Page 350 - Instrumentation Reference Book 3E

Photo-acoustic spectroscopy 333

Table 16.4 Analyzing crystals
Crystal Rejection 2d spacing Lowest citoinic
plane (A) (IA=o.I nm) number detectable
L series
Topaz 2.112 Ce (58)
Lithium fluoride 2.848 Ce (58)
Lithium fluoride 4.028 In (49)
Sodium chloride 5.639 Ru (44)
Quartz 6.686 Zr (40)
Quartz 8.50 Rb (37)
Penta erythritol 8.742 Rb (37)
Ethylenediamine tartrate 8.808 Br (35)
Ammonium dihydrogen phosphate 10.65 As (23)
Gypsum 15.19 CLI (29)
Mica 19.8 Fe (26)
Potassium hydrogen phthalate 26.4 V (23)
Lead stearate 100 Ca (20)

A further allied technique is photoelectron 16.4 Photo-acoustic
spectroscopy (PES) or Electron Spectroscopy spectroscopy
for Chemical Analysis (ESCA). In Figure 16.14,
showing the transitions within an atom to pro- An instrument marketed by EDT Research
duce X-rays, it is seen that some electrons are makes use of this technique to study both liquid
ejected from the various shells in the atom. The and solid samples. Figures 16.16 and 16.17 give
energy of these electrons is characteristic of that schematic diagrams of the instrument and cell.
atom and so by producing an energy spectrum of Radiation from an air-cooled high pressure
electrons ejected from a sample when the latter is xenon arc source, fitted with an integral parabolic
scbjected to X-ray or intense UV radiation, the mirror, is focused onto a variable speed rotating
presence of different elements and their concen- light chopper mounted at the entrance slit of a
trations can be determined. It should be pointed high radiance monochromator. Tne monochro-
out that this technique is essentially a surface mator has two gratings to enable optical acoustic
technique and will only analyze a few monolayers spectra to be obtained in the UV: visible, and
of sample. Instruments are manufactured by near-infrared. The scanning of the monochro-
Vacuum Generators. mator is completely automatic over the spectral
range covered and a range of scan rates can be
16.3.2 X-ray diffraction selected. The exit and entrance slits provide vari-
able band passes of width 2-1 6 nni in the UV and
This is a technique: which is invaluable for the
identification of crystal structure. In Section 3.3.1
it was seen that crystals diffract X-rays according
to Bragg's law: Dual .grating
monochromator
nX = 2d sin Q
power supply
Thus if a small crystal of an unidentified sample is
placed in an X-ray beam, the X-rays will be dif-
fracted equally on both sides of the sample to pro- 1-
duce an X-ray pattern on a film placed behind the 300 W xe arc. 1 ,L
sample. The position of the lines on the film
(i.e.. their distance from the central beam) is a func-
tion of the crystal lattice structure, and by reference
to standard X-ray diffraction data, the crystals in
the sample are identified. Again this is a specialized
technique and beyond the scope of this book.
Manufacturers of X-ray fluorescence spectro-
meters also make X-ray diffraction spectro-
meters. Typical uses for an instrument are the
identification of different types of asbestos, and X -Y recorder
corrosion deposit studies. Figure 16.1 6 Photo-acoustic spectrometer layout.