sides of an optically flat thin prismatic crystal of thallium bromide/iodide (KRS-5) through which
radiation from the source is passed. By directing the beam into one end of the crystal at the correct
angle of incidence it can be made to undergo multiple internal reflections before emerging from the
other end and passing through the spectrometer to the detector. At each reflection, the beam penetrates
the surface of the sample to a depth of a few micrometres. The intensity of the beam is attenuated
according to the absorption characteristics of the sample so enabling an absorption spectrum to be
recorded. Diffuse reflectance spectra are obtained using a specially designed curved or plain mirror
system to collect the radiation diffusely reflected from the surface of a sample over a wide solid angle
and focusing it onto the detector. It is particularly suitable for powdered solid samples, which are
simply placed in a small cup, and for liquids or gases adsorbed onto the surface of a non-absorbing
powdered substrate such as alkali halide. As little as a few micrograms of material can give an
acceptable spectrum.
Infrared microscopy combines an optical microscope with an FT-IR spectrometer enabling pico- to
femtogram (10–^12 – 10 –^15 g) quantities of substances to be characterized or very small areas of larger
samples to be analysed. Beam-condensing optics focus the radiation onto an area of the sample
identified using the optical microscope and either reflectance or transmittance spectra can be recorded.
The highly-sensitive MCT detector (p. 283) is normally used as its size can be matched to that of the
radiation beam to maximize its response.
Gaseous samples require long path length cells to produce absorption bands of reasonable intensity; up
to several metres of optical path are obtainable from cells incorporating mirrors which produce multiple
reflections. For GC-IR, light pipes provide the best sensitivity (p. 117).
Near Infrared Spectrometry
Overtones and combination bands from CH, OH and NH vibrations are found in the near infrared
region (NIR) between 4000 cm–^1 and about 12500 cm–^1 (2500 nm to about 800 nm). Although these are
weak bands (low molar absorptivities), sensitivity can be enhanced by the use of more powerful
radiation sources and more sensitive detectors. NIR spectra are generally lacking in prominent features,
consisting largely of broad overlapping bands of varying intensities that are strongly affected by H-
bonding. Some sharper bands may also appear, particularly in the region between 4000 and 6000 cm–^1
(2500–1667 nm) (Figure 9.26(a) and (b)). Band positions, profiles and intensities are highly dependent
on the physical characteristics of the sample, especially particle size and moisture content. The
comparison of spectra for identification purposes and calibration procedures for quantitative analysis
are both more complicated than in the mid-infrared region, being largely empirical in approach.
However, the development of Fourier transform (FT) instruments (pp. 281,