Page 76 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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The West rod atomizers were marketed for a while, but their production has also ceased. These
atomizers had the advantages of simplicity, low power requirements (less than half that required by a
furnace) and fast heating rate (2000 K s ). They were, however, considered to be extremely prone to
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interferences. This was attributed to the rapid cooling of the atoms once they had left the filament. This
was partially overcome by setting the light beam so that it grazed the surface of the rod.
Other modifications that have been produced include incorporating a cavity or 'minifurnace' into the
rod. This was accomplished by placing a hollow-cathode cylinder or a graphite cup between two
spring-loaded rods. These mini-furnaces could cope with samples of up to 20 mm , or in some cases
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solid samples could also be analysed.
Q. What are the advantages and disadvantages of metal furnaces compared with graphite furnaces?
Q. What are the advantages and disadvantages of filaments compared with graphite furnaces?
Q. How has the original design of the West rod been modified to minimize these disadvantages?
3.4 Atomization Mechanisms
An investigation of both thermodynamic and kinetic considerations is necessary in the understanding of
atomization in graphite atomizers.
3.4.1 Thermodynamic Considerations
Several possible reactions may be involved.
(i) Conversion of metal salts to the oxide. When heated strongly after deposition from aqueous
solution, nitrates, sulphates and some chlorides are usually converted to the oxide.
(ii) Evaporation of the metal oxide or metal halide prior to atomization. Most metal halides are
volatile and evaporate before atomization. Some metal oxides have measurable vapour pressures at
the temperatures at which atomization is first observed to occur (the so-called 'appearance
temperature'). Using the simple gas law, 1 ng of a metal oxide (molecular weight = 100), completely
vaporized into a volume of 100 mm at 1000 K, would exert a vapour pressure of 6 x 10 mmHg. Thus,
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unless this exceeds the saturated vapour pressure at 1000 K, complete evaporation could be expected.
