Page 24 - Inorganic Mass Spectrometry - Fundamentals and Applications

14 Smith

Schematic drawing of a multicollector mass spectrometer.

would be predicted to be element-dependent. At the time of writing detector arrays
are available only as Faraday cups; it has thus far not been possible to reduce the
size of electron multipliers sufficiently to use them in arrays, and position-
sensitive detectors lack the necessary dynamic range (>106) and tend to be too
noisy. The trade-offs between detector systems thus involve balancing precision
requirements against those of sample size.

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Either the ion source high voltage or the magnetic field can be swept to scan the
mass spectrum-unless, of course, a multidetector array is used, in which case the
spectrum need not be swept. Magnetic sweeping is in general to be preferred to
high-voltage sweeping because, when the voltage is swept, each isotope experi-
ences a slightly different extraction field; this introduces mass-dependent bias for
which correction must be made. There are circumstances, however, when the
advantages of voltage scanning outweigh the drawbacks. One occurs when the
mass spectrometer has two magnetic sector fields in tandem; it is difficult to make