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relatively small amounts of sample will be vaporized. Modern electrical discharges combine the
characteristics of both arc and spark (e.g. a unidirectional spark may be used so that the intermittent
discharge always excites the sample as the cathode) to obtain optimal detectability and precision for
quantitative work. Further improvements have been made by sheathing the electrodes with argon to
reduce the amount of self-absorption by the cooled sample, reduce the background and stabilize the
discharge. Attempts to reduce the influence of surface effects on the final result by careful pre-sparking
are also made. Such discharges are used very widely in polychromator systems, with computer control
of the data acquisition and calculation of the several corrections needed because of inter-element effects
and spectral interferences. These instruments are referred to as direct reading spectrometers and, in
conjunction with arc/spark sources, are widely used in the steel industry because they can be used for
the rapid analysis of solid samples. Mobile spectrometers incorporating arc and spark discharges have
recently become available and are used extensively for spot testing raw materials and scrap.

4.6.2 Glow Discharges

Glow discharges can be used to analyse both conducting and non-conducting samples. A glow
discharge is formed between two electrodes in an inert gas (e.g. argon) at low pressure (0.1-10 Torr).
The sample forms one of the electrodes, usually the cathode with the wall of the discharge chamber
forming the anode (Fig. 4.22). With DC glow discharges, non-conducting samples must be mixed with
a conducting material (e.g. graphite) and pressed into a pellet, while radiofrequency glow discharges
allow the direct analysis of non-conducting samples. When the discharge is initiated, argon atoms are
accelerated across the dark space towards the sample surface where they dislodge several atoms in a
process known as sputtering. The sputtered atoms are then ionised and excited in the negative glow
region of the discharge.
One popular configuration is the Grimm source, which accepts samples in the form of discs. Such
sources usually operate at 500-1000 V, 25-100 mA and 1-5 Torr, with detection limits of approximately
0.1 ppm. Another configuration is the hollow-cathode lamp in which the sample can be either
machined as a hollow cathode, evaporated to dryness (if a solution) or pressed (if a powder) into a
hollow cathode made of pure graphite. Typical operating conditions are 200-500 V, 10-100 mA, and
0.1-1.0 Torr, with detection limits in the range 0.1-10 ppm.