advanced Laboratories Conference Abstract Detail Page
Previous Page |
New Search |
||Mass Spectrometry: Quadrupole Mass Filter
||The mass spectrometer is essentially an instrument that can be used to measure the mass, or more correctly the mass/charge ratio, of ionized atoms or other electrically charged particles. Mass spectrometers are now used in physics, geology, chemistry, biology and medicine to determine compositions, to measure isotopic ratios, for detecting leaks in vacuum systems, and in homeland security. The first mass spectrographs were invented almost 100 years ago by A.J. Dempster, F.W. Aston, and others, and have been in continuous development ever since. However, the principle of using electric and magnetic fields to accelerate and establish the trajectories of ions inside the spectrometer according to their mass/charge ratio is common to all the different designs. Dempster's original mass spectrograph is a simple illustration of these physical principles.
In practice, it is difficult to achieve very stable and spatially uniform magnetic fields, especially with permanent magnets. These difficulties can lead to degradation of the mass resolution and drifts in the calibration of the instrument. In addition, the presence of stray magnetic fields can affect other instruments that may be used in conjunction with a mass spectrometer, for example, electron energy analyzers. In the early 1950's it was realized, by W. Paul (shared Nobel Prize in Physics, 1989) that use of magnetic fields could be eliminated altogether by a clever design which uses alternating quadrupolar electric fields rather than magnetic fields, hence the name Quadrupole Mass Spectrometer. This is the design that is currently in widespread use for residual gas analysis. It is highly stable and has excellent mass resolution. With high sensitivity electron multipliers, it can measure partial pressures down to 10-14 Torr! The operation of the Quadrupole Mass Spectrometer (QMS) is not quite so simple to understand as the magnetic sector design, but it is extremely elegant and involves some beautiful mathematics, and therefore the details are worth appreciating.
||Session V - Parallel Workshops
Faculty or Staff
University of Michigan