Gas Phase Ion Detectors
After the ions have been separated by a mass analyzer, they travel to a detector where their arrival is registered and converted into an electrical signal. The electrical signal is transformed into the digital graphics that is eventually displayed as a mass spectrum. There are two broad classes of detectors: focal point detectors and focal plane detectors. Focal point detectors, which monitor one m/z ion at a time, are used with scanning mass analyzers such as a quadrupole. Examples include: Faraday cup detectors, electron multipliers, and photomultiplier detectors. Focal plane detectors simultaneously monitor all spatially resolved ions. They are used with beam analyzers that spatially resolve ions such as a time-of-flight (TOF) mass analyzer. Multichannel plate (MCP) and multichannel array detectors are examples of focal plane detectors.
Faraday Cup Detector
Figure 1: Principle of operation of the Faraday cup detector
In a Faraday cup detector (Figure 1), the ion beam collides with a collector electrode housed in a Faraday cage. Colliding ions transfer their charges to the electrode. This generates a current that creates a voltage drop across the feedback resistor. The signal is then amplified with a high impedance amplifying circuit. Because of their slow response they are not very useful with fast scanning mass analyzers like a quadrupole. However, they are useful in situations that demand very stable signals (e.g. isotope ratio mass spectrometry). Another advantage is that their signal is independent of the energy of the ions.
Electron Multiplier Detectors
Electron multipliers are comprised of either discrete dynodes (Figure 2) or a continuous dynode (Figure 3). A dynode releases electrons when and ion or electron collides with it. They are called electron multipliers because the initial electrons released by ion collision (primary electrons) are accelerated and re-collide with the dynode causing more electrons to be released (secondary electrons). The secondary electrons in turn cause further release of more electrons upon colliding with the dynode. The result is a cascade effect that culminates in the production of a large number of electrons that are collected by a terminal anode that is positively biased relative to the dynode(s). The electron current is converted to a voltage drop and then amplified. Electron multipliers have fast response, high gain and high sensitivity.
Figure 2: Principle of operation of discrete dynode electron multiplier (DDEM)
Figure 3: Principle of operation of continuous dynode electron multiplier (CDEM).This is also known as the channel electron multiplier (CEM)
Multichannel Plate Detector
A multichannel plate detector (Figure 4) is a two-dimensional arrangement of microchannel electron multipliers similar to those in Figure 3. Each individual microchannel is about 10mm in diameter. Ions of different m/z strike different microchannels on the plate. This is an example of a focal plane detector and it can be used with a time-of-flight mass analyzer.
Figure 4: Principle of operation of multichannel plate (MCP) electron multiplier.