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| How Electron Multipliers Work |
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An electron
multiplier is used to detect the presence of ion signals emerging from the
mass analyser of a mass spectrometer. It is essentially the ‘eyes’ of the
instrument (see Figure 1).
The task of the electron multiplier is to detect every ion of the selected
mass passed by the mass filter. How efficiently the electron multiplier
carries out this task represents a potentially limiting factor on the
overall system sensitivity. Consequently the performance of the electron
multiplier can have a major influence on the overall performance of the mass
spectrometer. |
Figure 1.
Components of Mass Spectrometry
The general layout of a mass spectrometer consists of the
following elements; Sample introduction and separation system,
Ion source, Mass analyser, Ion detection system, Data
processing. |
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Figure 2.
Secondary Electron Emission
The average number of secondary electrons emitted from the
surface of a standard dynode in an ETP electron multiplier
plotted against the energy of the incident primary electron. |
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The
basic physical process that allows an electron multiplier to operate is
called secondary electron emission. When a charged or neutral particle, an
ion or an electron, strikes a surface it can cause electrons associated with
the outer layers of atoms to be released. The number of secondary electrons
released depends on the type of incident primary particle, its angle, energy
and characteristics of the incident surface (see Figure 2).
In general there are two basic forms of electron multipliers that are
commonly used in mass spectrometry; the discrete-dynode electron
multiplier, and the continuous-dynode electron multiplier (often
referred to as a channel electron multiplier or CEM). All ETP electron
multipliers are of the discrete-dynode type (see Figure 3). |
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discrete-dynode electron multiplier has between 12 and 24 dynodes and is
used with an operating gain of between 104 and 108,
depending on the application. In GC-MS applications, for example, the
electron multiplier is typically operated in analog mode with a gain of
around 105. For a new electron multiplier this gain is typically
achieved with an applied high voltage of ~1400 volts. |
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Figure 3.
Ion-optics of an ETP discrete-dynode electron multiplier showing
the electron gain at each successive dynode. This electron
cascading process results in gains up to 108 being
achieved with ~21 dynodes. |
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Features of ETP Electron Multipliers |
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Proprietary surface material with very high secondary electron
emission
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Air stable
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2 year shelf life guarantee
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Discrete dynode design results in extended operating life
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The electron multipliers manufactured by ETP use a proprietary dynode
material. This material has a number of properties that make it very
suitable for use in an electron multiplier. It has very high secondary
electron emission, which allows exceptional gain to be achieved from each
dynode. This material is also very stable in air, in fact an ETP multiplier
can be stored for years before being used.
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As a
direct result of the high stability of the active materials used in ETP
multipliers they come with a 2 year shelf life warranty. Many testing
laboratories take advantage of this long shelf life by keeping a replacement
ETP multiplier on hand, ready for immediate installation. This keeps
instrument down time to a minimum.
For a typical ETP electron multiplier for GC-MS, the total active dynode
surface area is ~1000mm2. This can be compared to a standard
continuous dynode multiplier that has a total channel surface area of only
around 160mm2 (for a channel with 1mm diameter and 50mm length).
This increased surface area spreads out the ’work-load‘ of the electron
multiplication process over a larger area, effectively slowing the aging
process and improving operating life and gain stability. |
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