Irwin Cohen
Youngstown University Youngstown, Ohio
Disposable Polyethylene Cells for Infrared Spectra
Infrared spectra of mulls, greases, and aqueous solutions can be obtained by several well known techniques, including the use of demountable cells, pressed KBr discs, and special cell material^.',^ A further method is suggested here which is more convenieut than these and which is particularly well suited for viscous or for strongly absorbing materials. The Dew technique consists of supporting the sample on a strip of very thin polyethylene. Thick samples may he pressed down until the desired degree of optical clarity is reached. Liquid films are supported by placing a few drops of the liquid about 4 or 5 cm from the end of a 7-cm X 15-cm strip of polyethylene and folding over the strip so as to form and enclose the film. The corners of the "cell" may he tucked in at the fold to keep excess liquid from running out. The polyethylene usually clings to the instrument cell-holder, hut if it does not it may he supported by a paper clip. The cell may be discarded after use. Some of these cells have been successfully cleaned, especially after contact with aqueous samples, hut since they are easily marred and since their cost is negligible (about 25 cells per penny) there is no reason to save them. The polyethylene garment hags formerly widely used by dry cleaners provided an excellent source of 10 p material, hut because of the clinging quality of this almost weightless film, thicker and less transparent material is now being used by these firms. However, Dow Chemical Company now produces "Handi-Wrap" (available a t food markets), which is even thinner and which makes a virtuallv ideal infrared cell. An infrared spectrums of a sample of Handi-Wrap (Fig. la) shows transmission above 85% except for these ranges: 3.3-3.6, 6.8-7.1, and 13.6-14.0 p. Except for the small peak a t 13.7 p, these absorptions are all duplicated by paraffin oil (Fig. lb). Calculation of film thickness from interference fringes, assuming a refractive index of 1.5, yields values averaging about 7 p. Three classes of applications can he distinguished: paraffin oil mulls, other viscous materials, and thin or volatile liquids, including aqueous solutions. The use of these cells for paraffin oil mulls is without significant disadvantage. Except for the small peak a t 13.7 p, these spectra are indistinguishable from conventional-cell spectra, and they are far more conveuiently made.
For other viscous materials, these cells suffer from the disadvantage of introducing the three narrow bands of polyethylene absorption. Where these regions are not specifically important, however, there is no more convenient way to obtain the spectrum. Excellent spectra have been recorded for materials ranging in viscosity from Dow Corning Silicone High Vacuum Grease down to triethanolamine (Fig. lc) and corn oil. The polyethylene interference has been reduced by placing an empty "cell" in the reference beam, hut this procedure cannot be relied on because of the variability in thickness of the material. However, it takes but a few seconds to vary the sample thickness, and a consequent variation or lack of it for these absorption peaks may indicate whether absorption in these regions was partly due to the sample or only to the polyethylene. Aqueous solutions, 95% ethanol, sulfuric acid, and several other liquids have given recognizable spectra. Since these samples run off or evaporate readily, a fast scan is advantageous, and interrupting the run to readjust or recharge the cell may he desirable. The difficultyis minimized by having one or two small wrinkles in the film, a circumstance which in fact is difficult to avoid. I n general, spectra of thin liquids are not as good as those obtainable by use of special cell materials. But where the cost or inconvenience of the latter is a factor, or where only quick, crude results are needed, the polyethylene cell may be the cell of choice. The application to aqueous solutions may perhaps he of especial interest to educational laboratories for the
STERNOLANZ, H., Appl. Spedmscopy, 10, No. 2 , 7 7 (1956). O LW. ~ L., AND BALLARD, S. S., P m .I. R. E., 47, 1540 (1959). a All of the spectra described here were made with a PerkinElmer model 137 Infracord.
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Figure 1. infrared rpeetr.: (01 solid line, Handi-Wrap polyethylene, 7 p dngle layer; lb) broken fine, paraffin oil, York brand, in 2 5 p NoCl cell; (c) triethmolomine, Rlm between two layers of Handi-Wrap.
demonstration of aqueous infrared studies without special techniques. In any laboratory, the quickness and convenience of this method may make it useful as a preliminary step. A further application has been the precise location of strong absorption peaks in the case of substances for which these peaks "mn off" the chart. The thin liquid film held by the polyethylene is well suited for this. For example, ally1 alcohol in a 0.028 mm NaCl cell
shows zero transmission for 9.54 to 10.13 p, whereas a polyethylene cell spectrum resolves this into two peaks, a t 9.74 and 10.08 p?respectively. Disposable polyethylene infrared cells thus provide a quick and convenient method for obtaining qualitative infrared spectra, especially for materials which are otherwise troublesome to handle. Figure 1 was prepared hy Henry Kacher, Youngstown IJniversity.
Volume 39, Number 5, May 1962
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