Page 22 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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1.3.3 Atomic Fluorescence.

In atomic fluorescence spectroscopy an intense excitation source is focused on to the atom cell. The
atoms are excited then re-emit radiation, in all directions, when they return to the ground state. The
radiation passes to a detector usually positioned at right-angles to the incident light. At low
concentrations, the intensity of fluorescence is governed by the following relationship:

where If is the intensity of fluorescent radiation, C is the concentration of atoms, k is a constant, I0 is
the intensity of the source at the absorption line wavelength and F is the quantum efficiency for the
fluorescent process (defined as the ratio of the number of atoms which fluoresce from the excited state
to the number of atoms which undergo excitation to the same excited state from the ground state in unit
time).

The intensity of fluorescence is proportional to the concentration of atoms, and hence the concentration
of the element in the sample, so a plot of concentration against fluorescence will yield a straight line.
There are several different types of atomic fluorescence as follows:

(i) Resonance fluorescence.
(ii) Direct line fluorescence.
(iii) Stepwise line fluorescence.
(iv) Thermally assisted fluorescence.

These are described in more detail in Chapter 6. Resonance fluorescence, i.e. the excitation and
emission are at the same wavelength, is most widely used. The others have very limited use
analytically.

1.3.4 Atomic Mass Spectrometry

The degree of ionization of an atom is given by the Saha equation:

where n i n and n are the number densities of the ions, free electrons and atoms, respectively, Z and Z
a
i
a
e
are the ionic and atomic partition functions, respectively, m is the electron mass, k is the Boltzmann
constant, T is the temperature, h is Planck's constant and E is the first ionization energy.
i
In atomic mass spectrometry, the rate of production of ions is measured directly. This is proportional to
the concentration of ions, and hence atoms. A plot of ion count rate against atom concentration will
therefore yield a straight line.