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How a quadrupole mass filter works (part 2)
A conventional quadrupole mass spectrometer works by scanning the voltages applied to the
rods of the quadrupole mass filter. The scan is arranged to follow the red dotted line on
the diagram, right. Therefore for most of the scan, the ion is unstable, and won't make
it to the other end of the filter. For a brief moment the scan-line passes through the
stable region, and during this time, the ion will pass through the filter unmolested, to the detector
at the other end.
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Different ions have different stability regions. The diagram left shows two ions, one larger
than the other. As the spectrometer scans its voltages, the scan-line passes through the
two stable regions one after the other.
The line above the red, dotted scan-line shows how the signal intensity at the detector at the end of the quadrupole mass filter will vary. There will normally be little signal, but as the scan-line passes through each stable region, there will be a peak as that ion emerges. |
At a simplistic level, this is more-or-less all there is to be said. Sorting out the scan-line is a part of the process of tuning the mass spectrometer. The scan-line must be arranged so it just slices off the very tip of each stable region, from the smallest to the biggest ions. It must also be calibrated in units of mass rather than voltage.
Ion-trap spectrometers have very similar stability diagrams, but use them in a slightly different way.