Page 65 - Inorganic Mass Spectrometry - Fundamentals and Applications

Glow isc charge ass Spectrometry 55

Y

HIGH MASS

ION
SOURCE

\
ION
0
SOURCE
DETECTOR

Schematic diagram of (a) a magnetic sector mass spectrometer and (b) a
quadrupole mass Spectrometer.

Once the ions exit the mass spectrometer, they must be detected. Several
different detection systems are used with glow discharge mass spectrometers,
including Faraday cups, electron multipliers, and microchannel plates. Choice of
detector is often independent of the kind of mass spectrometer, although some
combinations of mass analyzer and detector axe more common than others (e.g.,
microchannel plates are used extensively with time-of-flight mass spectrometers).
of
Faraday plates are the simplest type of detectors, These devices usually consist
a thin metal plate, electrically isolated from the housing and positioned to inter-
cept the charge flux emerging from the mass separating device (i.e., qua~pole
rods, magnetic sector, etc.). Collisions of ions with the plate induce a current that
can be amplified and, by dropping it across a resistor, converted to an output
voltage. This detector requires relatively large current flux (typically greater than
a
a
10 PA) and is used primarily for monitoring major species in spectrum. To detect
the trace constituents, an electron multiplier is often employed. The multiplier is
usually positioned off the ion axis to reduce high-energy ion and stray photon
noise. The aperture of the multiplier is held at a large negative potential to
accelerate the ions emerging from the mass spectrometer toward the surface of the
device. At the surface, ions impact and release free electrons that are multiplied
through a cascade process, generating gains of up to lo6. Some multipliers can be
operated in both the analogue and pulse-counting modes. In the analogue mode,
the current generated by the multiplier is output in a similar fashion to that
generated by a Faraday plate. In ion counting mode, each incident ion on the