Page 348 - Instrumentation Reference Book 3E

X-ray spectroscopy 331
sampled is passed through a tube located between Stepwise fluorescence This entails excitation of
source and detector, and the absorption is a mea- the atom to a high energy level. The atom is then
sure of the mercury vapor in the air. There are de-activated to the first excited state. There, it
many instruments manufactured for this purpose. emits resonance radiation on returning to the
and all are very sensitive with limits of detection of ground state, e.g., the emission of sodiilm fluor-
around 0.1 ppm by volume. escence at 589 nm, following excitation at 330.3 nm.

Sensitized fluorescence This occurs when the
162.3 Atomic fluorescence spectroscopy atom in question is excited by collision with an
excited atom of another species and normai reson-
This is a technique closely allied to atomic ance fluorescence follows. Thallium will fluoresce
absorption. To initiate atomic fluorescence, neu- at 377.6nm and 535nm following a collision of
tral atoms in a flame cell are excited as in atomic neutral thallium atoms with mercury atoms
absorption, Le., by absorption of a characteristic excited at 253.7nm.
radiation. Fluorescence occurs when these atoms An instrument used to determine trace
are de-activated by the emission of radiation at amounts of elements in solution by atomic fluor-
the same or a different wavelength. The fluores- escence very simply consists of (a) an excitation
cent wavelength is characteristic of the atoms in source which can be a high intensity hollow cath-
question and its intensity is proportional to the ode lamp, a microwave-excited electrodeless dis-
atomic concentration. In practice, initiation is charge tube, some spectral discharge lamps or
achieved with a high intensity source, and the more recently. a tunable dye laser; (b) a flame cell
fluorescent signal emitted by the atomic vapor is or a graphite rod as in atomic absorption: and (c)
examined at right angles by passing it into a a detection system to measure the fluorescence at
radiation detection system. Very briefly the basic right angles to the line between source and flame.
equation relating the intensity of a fluorescent The detection system is usually a simple mono-
signal to atomic concentration is
chromator or narrow band filter followed by a
F = 2.303dIoe~lcp photomultiplier tube, amplifier, and recording
device. Limits of detection are achieved which
where F is the intensity of fluorescent radiation, 0 are much lower than those obtained by atomic
the quan'rurn efficiency (which factor has to be absorption because it is easier to measure small
used to account for energy losses by processes signals against a zero background than to mea-
other than a fluorescence), Io is the intensity of sure small differences in large signals as is done in
the excitation radiation, eA the atomic absorptiv- atomic absorption. Detection limits as low as
ity at the wavelength of irradiation, 1 the flame 0.0001 ppm are quoted in the literature.
path length. c the concentration of the neutral
atom absorbing species, and p a proportionality
factor relating to the fraction of the total fluores-
cence observed by the detector. Thus: F = Kdoc 16.3 X-ray spectroscopy
for a particular set of instrumental conditions,
and c is proportional to F, and F will increase if 16.3.1 X-ray fluorescence spectroscopy
the intensity of the irradiating source is increased. Many books have been written about this techni-
There are four types of atomic fluorescence. que and only a brief outline is given here.
The technique is analogous to atomic emission
Resonance j%orescence This is the most intense spectroscopy in that characteristic X-radiation
type of fluorescence and most widely used in arises from energy transferences involved in the
practice. It occurs when the fluorescent and exci- rearrangement of orbital electrons of the target
tation wavelengths are the same, that is, the atom element following ejection of one or more eiec-
is excited from the ground state to the first excited trons in the excitation process. The electronic
state and then emits fluorescent energy on de- transitions involved are between orbits nearer to
activation to the ground state. the nucleus (see Figure 16.14).
Thus if an atom is excited by an electron beam
Direct line fluorescence Were. the valence elec- or a beam of X-rays, electronic transitions take
tron is excited to an energy level above the first place, and characteristic X-radiation is emitted for
excited state. It is then de-activated to a lower that atom. If. after collimation, these X-rays fall
energy ievel (not the ground state), and fluores- on to a crystal lattice-which is a regular periodic
cent energy is emitted. The wavelength of fluor- arrangement of atoms-a diffracted beam will
escence is longer than the excitation wavelength, only result in certain directions, depending on the
e.g., the initiation of thallium fluorescence at wavelength of the X-rays A, the angle of incidence
535 nm by a thallium emission at 377.6 nm. 0, and atomic spacing within the crystal d.