Page 195 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GRB/MBQ P2: GLM Final Pages
Encyclopedia of Physical Science and Technology EN007C-340 July 10, 2001 14:45

806 Infrared Spectroscopy

with only a small raised area of the flat surface that allows The sampling region can be viewed visually through the
spectra to be taken of quite small areas. Since there is only microscope, and selected areas can be isolated by mask-
one internal reflection, the crystal used, such as silicon, has ing off the unwanted parts of the field. Variable aperture
a very high index of refraction. masks are located in remote image planes of the sample
There are many applications for internal reflectance area,locatedaboveandbelowthesampletoreducediffrac-
spectroscopy, and only a few will be mentioned here. In- tion effects. Then, the optical path is changed so that the
ternal reflectance spectroscopy can be used to obtain the source radiation goes through the unmasked areas and the
spectra of rubbery materials that are hard to grind. The infrared spectrum of the sample is recorded. This can be
rubbery material is simply pressed against the internal re- divided by the spectrum of a similarly masked blank for
flectance plate, and it is ready to run. Carbon-filled rubber example, to give a percent transmission spectrum of the
or other polymers may be run using a high index of refrac- sample, or from this, an absorbance-type spectrum of the
tion germanium as the internal reflectance element. Inter- sample.
nal reflectance is used to obtain selectively the top few The types of applications are basically similar to those
micrometers of a sample surface where the composition handled by transmission spectroscopy, but with a signif-
may be different than that further down. It is also good for icant difference. The microscope can yield good spec-
water solutions because the controlled penetration keeps tra on much smaller sample areas. This means that many
the effective sample thickness small. new types of problems can be handled that were difficult
or impossible to solve with standard instruments. Sam-
ple areas that are heterogeneous are now easily measured
J. Diffuse Reflectance
in the chemical industry, forensic work, and biological
Diffuse reflectance is a technique usually used with FT- studies.
IR instruments. A powdered sample is placed in a small In the chemical industry for example, some polymer
container, where source radiation strikes it and is diffusely products may show some very small impurity areas whose
reflected in various directions. This radiation is collected chemical composition may be characterized with little
and measured by the spectrometer. Usually in the mid-IR difficulty by the spectra. Polymer films weathered by
region the finely powdered sample is diluted to 5 to 10% outdoor exposure may show chemical modification as a
with finely powdered KBr or KCl. The spectrum is ra- function of the layer depth below the surface. This may
tioed against a reference spectrum of pure powdered KBr be characterized by the infrared spectra of the various
or KCl. The ratioed spectrum is processed by a computer layers.
using a function f (R ∞ ) derived by Kubelka and Munk, In forensic work, heterogeneous fields can be ex-
which changes the reflectance spectrum into one resem- amined selectively for hair strands, polymer fibers, or
bling a linear absorbance spectrum: lint particles, for example, and their chemical com-
(1 − R ∞ ) 2 k R ∞ (sample) position can be characterized by their infrared spec-
f (R ∞ ) = = R ∞ = . tra. Cross-sections of paint chips can be examined and
s R ∞ (reference)
the various layers can be characterized, which may be
2R ∞
(14) distinctive.
In the biological field, heterogeneous areas of various
Here (R ∞ ) is the reflectance of a thick scattering layer, k
biological specimens can be examined, and the infrared
the molar extinction coefficient, and s a scattering coeffi-
spectrum of different microareas of the field can be taken.
cient, which is a function of particle size. The spectrum is
The spectrum of a single red blood corpuscle can be eas-
quite sensitive to particle size, which affects the radiation
ily measured. One difference that can be observed in these
scattering. Spectral distortion (compared with a transmis-
samples is the ratio of protein to fat. Proteins have bands
sion spectrum) may occur if the particle size is not uni- −1
near 1650 and 1550 cm for the O C-NH group, and fats
formly fine. Black, strongly scattering materials such as
have an ester C O band near 1740 cm −1 and an alkane
coal can be run by this technique.
−1
chain doublet near 2925 and 2855 cm , with an internal
unsaturation band near 3015 cm −1 for the CH group.
K. Infrared Microspectroscopy
Some studies may reveal chemical differences between
In this method of sampling, a special type of microscope adjacent tissues of various types. The spectral character-
is used to select very small sample areas for examination istics of two-dimensional areas may be mapped by running
by the infrared spectrometer. As in the macroscopic case, a series of spectra of a grid pattern. An array consisting
samples can be prepared with thicknesses on the order of a number of detectors can be used, each one of which
of 0.01 mm. Since infrared radiation must pass through generates the infrared spectrum of its own small area of
the microscope, all the optics are front surface mirrors. the grid pattern.