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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