Ion Detectors for Virtually all Mass Spectrometry Applications
Electron multipliers are used in a wide range of applications to
detect and measure small signals of ions, electrons or photons.
One of the main applications for an electron multiplier is for
the detection of ions in a mass spectrometer. ACTIVE FILM
Multipliers manufactured by ETP ( a division of SGE Group
of Companies) are the result of 15 years experience in
supplying high performance electron multipliers for use in
mass spectrometry applications. The performance of this
unique type of ion detector has seen it become widely used in
almost all fields of mass spectrometry (including ICP-MS,
GC-MS, LC-MS, TOF and SIMS).
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.
Principles of Multiplier Operation
The basic physical process that allows an electron multiplier to
operate is called secondary electron emission. When a
charged 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 energy and
characteristic 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 multiplier
Figure 1. Components of Mass Spectrometry
Gas Phase
Source
Ion Sorting
Electron
Multiplier
Analyser
Vacuum
Data
Inlet
Sample
Introduction
Ion Detection
Data Output: Mass Spectrum
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.
Figure 2. Secondary Electron Emission
6
Secondary Electron Emissions
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TECHNICAL ARTICLE
Mass Spectrometer Detectors
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4
3
2
1
0
0
100
200
300
400
Incident Electron Energy (eV)
500
The average number of secondary electrons emitted from the
surface of an ETP electron multiplier plotted against the
energy of the incident primary electron.
(often referred to as a channel electron multiplier or CEM).
The principal difference between these two types of multipliers is the
structure used to harness the secondary electron emission process. In
the case of the discrete-dynode multiplier electron, amplification is
carried out using an array of electron multiplying electrodes, called
dynodes. Ions striking the first dynode cause secondary electrons to be
emitted from the surface. The optics of the dynodes focuses these
secondary electrons onto the next dynode of the array as shown in
Figure 3. These in turn emit even more secondary electrons from the
surface of the second dynode. In this way a cascade of electrons is
developed between successive dynodes, each dynode increasing the
number of electrons in the cascade by a factor of 2 to 3, until the
cascade of electrons reaches the output electrode where the signal is
extracted.
In the case of the continuous-dynode multiplier, electron amplification
is carried out using a single continuous channel made from extruded
lead-silicate glass. The electron gain occurs by the ion striking the
channel wall and causing secondary electrons to be emitted. The
secondary electrons are then drawn down the channel, by an
electrostatic field, until they again impact with the channel wall,
emitting even more secondary electrons. This process is illustrated in
Figure 4.
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 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 (Figure 5).
Noise generated within an ETP electron multiplier is negligible, typically
the output equivalent of just a few ions per minute. In applications such as
GC-MS the noise on the output signal from the multiplier is influenced by
three factors:
Figure 3. Discrete Dynode Detector
ION
First
Dynode
1. Noise already existing on the ion signal (produced in the
separation column and the ion source).
2. The ability of the input optics of the detector to screen out
unwanted radiation (not ions).
3. The statistical noise associated with the number of ions in a
measurement (especially for low level ion signals).
Figure 6 shows the change in the applied high voltage required to
maintain the gain of a typical electron multiplier for GC-MS over the
course of its operating life. The electron multiplier power supply in most
GC-MS systems is limited to 3000 volts. When the applied high voltage
necessary to achieve the required operating gain begins to approach the
limit of the power supply, it is necessary to replace the multiplier with a
new one. A typical electron multiplier for GC-MS applications lasts 1 to 3
years.
Features of ETP Electron Multipliers
The electron multipliers manufactured by ETP are called ACTIVE FILM
Multipliers. Their name comes from the specialized surface materials
used on the dynodes. This proprietary dynode 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 the instrument down time to a minimum.
A major advantage of the discrete dynode design of ACTIVE FILM
Multipliers is increased operating life. This is a consequence of the
mechanism of gain loss in an electron multiplier.
One of the main mechanisms of multiplier gain loss is the deposition of
contamination on the surface of the dynodes. The rate of contamination
build-up on the dynode surfaces is influenced by two factors;
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.
Figure 4. Continuous Dynode Detector
High Voltage
Ion-optics of a continuous-dynode, or channel, electron
multiplier. The electron gain occurs as each electron
strikes the walls of the channel causing secondary
electrons to be emitted.
