pressed from powder. Homogeneity of the electrode is of prime importance. A pointed graphite rod or
tungsten is used as a counter electrode. For non-metals, high purity graphite electrodes are used, the
lower electrode, or anode, having a cup machined in the end to contain the powdered sample. Intimate
mixing of the sample with graphite powder and a spectroscopic buffer or carrier (vide infra) is
necessary to optimize and control excitation conditions. A laser is focused directly on to the sample and
will provide a signal from a very small part of it. Particularly valuable is the potential of the laser for the
analysis of very small specimens.
Qualitative and Quantitative Analysis
Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples
excited by arc and spark discharges are considerable. For this reason instantaneous measurements are
not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over
periods of up to several minutes. Modern instruments will be computer controlled and fitted with
VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the
spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay
routines for comparisons with standard spectra, detection of interferences and their correction (Figure
8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels
(ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading
spectrometers.
Figure 8.4
Wavelength characterization scan (from ARL laboratories).
5000 ppm Cr; - - - 1000 ppm Cr; -.-.- 10 ppm Co.