Page 66 - Inorganic Mass Spectrometry - Fundamentals and Applications

surface of the multiplier generates a pulse of electrons. This pulse is fed to an
amplifier and then to a discriminator that conditions the pulse, filters out the
background, and directs it into a counterlprocessor whose output consists of a
digital count rate.
One specific type of pulse-counting detector is called a Daly detector after
is
its developer [98]. In this system, the ion beam accelerated to 20 kV and directed
to a highly polished surface of a material with a large cross section for secondary
electron emission. Ejected electrons impinge on a plastic scintillator, whose
flashes of light are registered by a photomultiplier outside the vacuum system.
Data processing then proceeds as with the other pulse-counting detectors.

S has found widespread analytical application in trace element analysis, and
In
its greatest use in the routine analysis of metals, alloys, and semiconductors. the
last 10 years, GDMS has become a routine technique, providing reliable data in
commercial laboratories. Semiconductor materials compose the majority of sam-
ple types analyzed by GDMS, but as new methods are developed to analyze such
things as solution residues and nonconductors (e.g., glasses, ceramics, and soils),
analysis of other types of smples may become more common mong GDMS
practitioners. This section highlights some of the novel applications of GDMS;
most of the examples are from the past 10 years. The discussion is not intended to
be all-inclusive but to provide an overview of the direction in which the field is
moving.

The precise measurement of isotope ratios of solids has always been an important
area in inorganic mass spectrometry. These measurements are usually performed
by using thermal ionization [see Chapter l, as well as excellent reviews by Heu-
mann (99) and DeLaeter (loo)]. However, recent interest in environmental chem-
istry and the expanding role of mass spectrometry in nuclear technology have
of
~enerated a need for rapid, relatively precise measurement isotope ratios in bulk
solids. Most of the work in this field has proceeded in two laboratories: the
Institute for ~ransuranium Elements in Karlsruhe, Germany [loll, and the Oak
Ridge National Laboratory in Oak Ridge, Tennessee. The inst~~ent used in both
cases was the VG-9000 double-focusing magnetic sector with reverse Mer-
A
Jo~son geomet~. direct current discharge was used for all of the studies
reported; the ~arls~he group used a secondary cathode for nonconducting sam-
ples and the Oak Ridge group mixed their nonconducting samples with copper or
silver powder prior to analysis.