outer 'mantle' will be more characteristic of atoms. Each element is characterized by the wavelengths at
which radiation is emitted and the intensity of spectral lines is directly related to the concentration of
that element in the sample. The technique therefore provides a means of qualitative identification and
quantitative analysis.
Instrumentation
A diagram of an emission spectrometer is shown in Figure 8.3. The electric discharge occurs between a
pair of electrodes (preferably enclosed in a special housing for reasons of safety) one of which contains
the sample. Emitted radiation is dispersed by a prism or grating monochromator and detected by
photomultipliers.
Figure 8.3
Schematic diagram of an emission spectrometer.Excitation by Electric Discharge
During the many years that atomic emission spectrometry has been employed for chemical analysis a
variety of types of excitation sources have been used. In earlier times electric discharges, dc-arcs and
ac-sparks, found considerable favour. The inherent instability of the discharges has meant that as more
stable alternatives have been developed they have been progressively replaced by them. Where
electrical excitation is still employed it is achieved by an electrically controlled spark with far greater
stability and much improved precision for the analysis.
Excitation by Laser
A rather specialized emission source, which is applicable to the study of small samples or localized
areas on a larger one, is the laser microprobe. A pulsed ruby laser beam is focused onto the surface of
the sample to produce a signal from a localized area ca. 50 μm in diameter. The spectrum produced is
similar to that produced by arc/spark sources and is processed by similar optical systems.
Sample Preparation
For the simplest analyses of metals and alloys, electrodes made of the sample material itself may be
used, a disc or cylinder being cut, cast or