electron multiplier


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electron multiplier

A detection device inside the vacuum of a mass spectrometer which converts ions impacting on its front dynodes into a detectable, amplified electron current at the back lead of the device, with an overall gain (signal out/signal in) as high as 104–108.

Positive ions exiting the mass analyser impact the first dynode’s surface, triggering the release of several electrons, which are then accelerated through a potential to the next electrode, where each electron impact triggers the release of several secondary electrons; these are accelerated into the next dynode, leading to further impact (release events). A cascade of electrons is produced, generating a current that is further amplified and then sampled by an analog-to-digital converter to be recorded by the data system.
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During the RF sweep, which progressively destabilizes ions for ejection from the ion trap to the electron multiplier, the axial modulation RF enhances the movement of ions away from the center of the ion trap where magnetic field strength is low and ion density is high.
For increased sensitivity and faster scan rates, an optional electron multiplier is offered that detects partial pressures down to [10.
However, in most cases, running the electron multiplier at higher gain levels than the RGA makes up for the reduction in sensitivity.
Uniquely, by utilizing two detectors, a deep bucket Faraday and an ion counting electron multiplier on two separate spurs, this analysis can be carried out simultaneously.
Here, the ions are sorted by mass and detected using a scanning electron multiplier.
The RGA combines electron multiplier detection and high frequency quadrupole operation to give low helium detection limits and excellent hydrogen-helium separation.
It features mass ranges to 300 amu, Faraday cup and electron multiplier detectors, and a choice of open or closed ion sources.
They did this so they wouldn't get a high current coming from their electron multiplier, since that would have shortened its lifetime.

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