Section E – Spectrometric techniques
E7 ATOMIC ABSORPTION AND
ATOMIC FLUORESCENCE
SPECTROMETRY
Principles The energy levels of atoms are specific and determined by the quantum
numbers of the element. If ground state atoms are excited, as described in Topic
E1, some will be promoted to higher energy levels, the transitions being charac-
teristic of the element involved. Atoms may be excited by incident UV or visible
electromagnetic radiation, and if the wavelength (or frequency) corresponds to
that of the transition, it will be absorbed. The degree of absorbance will depend
on concentration, in the same way as with other spectrometric techniques. This
technique is known as atomic absorption spectrometry (AAS).
The sample is generally volatilized by a flame or furnace. The temperature is
not usually sufficient to produce ionization, so that the vapor contains largely
atoms. These atoms absorb the characteristic incident radiation resulting in the
promotion of their electrons to an excited state. They may then undergo
transitions to other energy levels and re-emit radiation of another, but still
characteristic, wavelength as fluorescence. This allows determination by atomic
fluorescence spectrometry (AFS).
Ideally, the lines of an atomic spectrum should be very narrow, of the order
of 10-^5 nm. However, thermal movement causes Doppler shifts andpressure
broadening due to collisions among the atoms, and also electrical and magnetic
fields in the sample vapor all tend to broaden the lines slightly to about 10-^2 nm.
If a source emitting a broad band of wavelengths were used, the fraction
absorbed by a narrow line would be small. It is therefore important to use a
source producing a sharp emission line characteristic of the element to be
Key notes
The absorption of electromagnetic radiation by atoms allows both
qualitative and quantitative determination of a wide range of elements.
Alternatively, fluorescence radiation may be emitted and measured.Narrow band sources of radiation specific to particular elements irradiate
the atomic vapor produced by flame, furnace or other methods and the
absorption is measured. For fluorescence, both continuum and element
specific sources are used.Atomic absorption spectrometry is used widely for the quantitative
determination of metals at trace levels. Atomic fluorescence spectrometry
is usually limited to mercury and other volatile species.Related topics Flame atomic emission Inductively coupled plasma
spectrometry (E4) spectrometry (E5)PrinciplesInstrumentationApplications