Page 20 - Inorganic Mass Spectrometry - Fundamentals and Applications
P. 20
between them is made on the basis of the needs of the experiment. ~ultifilament
configurations are most common. mere can be either two or three filaments, but
the principle of operation is identical for each. The sample is loaded on a side
filament, which is often made of tantalum rather than the more expensive rhenium.
The sahple is evaporated from the side filament at a temperature that depends on
the volatility of the element undergoing analysis. This vapor, which is not neces-
sarily atomic, impinges on the ionizing filment, which is almost invariably made
of rhenium and maintained at a constant high temperature; 12100°C is common.
Ions generated at this surface are subjected to the extraction field of the ion source,
are accelerated, and undergo mass analysis. The temperature required for the
evaporating filament is substanti~ly lower than that required for ionization when
only a single filament is used; for uranium, for example, the two temperatures are
1500°C for the evaporator and 1700- 1800°C for single-filament ionization. Sam-
ple size is usually in the microgram range when Faraday cup detection is used; as
a result, such detectors are comon in multifilament systems.
It is necessary to obtain stable emission from the evaporator if results of
high quality are desired. In general, each element requires a different procedure,
although of course some elements behave similarly. Samples and standards are
treated as much alike as possible to ensure that data are taken on that portion of
the fractionation curve for which calibration is valid; this topic is treated in greater
detail later.
In the single-filament configuration, the same surface serves both to evapo-
rate and to ionize the sample. It is more efficient than multifilament con~gurations
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