Page 272 - Handbook of Instrumental Techniques for Analytical Chemistry

262 Handbook of Instrumental Techniques for Analytical Chemistry

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for Liquid Evaluation (CIRCLE ) cell.
The resulting ATR-IR spectrum resembles the conventional IR spectrum, but with some differenc-
es: The absorption band positions are identical in the two spectra, but the relative intensities of corre-
sponding bands are different. Although ATR spectra can be obtained using either dispersive or FT
instruments, FTIR spectrometers permit higher-quality spectra to be obtained in this energy-limited sit-
uation.
Specular reflectance provides a nondestructive method for measuring thin coatings on selective,
smooth substrates without sample preparation. It basically involves a mirrorlike reflection and produces
reflection measurements for a reflective material, or a reflection–absorption spectrum for the surface
film on a reflective surface. Thin surface coatings in the range from nanometers to micrometers can be
routinely examined with a grazing angle (typically 70 to 85°) or 30° angle of incidence, respectively.
For example, lubricant thickness on magnetic media or computer disks is conveniently measured using
this technique.
Diffuse reflectance technique is mainly used for acquiring IR spectra of powders and rough surface
solids such as coal, paper, and cloth. It can be used as an alternative to pressed-pellet or mull techniques.
IR radiation is focused onto the surface of a solid sample in a cup and results in two types of reflections:
specular reflectance, which directly reflects off the surface and has equal angles of incidence and re-
flectance, and diffuse reflectance, which penetrates into the sample, then scatters in all directions. Spe-
cial reflection accessories are designed to collect and refocus the resulting diffusely scattered light by
large ellipsoidal mirrors, while minimizing or eliminating the specular reflectance, which complicates
and distorts the IR spectra. This energy-limited technique was not popular until the advent of FTIR in-
struments. This technique is often called diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS).
The sample can be analyzed either directly in bulk form or as dispersions in IR-transparent matrices
such as KBr and KCl. Dilution of analyte in a nonabsorbing matrix increases the proportion of diffuse re-
flectance in all the light reflected. Typically the solid sample is diluted homogeneously to 5 to 10% by
weight in KBr. The spectra of diluted samples are similar to those obtained from pellets when plotted in
units such as log 1/R (R is the reflectance) or Kubelka–Munk units. The Kubelka–Munk format relates
sample concentration to diffuse reflectance and applies a scattering factor.
Photoacoustic spectroscopy (PAS) is a useful extension of IR spectroscopy and suitable for exam-
ining highly absorbing samples that are difficult to analyze by conventional IR techniques. The size and
shape of the sample are not critical. PAS spectra can be obtained with minimal sample preparation and
without physical alteration from a wide variety of samples such as powders, polymer pellets, viscous
glues, single crystals, and single fibers.
Typically, the modulated IR radiation from an FTIR interferometer is focused on a sample placed

Figure 15.11 Schematic representation of multiple
internal reflection effect in Attenuated Total Reflectance
(ATR). (Reprinted from 1988 Annual Book of ASTM
Standards by permission of American Society for Testing
and Materials.)