Mass analysis - i.e.
the separation of bunches or streams of ions according to their
individual mass-to-charge (
m/z)
ratio, is only part of the job of a mass spectrometer. Without some
form of accurate and reliable ion detection, everything that happens
previously would be pointless. All mass spectrometers, apart from
FT-ICR - which by it's very definition
is a combined mass analyser and detector, require an ion detector. The
choice of detector depends on the design of the instrument and
the type of experiment it was designed to perform. The detector
generates a signal from
incident ions by either generating secondary electrons, which are
further
amplified or by inducing a current generated by a moving charge
(similar to
FT-ICR). The earliest
ion detectors (back in the days of Thomson and Aston)
consisted of photographic plates located at the end of the mass
analyser. All ions of a given
m/z
would impact at the same place on the photographic plate making a spot.
The darkness of the spot was indicative of the intensity of that
particular
m/z. The most
common types of ion detector used in modern instruments are described
below.
1. The Faraday Cup or Cylinder
The Faraday cup or cylinder electrode detector is very simple. The
basic principle is that the incident ion strikes the dynode surface
(see
Fig. 1) which emits electrons and induces a current which is amplified
and recorded. The dynode electrode is made of a secondary
emitting material like CsSb, GaP or BeO. The Faraday cup is a
relatively insensitive detector but is very robust. It is ideally
suited to isotope analysis and IRMS.
Fig 1. A schematic of a Faraday Cup or
Cylinder Electrode.
2. The Electron Multiplier
Electron multipliers are probably the most common means of detecting
ions, especially when positive and negative ions need to be detected on
the same instrument. Their are two types (Fig. 2) of electron
multiplier, but they both work essentially by extending the principles
of the
Faraday cup. A Faraday cup uses one dynode and as a result produces one
level of signal amplification. One type of electron
multiplier (Fig. 2a) has series of dynodes maintained at
increasing potentials resulting in a series of amplifications. The
other type (the channel multiplier, Fig. 2b) has a curved ('horn'
shaped)
continuous dynode where amplifications occur through repeated
collisions
with the dynode surface. In both cases, ions pass the conversion
dynode (depending on their charge) and strike the initial amplification
dynode surface producing an
emission of secondary electrons which are then attracted either to
the second dynode, or into the continuous dynode where more secondary
electrons are generated in a repeating process ultimately resulting in
a cascade of electrons. Typical amplification is of the order of one
million to one.
Fig. 2. Schematics of the two types of
electron multiplier, showing the cascade
of electrons that results in
amplification.
3. The Photomultiplier or Scintillation Counter
In the photomultiplier (or scintillation counter) the ions initially
strike a dynode which results in electron emission. These electrons
then strike a phosphorous screen which in turn releases a burst of
photons. The photons then pass into the multiplier where amplification
occurs in a cascade fashion - much like with the electron multiplier.
The main advantage of using photons is that the multiplier can be kept
sealed in a vacuum preventing contamination and greatly extending the
lifetime of the detector. Photomultipliers are now probably the most
common detectors in modern mass spectrometers.
Fig. 3. Schematic of a photomultiplier,
showing the
conversion of the ion/electron signal into photon(s)
which are then amplified and detected by the photomultiplier.
