Page 108 - Inorganic Mass Spectrometry : Fundamentals and Applications

98 Olesik

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signals either an analogue detection system must used or the ion transmission to
the detector can be reduced by defocusing or other means. Channeltron electron
multipliers, discrete dynode electron multipliers, Daly detectors, and Faraday
collector detectors have been used.
Continuous dynode electron multipliers (such as the Channeltron) are horn-
shaped detectors (Fig. 3.14.b). A high voltage is applied between the input and
output ends of the detector, When an ion strikes the detector, secondary electrons
are produced. These electrons in turn strike the wall of the detector, generating
more electrons. Up to 108 electrons are produced and collected at a collector
electrode at the output end of the detector for each incident ion, depending on the
applied voltage.
Discrete dynode detectors (Fig. 3.14a) are similar in concept except that
there are separate dynodes rather than a continuous dynode surface. The voltage
applied to each dynode stage can be controlled. A dual-mode variation of the
discrete dynode detector (Fig. 3.14~) allows signals to be measured partway down
the dynode chain (with analogue detection for large signals) or at the end of the
dynode (with pulse-counting detection for signals of less than about 106 counts/
sec). The two signals can be monitored simultaneously. Furthermore, the voltage
applied to the dynodes past the point of analogue detection can be turned off to
prevent damage when signals are large. The analogue gain of the detector is a
function of mass and can. change over time. Therefore, the analogue signal must
occasionally be calibrated relative to the pulse-counting mode.
Current from a Faraday cup detector (Fig. 3.14d) is measured with an
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analogue amplifier. The resulting analog output voltage can converted to pulses
by using a voltage-to-frequency converter. The pulses can then be counted with
the same electronics used for ion counting. Ion signals corresponding to a mini-
mum of about IO4 ionslsec can be detected.
A Daly detector [ 138,1391 (Fig. 3.14e) is also being used in some instru-
ments. Ions strike a negatively charged target (an aluminum surface, example)
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to produce secondary electrons. The electrons are accelerated to a thin metal film
held at ground on top of a scintillator. Light is produced when the secondary
electrons strike a scintillator. The light is detected by a p~otomultiplier tube.
Advantages of the Daly detector include long lifetime, a wide linear dynamic
range, and lack of gain loss after monitoring a very strong signal.

ass spectra produced by ICP-MS consist of elemental ions, polyatomic ions
produced from plasma and solvent species, and polyatomic ions produced from