Teflon Tape as a Sample Support for IR Spectroscopy Keith A. Oberg and Daniel R. ~ a l l e r o s ' Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064 Infrared spectroscopy (IR) is one of the most ubiquitous techniques used in the introductory organic chemistry laboratory. This is true, in part, due to the simplicity ofthe method and the availability of instrumentation. Despite this, some mishaps are unavoidable in classes with a large number of students. One of the most common casualties when running IR spectra are the sodium chloride windows used a s sample support. Even with proper care these plates require frequent polishing, they wear out, and eventually must be replaced. I n our experience, in a class of over 200 students, each of whom collects a t least six IR spectra over a 10-week period, 15 pairs of windows typically have to be replaced. This amounts to over $300 a auarter. Given the calamitous budgetaw situation in the taining IR spectra. Any alternative to NaCl plates should be transparent in the IR region to be analyzed, simple to use, and give reproducible results. I n addition, the support should have minimum scattering of IR light. There has been a n early report (1) on the use of Teflon tape a s a n IR window material. Recentlv. .,dis~osablesample c a r d s m a d e of microporous
adhesive tape
rection (See Fig. 1). I n a typical preparation for a n IR run, the Teflon tape is fastened to a holder with standard dimensions: 5 x 7 x 0.15 an,made of aluminum, plastic or cardboard, with a 2.0-cm diameter hole cut in the center. We found that aluminum is superior to cardboard or plastic because it is not attacked by solvents. A4-cm length of Teflon tape is secured with adhesive tape placed so that it covers a 2-3 mm strip along the top of the Teflon (Fig. 1A). A second piece of adhesive tape is then attached along the bottom of the Teflon leaving a strip about 5 mm wide uncovered. The Teflon tape is then stretched by pulling i t down a s shown in Figure 1B. Finally, the bottom piece of adhesive tape is pressed down onto the support to hold the Teflon in place (Fie. 1C). We define the stretching ratio, S,
-
Teflon tape (50 mm)
-
A
pO'ytetrafluOmethy'ene (pTFE' i r e 1 Preparing the Teflon tape as sample support. See text for explanation. Marks indicating It and polyethylene have position of the Teflon tape can be etched permanently on the backing to help students obtain consiste become available commercially. stretching ratios. They consist of a polymer film secured on a piece of cardboard with a round hole in the center. These cards provide a fast and clean method to run IR spectra. Unfortunately, the cost of using them ($1.80 each) in a class of about 200 students would be about $2000 a quarter, six times more than our expenditure in NaCl windows. here fore, we set off to make similar cards using Teflon tape available in most hardware stores for only $2.50 per 100-ft roll (enough for a n entire quarter of IR spectra). We found that the homemade cards give IR spectra a s good a s those obtained with NaCl windows. nrovided certain precautions are taken in I n this paper we describe a quick and their reproducible way to make Teflon IR-cards and to obtain good quality spectra using them. Our first attempt a t making these cards was with a short strip of Teflon tape, cut directly from the roll, and taped to a piece of cardboard. This homemade card, without sample, was placed in the path of the IR beam and the spect r u m collected. Strong scattering was observed in the 16004000 cm-' region. I n a previous report this problem was overcome hv the use of special ootics, the cost of which Wavenumber (cm') would completeiy offset theLsaving8in 10, Fig. 3). The observed scattering is caused by the particulate nature of the Teflon tape. As the scanning electron micrograph in Figure 4A shows, the unstretched tape con.iists~i'in~tel~linkrb part~clesofahour 5 - 1 0 ~ min width and 10-30 cm in lrnmh. When the tape is stretched, the Damcles unravel into fibers with a width of about 3 pi (Fig. 4C). This unraveling in conjunction with the reduced density of the polymer, lessens the scattering of IR radiation and allows higher throughput. As can he observed in Figure 2, in the 700-4000 em-' range, Teflon has strong absorption bands a t 1160 a n d 1222 cm-'. Because of total absorption, these bands cannot be removed completely by background correction, but they do not pose a problem because they lie in the fingerprint region of the IR spectrum that usually is not assigned a t the introductory level. Below 700 em-', a region of limited applications i n the organic chemistry laboratory, Teflon absorbs a t 639,625,554, and 480 c m . Two different kinds of Teflon tape were investigated-a generic b r a n d purchased from local hardware stores (PTFE Thread Seal Tape, Mil Spec T-27730 A, distributed by William H. Harvey Co., Omaha, NE) and one from Fisher Scientific. Both types of tape were 0.5-in. wide but of different linear density. The linear density of the generic brand was 0.4 glm while that of the Fisher tape was 1.1 glm; being denser and thicker, the Fisher tape was less malleable than the generic brand. We observed that while stretching ratios as large a s 20 were obtained easily with the generic tapes, the maximum stretching ratio reached with the Fisher tape was about six. As a result, the quality of IR spectra taken using the Fisher tape was lower than that of the IR obtained using the generic brands. We also observed that besides the normal IR bands, the tape pryvided by Fisher showed additional hands a t 2200 cm- , which are probably due to a nitrile-containing additive; the generic tapes did not have this contaminant. All the results
