Page 192 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN007C-340 July 10, 2001 14:45

Infrared Spectroscopy 803

also stored in the memory. These two single-beam spectra can be polished the same way, but with alcohol substi-
are ratioed by the computer to give a percent transmittance tuted for water. The best polish comes when the lap is
spectrum. nearly dry.
The computer can be used to modify the spectrum fur-
ther. For example, the vertical or horizontal scale can be
C. Liquid Samples
expanded, the background can be straightened, or a linear
absorbance scale can be generated. A useful procedure is The easiest samples to run on IR instrumentation are those
spectral subtraction whereby, for example, a solvent spec- in the liquid state. Slightly viscous samples can be simply
trum can be subtracted from a solution spectrum to yield squeezed between two polished IR-transmitting plates and
the pure solute spectrum. run as a thin film. A typical film thickness is ∼0.01 mm.
If the liquid is not viscous, usually a spacer is added be-
tween the plates to keep the plates apart at the appropriate
III. SAMPLE HANDLING TECHNIQUES spacing. Spacer material can be metal foil or an insolu-
ble polymeric film. Two strips roughly 10 by 2 mm can
A. Infrared-Transmitting Materials be used, for example, one on each side of the area the
IR beam will pass through. These are called temporary
One of the features of IR spectroscopy is that solids, liq-
cells and are disassembled and cleaned after each use.
uids, and gases can be run without special difficulties.
The thickness cannot be accurately reproduced.
Usually, some sort of IR-transmitting material is needed
Fixed cells are not disassembled after use but instead
to support or enclose the sample. Materials such as glass
are filled, emptied, and cleaned with solvent through ports
and quartz are useful as windows in the near-IR but even
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thin windows do not transmit much below 3000 cm . The on the cell assembly. The liquid enters the leak-proof sam-
ple area between the plates through holes in the cell win-
low wave number transmission limits of IR-transmitting
dow. These are used for volatile liquids or when the thick-
materials are not sharply defined but depend on the win-
ness needs to be accurately known or held constant as
dow thickness. Four commonly used materials and their
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approximate low wave number limits are NaCl, 600 cm ; in quantitative analysis. Many commercially available de-
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KBr, 350 cm ; CsBr, 250 cm ; and CsI, 200 cm . signs are used, and cells come in thickness from 0.01 to
4 mm.
These materials are all water-soluble. Water-insoluble ma-
Ifthecellwindowsaresufficientlyflat,thecellthickness
terials and their low wave number limits include CaF 2 ,
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1200 cm ; BaF 2 , 850 cm ; Irtran-2, 700 cm ; AgCl, can be measured by running the IR spectrum of the empty
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350 cm ; and KRS-5 250 cm . Irtran-2 is made of zinc cell and observing interference fringes in the form of per-
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cent transmission undulations. Wave number ¯ν 1 (cm )is
sulfide an is often used for water solutions or for making
films from water solution. Silver chloride is useful but is read at one transmission maximum, and wave number ¯ν 2
is read at another transmission maximum that is 1, 2, 3, or
soft, deforms easily, and darkens with exposure to light.
more generally n maximum away from the first. The cell
KRS-5 is thallium bromide iodide and is often used in the
thickness t is
internal reflection technique to be discussed later. In the
far-IR, high-density polyethylene transmits to as low as 5n
t (mm) = . (12)
30 cm −1 but cannot be used above 600 cm −1 because of ¯ ν 1 − ¯ν 2
its absorption. Interference results because part of the beam is twice re-
flected inside the cell and is retarded by twice the cell
B. Salt Polishing thickness relative to the transmitted beam with which it
interferes.
Sodium chloride can be easily polished between use. The
crystal is sanded flat with a finegrade sandpaper if it is D. Gas Samples
freshly cleaved, scratched, or damaged by water. Two pol-
ishing laps are prepared. There are different types, but the Gas cells used for IR spectroscopy come in a variety of
wet lap can be simply two thicknesses of fine nylon cloth types. The simplest is a basic cylinder 10 cm long with
stretched over a flat surface. The wet lap is wetted with IR-transmitting windows on each end. These may be ce-
water and sprinkled with a little fine polishing powder mented on or clamped in place, with vacuum-tight gaskets
such as aluminum oxide or cerium oxide. This is rubbed providing the seal. Entrance and exit tubes are provided
smooth and all excess water is wiped off. The flat salt plate and fitted with stopcocks. The cell is filled and emptied
is rubbed about 20 strokes on the wet lap and then, without with a gas handling system.
delay, is buffed about 7 strokes on the dry lap, which can be The sampling chamber of most IR spectrometers is not
simply a layer of diaper cloth held flat. Cesium bromide large enough to accommodate longer cell lengths directly.