control. For example, a complete analysis on a steel sample just drawn from a furnace can be completed
in two or three minutes and the composition of the bulk melt adjusted, if required, before casting. In the
field of geochemistry, an important application is in the charting of areas of land with respect to the
composition of underlying rock strata. The rapid screening of hundreds of samples taken at
predetermined positions enables the geochemist to produce a detailed map of the distribution of
valuable mineral and ore deposits. However, the development of plasma emission and atomic
absorption spectrometry has steadily displaced arc/spark methods from many applications.
Notwithstanding the need to take samples into solution, the former techniques are often preferred
because of greater sensitivity, better precision and relative freedom from interferences. Nevertheless,
arc/spark emission remains important especially in metallurgy where speed of sample preparation and
analysis may outweigh other considerations.
8.2—
Glow Discharge Atomic Emission Spectrometry
Summary
Principles
Emission spectrum generated by bombardment of sample by argon ions in a 'sputtering process'.
Instrumentation
Glow discharge lamp (analogous to hollow cathode lamp) in which the sample acts as the cathode.
Attached to a standard atomic emission spectrometer.
Applications
Limited to samples that are electrically conducting. Gives high stability signals. Valuable in metallurgy.
Disadvantages
Limited scope for sample types. Gives analysis of surface only.
A recent development in emission spectroscopy has been the glow discharge lamp (GDL) as an
excitation source. This source overcomes the reliability variations discussed in section 8.1 which lead to
errors and poor precision. For electrically conducting samples such as metals and alloys it shows an
impressive performance. Apart from the modified source, the overall instrumentation remains similar to
that discussed in section 8.1. A silent electric discharge through an argon atmosphere will lead to the
formation of positive argon ions. Under the influence of a potential gradient