Page 128 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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the angle of the 'Y'. The excitation temperature is about 5500 K. The DC plasma is comparatively
economical to run, consuming less than 1000 W and about 81 min of argon. Unfortunately there may
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be a tungsten background spectrum from the electrodes. A very troublesome problem with this plasma
appears to be the substantial enhancement of emission caused by easily ionizable elements. Buffering
with lithium or barium to overcome the problem has been proposed. An echelle monochromator (or
polychromator if the exit slit is replaced with a cassette of about 20 elemental slits) has been used
extensively with this type of plasma, offering excellent resolution, precision and accuracy, with lower
detection limits than with flame emission, although not as good as with ICP-AES.
Q. What are the main differences between the MIP and ICP?
Q. What interference problems may be encountered with microwave plasmas?
Q. Why are microwave plasmas used widely as detectors for gas chromatography?
Q. Where is the main analytical observation zone in the DCP?
Q. What are the advantages and disadvantages of the DCP?
4.6 Solid Sampling Plasma Sources
4.6.1 Arcs and Sparks.
An arc is a continuous electrical discharge of high energy between two electrodes. In the DC arc, the
sample is usually packed into an anode electrode, or the sample itself becomes the anode as, for
example, in metal analysis. The arc is struck and the sample is vaporized into the discharge region,
where the excitation and emission occur. Because relatively large amounts of sample are excited, the
detection limits are low, but the unstable nature of the discharge leads to poor precision. Additionally,
the intensity of emission is highly dependent on the matrix. If the light from the discharge is dispersed
by a prism, and a camera of long focal length (e.g. 1 m) is used, a photograph of the spectrum gives
good resolution and can be used for the rapid qualitative identification of the elements in an unknown
sample.
If an intermittent discharge is used (e.g. an AC spark), the precision can be greatly improved.
However, the detectability is poorer because only
