A Rapid Method for Preparing Solid Compounds for Infrared Analysis For routine analysis of solid unknowns in our third term organic chemistry laboratory, we have developed a rapid method for preparing samples on inexpensive NaCl discs. For this technique, which avoids the time-consuming preparation of KBr pellets or Nujol mulls (with subsequent C-H and C-C interference), we use spectro-grade earbon tetrachloride3 to dissolve or make a slurry of a few crystals of the solid on a spot depression plate. A drop or two of the solution or slurry is transferred to one of a pair of clear NaCl discs. Just before the carbon tetrachloride is completely evaporated, the second disc is put into place, sometimes with a little circular motion to flatten the slurry. If the solid crystallizes, or has dried between the discs with too much reflectance, a drop of carbon tetrachloride may be added to the discs supported in the holder and simple capillary action often promotes transmission of the infrared signal. As the carbon tetrachloride evaporates, some solid compounds form nice, flat, crystalline sheets while others require the presence ofthe solvent far translucence. T h e amount of sample and carbon tetrachloride is determined by trial. In addition, various tricks may be required to get the sample t o stay centralized during reworking. Using this technique, the time required for sample preparation is often less than one minute. As a result, we can obtain an infrared spectrum in less than four minutes, including the three minute infrared scan. A representative spectrum which illustrates the technique described is shown below. I
nmENC",/I*',
T h e price of a typically good infrared spectrum in the 4000-600 cm-' range is an initial strong interference between 800 and 700 em-'on the first pass due to C-CI absorption. However, the NaCl discs can then be separated, sometimes with blowing, topurge the carbon tetrachloride from the sample and the 800-600cm-' region can then be rerun tocheek for i m ~ portant aromatic substitution patterns. A cloudy NaCl disc can be cleared by first rubbing it on a wet paper towel and then polishing it on a dry paper towel or piece of felt. Any slight unevenness, due to moisture erosion, on the surface of the NaCl disc can he advantageous when running infrared spectra of liquid compounds of low viscosity or high volatility.
' Obviously the use of carbon tetrachloride requires working in a well-ventilated area. Undergraduate research assistant who ran the spectra.
G. Olof Larson, Jeff W. Miller,2 Everett J. Nienhouse Ferris State College Big Rapids. MI 49307
80 I Journal of Chemical Education
