Page 143 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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50-100 mV and 10 ns in duration. This is almost identical with the way in which a photomultiplier
works, the main differences being that instead of discrete dynodes, there is one continuous dynode,
which is the electron multiplier tube itself, and the electron cascade is initiated by an ion instead of a
photon. In fact, the electron multiplier is equally sensitive to photons, and because of the intensely
bright ICP source, photon noise is the major source of background in ICP-MS. Hence the multiplier is
positioned off-axis from the ion beam, positive ions being defelected into it by a negatively charged
plate opposite the mouth, and the uncharged photons passing by unaffected. Another precaution
commonly adopted is the inclusion of a photon stop on-axis in the ion lenses, which shades the
multiplier from the photon-rich source but allows ions to be focused around it.

Alternatively, the multiplier can be operated in the analogue mode with a gain of only 10 -10 , so that
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the multiplier does not become saturated and the pulses vary greatly in size. In this mode the applied
voltage is between -500 and -1500 V and the electron pulses are read at the collector electrode (Fig. 5.8)
where they are amplified and averaged over a short time interval to allow rapid data acquisition. The
greatest sensitivity is achieved with the detector in the pulse counting mode, but the detector will
become saturated at counting rates above 106 Hz, which are encountered when the analyte is at a high
concentration in the sample. If the detector is switched to the analogue mode it is less sensitive, but can
be used for analyte concentrations which are much higher, typically up to three orders of maginitude
higher than for pulse counting. Such dual-mode operation results in an extremely large linear dynamic
range of up to nine orders of magnitude but requires that two analytical scans be made to acquire all the
data. More recently, electron multipliers with discrete dynodes have come on the market which offer the
advantage of simultaneous pulse counting and analogue detection.

Another commonly used detector is the Faraday cup. This detector is an analogue detector and so has
poorer sensitivity than a pulse counting electron multiplier. However, it has the advantage of simplicity
(it is essentially only a metal plate used to measure ion current), and it does not suffer from burn-out
like an electron multiplier (which must be periodically replaced).

Q. What are the main differences between the scanning and peak-hopping mode of operation of a
quadrupole mass analyser?

Q. What are the advantages and disadvantages of magnetic sector mass analysers compared with
quadrupoles?

Q. What are the main differences between an electron multiplier operated in pulse counting and
analogue mode?