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Journal of Chemical Education
Figure 4. Scanning electron micrographs of Teflon tape prepared as Shown in Figure 1 at ditterent stretching ratios. The direction of stretching is from top to bottom. A: unstretched tape; 6: S = 5; C: S = 20.The bar represents 10 pm; the scale is the same for all three panels. Teflon tape was sputter-coated with Au-Pd. shown i n this paper were obtained using generic Teflon tape. Figure 5 shows the spectra of 4-chlorobenzaldehyde, phenacctin 4-ethoxvac(!t:1nilide!, bcnzophenonc, and carvone on 'l'!flon tape stretched to at lc:~stS = 10. Their cumpounds are used frequently in the introductory organic chemistry laboratory. The spectra showed here match those reported i n the literature ( 2 )except in the 1200 em-' region where Teflon absorbs. A comparison of the IR spectrum of eugenol run according to the protocol described in this paper and a spectrum from NaCl windows i s shown in Figure 6. Other than the absorption bands from Teflon a t 1160 and 1220 cm-', the spectra are of comparable quality. We found t h a t sample preparation and application on the Teflon tape is of paramount importance in obtaining high quality spectra. I t is important that each student collects a n individual background spectrum because S ratios will vary from student to student. Obtaining a good IR spectrum on Teflon tape involves the following steps:
benzophenone
I . . . . , . , . . . . . . . . . ' 4000 3600 3200 2800 2400 2000 1600 1200 800
Wavenumber (cm-')
I
0 3600 3200 2800 2400 2WO 1600 1200 800
Wavenumber (cm") Figure 5. IR spectra of typical chemicals used in the organic chemistry introductory teaching laboratory. About 15 1L of a solution of each comoound in methanol (10% wlv) was apDlied on the Teflon tape and the~-~ iolvent allowed to e;aoorate com~let& . . before the spectrum was co lecleo In !he case ol phenacel n f ve a q-01s (15"- eacn) of a 1'0 (W Y) SOILI on n metnanol were appleo. Ine sovent was allowed lo evaporate between applications; one application (15UL) of a 10% solution did not aive a satisfactowspectrum due to crvstallization of phenacel n on lne tape s~rfaceIn a cases S 2 10 Bacdgro~nawas stretcned Tef on !ape ooforc samp e app cat on, other experlmenla oeta Is were as descr be" n F gdre 2 egena Tne pcads aro~nd2350 cm-' are due to carbon dioxide ~
~
1. stretching the Teflon tape as discussed above; 2. ealleetine a backmound soectrum of the Teflon taoe: " .. 3. preparing the sample as discussed below; 4. applying the sample and evaporating the solvent; and 5. scanning the sample and correcting against background.
To avoid overloading or crystallization on the tape, all samples should first he dissolved i n a good solvent and then applied on the Teflon tape. I n applying solutions of solid compounds, crystallization should he avoided because it reiults in undesirable sr:lttt:ring. This can be don(, by using a surnplc concentration far hclow saturation, so the wlll not nrf,ciuitatc until very Intc: in thc d n" i n c ~ - - solute -~~-~ process. This allows the sample to penetrate the polymer fibers, rather than to be deposited on the surface of the tape. A 10% solution for both liquid and solid compounds is usually satisfactory; however, for those compounds with a high tendency to precipitate during drying lower concentrations are recommended and several applications mas .. be necessary to deposit sufficient material for a good spectrum; the solvent should he allowed to evaporate between ~
.
A
Figure 6. Comparison of IR spectra of eugenol obtained on NaCl plates (Neat) and on Teflon tape: NaCl plate: one drop of neat euaenol was SDread between two NaCl plates: backaround was open beim Tet on bne orop from a Pasle~rp pel (appro;marcly 30 ,.L) of a to0. so LI on of e~genoln melhanol was app eo to me Teflon!ape (S 2 10, an0 !he spectrm co ectco after comp ele evaporal on of the solvent; background was stretched Teflontape beforesample application applications. This i s illustrated in the case of phenacetin a s explained in the legend of Figure 5 . We found that solvents such a s methanol, 2-propanol, ethyl acetate, toluene, diethyl ether, and methylene chloride are all appropriate solvents for use with Teflon tapes. Acetone, although a very good solvent for most organic chemicals, does not "wet" the Teflon tape and should he avoided; water should not he used for the same reason. To load a sample, a small volume (15-20 WLor approximatelx one drop) i s applied a n d spread over a n area of 1 cm . The solvent should he allowed to evaporate completely before collecting the spectrum. Failure to do so will result i n strong interfering bands. Volatile samples should be rnn immediately after solvent evaporation to avoid unnecessary losses. We found that heating with a blow drier to accelerate the evaporation process d i s t o r t s t h e s p e c t r u m because h e a t i n g changes the properties of the stretched Teflon tape, thus altering the background signal. Summary We show i n this paper that Teflon tape can be used effectively a s a sample support to obtain IR spectra. The technique is simple, rapid and economical and produces good quality spectra if sample preparation is done carefully. We have been using this method i n our teaching labs for over a year with very satisfactory results. Literature Cited 1. Lenhardt. J. 0.Am&d Smdrosouv 1980.34,702-704 .. . 2. Pouehert. C.J. The Aidrich Libmry dFP-IR Spectra. 1st ed.: Aldrich, Milwaukee. 1985;Vols. 2 and 3. ~
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Volume 72 Number 9 S e ~ t e m b e 1995 r
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