one atom emits a photon there is a chance some other atom will absorb
it. To absorb the photon the energy of the photon must match a possible
electron transition in the atom, normally this means the absorbing
atom must be of the same type as the emitting atom, hence the term
This means, for example, emissions from iron atoms can only be
absorbed by other iron atoms, because only other iron atoms have
matching electron energy levels. Also, to match energy levels, the
emitting and absorbing atoms must normally be in the same excitation
state, and since most atoms in analytical plasmas are in or near
the ground state, self-absorption is normally only seen for electron
transitions involving the ground or near-ground states. These transitions
are called resonance or near-resonance transitions.
The effect of self-absorption is to rob signal from
the emission line. As the number of emitting atoms increases, the
likelihood of self-absorption increases and the there is no longer
a linear relationship between the number of emitting atoms and the
In simple models of self-absorption, it possible to
show that the severity of self-absorption depends on the product
of the number of emitters and the effective absorption cross-section,(1)
given by (2)
where M is the atomic weight of the emitting
atom or ion, T is the absolute temperature of the plasma
gas, l0 the wavelength
emitted, and f the oscillator strength. Self-absorption then
tends to be highest when there is a large number of emitters, of
high atomic weight, at lower gas temperatures, longer wavelengths,
and higher oscillator strengths. Clearly the problem of self-absorption
will vary greatly from one emission line to another and one emission
source to another.
Spark OES is known to have severe self-absorption
on some lines, ICP-OES has moderate problems at high concentrations,
and GD-OES has less severe problems, though still present
for some important lines in some materials, e.g. Zn I 213.8 nm
and Cu I 327.3 nm in brass.
For more details on self-absorption, especially related
to glow discharge, the GDOES
- R Payling, M S Marychurch and A Dixon, in Glow
Discharge Optical Emission Spectrometry, R Payling,
D G Jones and A Bengtson (Eds), John Wiley &
Sons (1997), pp 376-91.
- N P Ferreira and H G C Human, Spectrochim.
Acta 36B, 215 (1981).
- Th. Nelis R. Payling, A practical Guide Glow Discharge Optical Emission Spectroscopy,RSC, 2004
First published on the web: 15 November 1999.
Author: Richard Payling