8.3—
Plasma Emission Spectrometry
Summary
Principles
Emission of electromagnetic radiation in the visible and ultraviolet regions of the spectrum by atoms
and ions after electronic excitation in a high temperature gas plasma. Laser excitation sometimes used.
Instrumentation
Emission spectrometer incorporating a sample nebulizer, grating monochromator, photomultiplier
detection system and microprocessor controller. Excitation by dc-arc plasma jet, or inductively coupled
plasma. Laser excitation sometimes used.
Applications
Very widespread for qualitative and quantitative analysis of metals and some non-metals, particularly at
trace levels. Relative precision 0.5–2%.
Disadvantages
Samples require dissolution before analysis. Instrumentation is complex and expensive to purchase and
operate.
At high temperatures (~ 6000 K) a gas such as argon will contain a high proportion of ions and free
electrons constituting a plasma. Additional energy may be supplied to the electrons in the plasma by the
application of an external electromagnetic field. By collisions between the electrons and other species in
the plasma this additional energy is uniformly distributed. If the gas pressure is high the mean free path
of the electrons decreases and the probability of collision increases making the energy transfer more
efficient and leading to a substantial temperature enhancement. Small plasmas suitable for emission
spectrometry sources with power densities of 1 kW cm–^3 and temperatures of 8000–10 000 K operating
at ambient pressures can be produced in this way. Plasma excitation offers an attractive alternative to
that of arc/spark because of the high temperatures attainable and the homogeneity of the source. The
plasma provides a substantially better signal stability and thus analytical precision as well. Two types of
plasma sources are in widespread use. These are based on a dc-arc discharge in one case, and on radio
frequency induction heating in the other.