COMPONENTS OF A MASS SPECTROMETER
The layout of a mass spectrometer generally consists of the following elements: sample introduction and separation system, ion source, mass analyser, ion detection system and data processing.
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.
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, increasing the size of the signal.
SECONDARY ELECTRON EMISSION
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, the primary particle, its angle and energy, and characteristics of the incident surface.
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.
A typical 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 analogue mode with a gain of around 105.
FEATURES OF AN ETP ELECTRON MULTIPLIER
Proprietary surface material with very high secondary electron emission
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.
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, improving operating life and stability.