2
compounds analyzed by this technique must be free of water. The pair of plates is inserted into a
holder that fits into the spectrometer.
Solids
There are several methods for determining infrared spectra for solids. One method of choice has
been to mix a finely ground sample with powdered potassium bromide and press the mixture
under high pressure. Under pressure, the potassium bromide melts and seals the sample into a
matrix. The resulting KBr pellet is inserted in the instrument. If a good pellet is prepared, the
spectrum obtained will have no interfering bands since potassium bromide is transparent down to
400 cm
−1
.
Another method, a Nujol mull, involves grinding the compound with mineral oil (Nujol) to
create a suspension of the finely divided sample dispersed in mineral oil. The thick suspension is
placed between two salt plates. The main disadvantage of this method is that the mineral oil
obscures bands that may be present in the analyzed compound. Nujol hands appear at 2924,
1462, and 1377 cm
−1
.
The best method of determining a spectrum of a solid is to make use of an attenuated total
reflectance (ATR) accessory. Modern FT-IR instruments now offer this accessory along with
the typical transmittance module. The ATR method provides a powerful sampling technique that
virtually eliminates sample preparation with both liquids and solids, thus leading to rapid
analysis of samples. Although manufacturers offer multiple crystal options, the diamond ATR
offers the best option for maximum durability in the organic chemistry laboratory. With the ATR
accessory, one simply places a small amount of a liquid or solid directly on the diamond without
any previous preparation. The diamond is cut and mounted at precise angles so that the incoming