Page 346 - Instrumentation Reference Book 3E

Atomic techniques: emission, absorption, and fluorescence 329
dard sample of known purity must be analyzed (parts per lo9) with an accuracy of about
under exactly the same conditions as the 10 percent.
unknown samples and the intensity of the spectral
lines compared. Thus a spectrochemical labora-
tory may have many thousands of standards cover- 16.2.2 Atomic absorption spectroscopy
ing the whole range of materials likely to require In emission spectroscopy, as we have already
analysis. seen, the sample is excited, the emitted radiation
dispersed and the intensities of the selected lines
in the emission spectrum measured. If self-
16.2.1.5 Applications
absorption and induced emission are neglected.
There are very few on-line instruments employing then the integrated intensity of emission of a line
atomic emission techniques. but mention should is given by
be made oi' a continuous sodium monitor for
boilerlfeed water. The water is nebulized into a
flame. the sodium emission is isolated by means
of a monochromator, and the intensity measured where Nj is the number of atoms in the higher-
by means of a photomultiplier and associated energy level involved in the transition responsible
electronics. Standard solutions are automatically for the line, F is the oscillation strength of the
fed into the instrument from time to time to check line, and C is a constant dependent upon the
the calibration. dispersing and detecting systems. Assuming that
In both the steel and non-ferrous alloy indus- the atoms are in thermal equilibrium at tempera-
tries. large grating spectroscopes are used to con- ture T, then the number of atoms in the excited
trol the composition of the melts before they are state: of excitation energy Ej is given by
finally poured. A complete analysis for some 30-
40 elements can be made within 2 minutes of a p,
small sample being taken. Suitable additions are N, = No - exp ( EjIKT)
PO
~
then made to the melt to satisfy the required
composition specification. In these cases the out- where No is the number of atoms in the ground
put from the instrument is fed to a computer, state, P, and Po are statistical weights of the
which is programed to produce actual elemental excited and ground states respectively, and K is
concentrations and also the necessary amounts Boltzmann's constant. For a spectral term having
required to be added to known weights of melts a total quantum number J,, P is equal to 2J1 + 1.
in the furnaces for them to be of the correct From the above equations, it can be seen that the
composition. Analysis of water samples or sam- emitted intensity depends on T and E,. Examples
ples in solution can be carried out using an in- of the variation of N,INo with temperature are
ductively coupled plasma direct reading given in Table 16.3.
spectrometer. Some 60 elements can be deter- In nearly all cases, the number of atoms in the
mined in each sample every two minutes. The lowest excited state is very small compared with
source is ionized argon pumped inductively from the number of atoms in the ground state and the
an r.f. generator into which the sample is nebu- ratio only becomes appreciable at high tempera-
lized. Temperatures of about 8500°C are tures. The strongest resonance lines of most ele-
achieved. Many instruments of this type are ments have wavelengths less than OOOnm and as
now rnanufactured and have been of great value temperatures in the flames used are normally less
to the water industry and to environmental che- than 3000"K, the value of N, will be negligible
mists generally-in particular, those instruments compared with No.
manufactured by ARL, Philips, and Jarrell Ash. In absorption, consider a parallel beam of
Limits of detection are of the order of lppb radiation of intensity Io, frequency v incident on

Table 16.3 Values of Nil& for various resonance lines
_____
Resonance line Transition P,IPo hrjlNo
T = 2000 K T = 3000K T = 4000K T = 5000K
Cs 852.1 nm 2s,,'-2P1,2 2 4.4 x 10-4 7.24 x 10-3 2.98 x lo-' 6.82 x IO-'
K 766.5 nm 2s,#2-2P1,2 2 2.57 x 10-4 4.61 x 1.65 x 3.66 x lo-'
Na 589.0nm 2s,li-2Pi,2 2 9.86 x 5.88 x 10-4 4.44 x 1.51 x lo-'
Ca 422.1 nin Is"-lP, 3 1.21 x 10-7 3.69 x 10-5 6.03 x 3.33 x
Zn 213.8 nm lso-lP, 3 7.29 x 5.58 x lo-'' 1.48 x 4.32 x