Figure 5. Gain vs Voltage
109
108
Typical Gain
As a multiplier is used, the surfaces of the dynodes slowly become covered
with contaminants from the vacuum system, causing their secondary
electron emission to be reduced and the gain of the electron multiplier to
decrease. To compensate for this process, the operating high voltage
applied to the multiplier must be periodically increased to maintain the
required multiplier gain.
107
106
105
104
1200
1400 1600 1800 2000 2200 2400 2600
Applied Voltage
A gain curve for an ETP electron multiplier to suit a
Hewlett-Packard 5972 MSD. Typical operating gain
in this instrument is ~2x105 which is achieved with an
applied high voltage of ~1400 volts.
The active dynode surfaces of an Active Film Multiplier are
composed of stable-in-air materials and can be repeatedly exposed
to the air with no loss in performance. The original packaging is
designed for long term storage. The multiplier is delivered in two
sealed plastic bags, the outer bag containing silica gel to absorb any
moisture. If the multiplier is to be stored for long periods it is best
left in its original packaging until required. In its original
packaging, the shelf life of the Active Film Multiplier is guaranteed
for two years from ETP's shipping date. If it is necessary to store
the multiplier without its original packaging, it should be kept in a
dust free, dry environment. Ideally it should be stored in a glass
desiccator containing silica gel.
Figure 6. Applied Voltage vs Operating Life
3500
Applied High Voltage
3000
2500
2000
1500
1000
500
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Fraction of Operating Life
0.8
0.9
1.0
Change in the applied high voltage required to maintain the gain of
an electron multiplier over the course of its operating life (for a typical
GC-MS). The applied high voltage must be periodically increased to
maintain the operating gain of the multiplier.
1.
Partial pressure of contaminants in the residual gas of the
vacuum chamber. This means that a cleaner vacuum
environment will allow the multiplier to achieve a longer
operating life time.
2.
The electron density incident on the surface of the dynode.
Because ACTIVE FILM Multipliers use large area dynodes in
their construction, the amount of surface contamination per unit
dynode area is decreased, resulting in the multiplier having
increased operating life and improved gain stability.
For a typical ACTIVE FILM Multiplier for GC-MS, the total
active dynode surface area is ~1000mm2. This can be compared to
a standard (chrome) 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.
The electron-optics used in ACTIVE FILM Multipliers were
designed using highly specialized CAD software developed by ETP
specifically for this task. This has resulted in a performanceoptimized design that efficiently focuses all the ions that enter the
input aperture of the multiplier onto the first dynode. This very
high ion detection efficiency results in an improved signal to noise
ratio on the output spectra.
Installation, Storage and Handling
Active Film Multipliers are individually designed for each specific
mass spectrometer and require no modifications to the instrument.
They are designed to be fully compatible with the instrument for
which they were designed and can be easily installed.
An Active Film Multiplier requires no preconditioning. However, it
is recommended that the applied high voltage be limited to 2200
volts for the first day of operation.
Any multiplier leads should be positioned to have a minimum
clearance of 3 millimeters between the lead and any metal parts of
the multiplier mounting or mass spectrometer system. This clearance
prevents the risk of noise due to arcing or electrical breakdown.
The multiplier should be handled using normal high vacuum
methods, keeping the multiplier clean and free of contamination.
Powder-free gloves should be used to prevent finger-oils from
contaminating the multiplier via direct contact with skin. All tools,
mountings and equipment should be cleaned before coming into
contact with the multiplier. Care should be taken to minimize the
time that the multiplier is exposed to airborne particles of dust or
lint which would be expected in a typical laboratory environment.
Dust particles within the multiplier can cause increased background
noise.
Exposure of the multiplier to a high humidity environment should
be avoided as it can cause noisy operation. In the event of this
situation occurring, the multiplier can be restored by baking for 3
hours in vacuum at 150ºC, or simply by leaving it in vacuum for a
day or more. After baking, the multiplier should be allowed to cool
in the vacuum chamber to avoid the possibility of damage due to
thermal shock.
The rugged design of the Active Film Multiplier greatly reduces the
chance of damage through rough or careless handling.
Nonetheless, an electron multiplier is a precision instrument and all
reasonable care should be taken when handling.