solutions in solvents which do not absorb in the region of analytical interest,
such as CCl 4 or CS 2 in NaCl cells, with a known path length provided by a
spacer, may be used. Most of these are also applicable to NIR, and short path
length silica cells may also be used there.
Since Raman spectra are recorded in the visible and NIR regions, glass or
quartz cells may also be used. The routine setting-up and calibration of a Raman
spectrometer can be carried out using liquid CCl 4 in a thin glass tube, which
gives a strong peak at a Raman shift of 458 cm-^1 and weaker peaks at 218, 314
and 760-790 cm-^1. Since water is a weak Raman scatterer, aqueous solutions are
readily studied.
If a solid organic powder sample is placed in an IR beam, the particles scatter
the light, and little is transmitted. Therefore, for routine analysis, the sample is
usually ground to a fine powder and mixed with paraffin oil (‘Nujol’) to form a
paste or mull. This reduces the scattering at the powder surface and gives a
good spectrum, with the disadvantage that the bands due to the oil (at approxi-
mately 2900, 1450, 1380 and 750 cm-^1 ) are superimposed on the spectrum.
Alternatively, the fine powder may be mixed with about 10-100 times its mass
of dry, powdered KBr and the mix pressed in a hydraulic press between smooth
stainless steel dies to give a clear KBr disk.
Solutionsof solids may also be used, and tetrachloromethane, CCl 4 is often
used as solvent, since it has few IR-active bands, mostly at the low wavenumber
end of the spectrum. These must be ignored when the spectrum is interpreted.
Thin films of solids such as polymers may be supported directly in the IR beam.
Polystyrene is a useful calibration sample to check the performance of an IR
spectrometer (see Fig. 2in Topic E11).
Raman spectra of solids may be obtained by placing the sample directly in the
beam so that the radiation is scattered correctly into the dispersion system.
Reflectance spectra can be measured in three ways. A powder is placed in the
incident beam and allowed to interact by diffuse reflectance. The reflections are
collected by a mirror, as shown in Figure 5 (a), or for NIR by an integrating
sphere surrounding the sample.
If the beam is reflected off a flat sample surface, specular reflectance results,
and this may give good spectra.
If the sample is placed in good contact with the surface of an optical device of
high refractive index (such as a prism of KRS-5) and illuminated through the
prism by IR, the beam passes into the layers in contact and is attenuatedbefore
being totally internally reflected by the system, as shown in Figure 5(b). This is
called attenuated total reflectance orATR. If the beam interacts several times,
then we have multiple internal reflectance (MIR)and if the surface is hori-
zontal, which is an advantage in setting up the sample, then it is horizontal
attenuated total reflectance (HATR).
It should be noted that the detail of spectra obtained by reflectance methods
might be different from that obtained in solution or with KBr disk techniques.
Modern instruments possess software to convert reflectance spectra to resemble
the more usual transmission spectra.
Analysts must often deal with samples of very small size, or analyze a small
area of a large sample. One technique is to reduce the size of the IR beam using
a beam-condensing accessory. This has lenses of CsI, which focus the beam
down to about 1 mm diameter and permit the study of micro-KBr disks.
A more versatile modern development is the IR microscope,or Raman
microscope. This is an adaptation of a conventional stereo microscope which240 Section E – Spectrometric techniques