Page 193 - 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

804 Infrared Spectroscopy

However, cells of much longer path length are available can be put onto a plate and the solvent evaporated to form
that use mirrors to deflect the IR beam and to reflect it back a film. This is a good technique for running water-soluble
and forth many times in the cell chamber before it leaves polymers, for example. Sometimes a film can be prepared
the cell and reenters the spectrometer. These long-path- on a substrate and stripped off and run as an unsupported
length cells are used for detecting very small quantities of film. If a film is too uniform in thickness, interference
gas in pollution studies, for example. fringes similar to those from an empty cell may be seen,
A technique often used with FT-IR instruments is gas as discussed earlier. If a film is too irregular in thickness,
chromatography, or GC–FT-IR. Here the effluent from a a spectrum with a false percent transmittance will result
gas chromatography column is fed through a heated light from the fact that different parts of the beam go through
pipe with IR-transmitting windows on the ends. Source ra- sample areas with different thicknesses.
diationpassesthroughthecellintotheFT-IRspectrometer.
The gas chromatography column separates the gas-phase G. Mulls
components and ideally sends them one by one through
One of the best techniques for running crystal-line solids
the light pipe, where the high speed of the FT-IR instru-
is the use of a mineral oil or Nujol mull. Here a few mil-
ment is utilized to get the spectrum of each component
ligrams of sample are finely ground with a small amount
“on the fly,” so to speak.
of mineral oil to make a thick paste like cold cream, for
example. The paste can be prepared with a mortar and
E. Solution Spectra pestle and spread between two IR-transmitting windows.
A well-ground sample has a brownish color like smoke
The techniques for running solids in IR are quite varied.
when one looks through it. Most beginners do not grind
In the first case a solid can be dissolved in a suitable sol-
the sample well enough and use too much oil. Mineral oil
vent and run as a liquid. Unfortunately, no solvent is free
has only a few bands in narrow regions. The CH stretch
of absorption in the IR region and, usually, the better the −1
region between 3000 and 2800 cm and the CH bend re-
solvent, the greater its absorption. This means that more −1
gion at about 1460 and 1375 cm are obscured, however.
than one solvent must be used to get the whole IR solution
If information is needed in these regions a second mull
spectrum in all regions. A commonly used pair of solvents must be prepared using a halogenated oil such as Halocar-
−1
are CCl 4 above 1330 cm −1 and CS 2 below 1330 cm .
bon or Fluorolube, which contain CF 2 and CFCl groups
These can be used in cells 0.1–1 mm thick, for example, −1
but no CH. These have no bands from 4000 to 1300 cm
with solute concentrations in the range 10–1%. In double- −1
but have strong bands below 1300 cm . Some people
beam grating spectrophotometers a cell of matching thick- −1
use the halogenated oil spectrum above 1300 cm and
ness containing solvent only can be put into the reference
the mineral oil spectrum below. In this case care must be
beam to compensate for the solvent bands. In FT-IR in-
taken to ensure that the sample thickness is the same in
struments, a reference solvent spectrum can be subtracted
both preparations.
from the solution spectrum to remove solvent bands. An-
other common solvent for solution spectra is CHCl 3 often
H. Potassium Bromide Disks
used in 0.1-mm cells with 5 to 10% solute. CHCl 3 has
−1
strong bands at 1216 and 757 cm , where solute infor- A very popular technique for running solids is the KBr
mation is often lost or inadequately presented. Even water disk technique. Here a few milligrams of sample are very
has been used as a solvent for some applications. Here finely ground and then mixed with 50 to 100 parts of dry
the cell thickness must be kept small, as water is a very KBr powder. The mixture is placed in a special device and
strong IR absorber. The internal reflection technique de- compressed into a disk at high pressure. If all goes well, a
scribed in Sec. III.I has been successfully used for water transparent disk results, which is put into the spectrome-
solutions. ter and run. Commercial KBr disk makers are available in
many forms. Some are activated with wrenches or levers,
while others are used with a hydraulic press. Some can be
F. Films
evacuated, which gives the disk transparency a longer life-
Solid-state films of suitable thickness can be prepared time, but this is not necessary if the disk is used promptly.
from melts or solution. Such films are most suitable Advantages of the disk over the mull include the fact
−1
for amorphous materials, especially polymers. Crystalline that KBr, unlike mineral oil, has no bands above 400 cm .
films may scatter light and show nonreproducible orienta- Also, many polymers are more easily ground in KBr.
tion effects from special orientations of the crystal on the Microsamples are easier to prepare with the KBr disk.
IR window surface. A sample can be heated between two The KBr disk has disadvantages compared with the mull,
salt plates until molten and allowed to solidify. Solutions however. The biggest problem is that KBr is hygroscopic,