Rapid IR Sampling Methods for the Organic Chemistry Laboratory Using a Diffuse Reflectance Accessory Richard Scamehorn Ripon College, Ripon. WI 54971 While routinely taking the infrared spectrum of a liquid sample in organic laboratories has been standard for many years, taking the spectrum of a solid sample always has been a problem. Standard methods for obtaining the IR spectrum of a solid include nujol mulls, solutions in organic solvents, liquid melts, and KBr pellets (1,2).All of these methods have limitations and drawbacks, and none is easy to apply to organic classes of 18-24 students, all of whom need to take a spectrum during the last hour or so of the laboratory period. The most commonly used method has been the use of the minipress to make KBr pellets. Making acceptable KBr pellets, however, is a time-consuming task for most students. Some students can work for a n afternoon and never obtain a eood one. Indeed, some compounds simply will not form pelrets that do not stick to the press. The result has been that in most orwnic classes students make one KBr pellet but forgo taking IR spectra of most other solid samples. The widespread availability of FT-IR spectrometers in the organic laboratory now makes possible the use of different sampling techniques for both solids and liquids. In oarticular. the use of a diffuse reflectance accessorv allows students to obtain the IR spectra of milligram samples of both solids (3)and liauids in about the same time reauired to obtain the spectrum of a liquid on NaCl plates. i n the diffuse reflectance technique, the infrared beam is reflected off the solid sample rather t h a n transmitted through it. The higher power and multiple scanning of FT instruments allow spectra with high signal-to-noise ratios to be obtained readily despite the fact that less than 15% of the incident radiation is reflected generally (4). The sample is prepared by mixing 1-5 mg of compound with 60-70 mg of KBr. We ask students to weigh these out because they are difficult to estimate, and the method rarely fails to give a good spectrum if these proportions are used. Our students make these samples usine a small mortar and pcsrlt,, hut t:xtcnsivr kvinding is not necewlly. Thev only need to he mixed intimntclv. If the compnund is crystalline, it should be crushed or prioE to mixing with the KBr. To obtain a spectrum, the diffuse reflectance sample cup is filled with pure KBr and a background spectrum is recorded. The cup is then filled with the sample mixture, and the spectrum obtained. An automated macro is used in which students only need to enter a file name to obtain the snectrum. We usuallv take 16 scans. bur mght to 12 scans generally w~llgl\.e an acceptable specJI spectrum 1s gen(wllg suflitrum One I ~ I ~ I background
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cient for an entire class. The Bio-Rad diffuse reflectance accessory has a tray with four sample cups.' This allows four samples to be placed in the sample comoartment a t one time:After the spectrum of each sample i's taken, the tray is advanced to the next cup. Two trays allow students to be continuously preparing samples wich little holdup. I t takes about three minutes to obtain a spectrum, including printing? Excellent spectra a r e obtained using t h i s method, and it is seldom necessary to repeat an analysis thus allowing organic classes to obtain IR spectra of solid samples routinely We also have found the diffuse reflectance accessory to be extremely valuable for the analysis of liquids. We have been using microscale experiments in our second organic course. In some of these experiments student yields of less than 10 mg are not unusual. Even with sample amounts of only 3 or 4 me, excellent IR spectra can be obtained usine the diffuse reflectance acces'ory. The diffuse reflectance Hample cup is filled with KBr. A few drops of dichloromethane i r e added to the vial containing theAsample,and one drop of the solution is placed on the KBr. After a minute or two, the dicbloromethane has evaporated and the sample is placed in the spectrometer, and a diffuse reflectance spectrum is obtained. The method is very rapid and gives excellent results. In two of our experiments in which small amounts of liquid product are often obtained, students use the same solution to obtain an IR spectrum and to do a GLC or GC-MS analysis. Acknowledgment The National Science Foundation Division of Undergraduate Education has provided partial support for this project through Grant No. USE-8851154. Literature Cited 1. Maya, D .W: Pike, R .M.: Butcher S. S.:Trumper. P K Micmmole Whniquesfor the Orgnnic Laboroiory; John Wiley & Sons:New York, 1991: p 237. 2. Pa*% D. L.: Lsmpmsn,G.M.:Knz, G. S.:Engle.R. Glntmduclion l o O r g a n i e L a b o ~ mtou Tpchni0ues:A Micmscole Approach; Saunden: Ph!ladelphia, 1990: p 770. 3. Raymond, K. W: Corkill, J. A. J. Chern. Educ 1994, 7l,A204. 4. Gritlitha. P. R.;daHamth.J. A. Fouriii ~ n 6 , f o r ml ~ f f f f dS p e ~ f f f f t " i ; WiIey: New York, 198% p 194.
'Bio-Rad Digilab Diffuse Reflectance Asccessory used with a Model FTS-7 Bio-Rad FT-IR spectrometer system. We also have used the accessory in a Mattson Genesis FT-IR spectrometer with good results. 'An HP LaserJet I l l printer is used.
