peratures (up to 10 000 K) produced by electrical discharges and radio frequency heated plasmas
involve excitation to many more levels and consequently result in more complex spectra, including
contributions from ionized species. Conversely, excitation by absorption of ultraviolet or visible
radiation, the principle upon which atomic absorption and atomic fluorescence spectrometry depends,
usually involves only resonance transitions (vide infra) and therefore results in very simple spectra.
Finally, X-radiation excites electrons occupying inner orbitals and results in fluorescence emission in
the X-ray region, the spectra again being relatively simple. The various means of excitation will be
discussed more fully in later sections.
Intensity of Spectral Lines
The intensities of atomic spectral lines are determined by their transition probabilities, which are largest
for transitions to and from the ground state giving rise to the so-called resonance lines. Weak lines,
arising from nominally 'forbidden' transitions, may also appear if the selection rules are relaxed by
perturbation effects which alter orbital symmetries. Line intensities are also governed by the time atoms
spend in the excitation region, i.e. the flame, the electrical discharge or the beam of electromagnetic
radiation. This residence time can be very short for volatile analytes and where thermal excitation is
used because strong vertical thermal currents tend to sweep the sample rapidly out of the excitation
zone. Clearly, the longer the residence time, the more sensitive the measurement process becomes and
this is an important parameter that needs to be optimized in developing an analytical procedure.
Instrumentation
The basic instrumentation used for spectrometric measurements has already been described in the
previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic
emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated
from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence
spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and
plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or
prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where
a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are
sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors
are used in every technique except X-ray fluorescence where proportional counting or scintillation
devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by
optical emission spectrometry, but is now rarely used.