Page 344 - Instrumentation Reference Book 3E

![ Atomic techniques: emission, absorption, and fluorescence 327

trons follow orbits immediately adjacent to the
nucleus. If energy is imparted to the atom by
means of a flame or an electric arc or spark, then
it undergoes excitation and its electrons move
into orbits further removed from the nucleus.
The greater the energy, the further from the
I
I
nucleus are the orbits into which the electrons
are moved. When sufficient energy is imparted
to the electron, it may be torn from the atom.
and the atom becomes a positively charged ion.
Atoms will not remain in this excited state, espe-
'I cially when removed from the source of energy,
and they return to their original states with elec-
trons falling to lower orbits. This electron transi-
iion is accompanied by 2 quantum of light
energy. The size of this pulse of light energy and
its wavelength depend on the positions of the
orbits involved in the transition.
The energy emitted is E = Izv
where h is Planck's constant, and v is the fre-
quency of the radiation. Or
Figure 16.9 Backscatter infrared gauge. Courtesy Infra-
red Engineering Ltd. E = IZCIX
or flake, refractory mixtures, paper, textiles, feed- where c is the velocity of light and X the wavelength.
ing stuffs, and a wide range of other materials may Hence the greater the light energy quantum, the
be undertaken with an accuracy of il percent of shorter is the wavelength of the light emitted.
instrument full scale. Only the outer, valence electrons participate in
the emission of spectral lines. The number of
16.1.5 chemiluminescence valence electrons in an atom differs for chemical
elements. Thus the alkali elements, sodium,
When some chemical reactions take place, energy lithium, potassium, etc., contain only one electron
may be released as light. This phenomenon is in their outer shell and these elements have simple
known as chemiluminescence. There are many spectra. Such elements as manganese 2nd iron
instruments which make use of this effect for the have five or six valence electrons. and their spectra
determination of the concentration of oxides of are very complex. Generally speaking, the struc-
nitrogen and for ozone. The principles are ture of an atom is closely bound up with its
described in Chapter 18. optical spectrum. Thus if a mixture of atoms (as
found in a sample) are excited by applying
16.2 Atomic techniques: energy, then quantities of light are emitted at
emission, absorption, and various wavelengths, depending on the elements
dlUOa@sCenC@ present. The intensity of light corresponding to
one element bears a relationship to the concentra-
16.2.1 Atomic emission spectroscopy tion of that element in the sample.
In order to sort out the light emitted, use is made
This is one of the oldest of techniques employed of a spectroscope. In Figure 16.10-16.12 are shown
for trace analysis. Because of its relative simplicity,
sensitivity, and ability to provide qualitative infor- P
mation quickly, it has been widely used in both
industrial and academic analytical problems. It
can be used for the analysis of metals, powders,
and liquids and is used extensively in the steel and
non-ferrous alloy industries: and the advent of
inductively coupled plasma sources for producing
spectra has made the technique invaluable for the
analysis of some 70 elements in solution4own to
concentrations of 1 ppb and less. The basic princi- Figure 16 .I 0 Optical system of a simple spectroscope
ples of the technique are as follows. S, slit; C, collimator lens; P, prism;T, telescope lens;
Each atom consists of a nucleus around which F, curve along which the various parts of the spectrum are
in focus; B, blue or short wavelength part; R, red or long
revolve a set of electr'ons. Normally these elec- wavelength part.