Using High Performance Liquid Chromatography to Determine the C60:C70 Ratio in Fullerene Soot An Undergraduate Chemistry Lab Michael C. ~umwalt' Department of Physics, University of Arizona, Tucson, AZ 85721 M. Bonner Denton Department of Chemistry, University of Arizona, Tucson, AZ 85721
Background The discovery of a technique for fullerene production in macroscopic quantities (1)has led to an explosion of research, particularly in chemistry, over the past few years. The technique for producing the fullerene molecules used in this work involves a carbon-arc vacuum chamber shown schematically in Figure 1. Though its simplified architecture has allowed for minor modifications (e.g., refs. 2 and 31, it is still the most widely used form for fullerene production. After this chamber is evacuated, it is filled to -200 Tom (V4 atm) with helium. Apower source then supplies up to 225 amps, at 35-50 V, into the electrodes that hold the carbon (graphite) rods. Because there is no oxygen present, this heating process does not result in a bum (e.g., fireplace), but rather, vaporization of the graphite. The produced soot is then harvested from the chamber and partially dissolved into toluene. That which is dissolved is made up primarily of CsOand C70.The yield of C&C7,, tends to be 5-12% of the chamber soot, by mass, and the typical ratio of Csoto C 7 is ~ 4:l. To obtain pure samples of Csoand Cvoeach, it is necessary to I 1. Fullerene soot production chamber. ~erform~ r e ~ a r a t i v scale e liauid chromatoma- Fiqure phy. ~ h e b r e ~ a r a t icolumnused ve in the faman laboratory is packed with alumina and is based on the design of Koch, et al. (4) The CsdC70mixture is then placed on top and the whole column flushed with hexanes. Two separated bands of each molecule appear along the column with CeOleaving first. Dissolved CeOappears purple while C70is reddish-brown. Determination of purity in these separated bands can then be camed out by HPLC. Safety is an important consideration especially in the handling of new materials. However, in the case of fullerene molecules there have been no significant gains in understanding or determining any mutagenic, teratogenic, carcinogenic, or toxic properties. Experimental Procedure A standard analytical HPLC setup is used. The 1:l acetonitrileltoluene mobile phase flows a t 3 mumin through an Alltech C18 column using a Waters 600E multisolvent delivery system. Absorption detection is accomplished using a Waters 484 tunable absorbance detector set at 357 nm. The calibration procedure used to determine the rela-
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Figure 2. Calibration line for Cso:Cm ratio. Volume 72 Number 10 October 1995
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tive ahsorption amounrs begin;; with pure samples ,99+'7, dissolved in cqual cnncentrstions u,irh toluene. Takinra 1:l volume mixture ofrhe tu,osolutions and injecting pL amounts onto the column, resultinx UV absorption peak-area ratios are determined as a function of wa\elen&h using a Spectra Physics SP4270 integrator. Alinear fit of this data is shown in Figure 2 where 356.6nm is the resulting wavelength a t which both fullerenes absorb equally. Because the UV absorption detector used in this experiment has only nanometer resolution we settle for 357 nm and use the appropriate correction factor in analyzing the ratio of Cs0 to C 7 0 for unknown mixtures. of'C,, and !9DrCiI of
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Results and Discussion A chromatogram of separated and well-resolved Cso and c 7 0 is shown in Figure 3. According to the integrator the peak-area ratio of Ceo:C,o is 4.04 k01.From Figure 2 the correction factor a t 357 n m is 11.983 resultine in a -n adjusted ratio of 4.10.The labeling of the appropriate peaks is based on the results of Diederich, et al. ( 5 )in which a 1:l acetonitrileltoluene mobile phase also was used on a C18 column, hut followed by absorption a t 310 nm and mass spectrometry. Although the higher-mass fullerenes typically comprise 1%of the total mixture, such peaks are of great use for analyzing conditions favorable to their production andor separation. As a matter of fact, HPLC has been recently used to collect -2 mgeach of the CT6,CT8,Cad, and mixtures of the Ca6CIo2fullerene molecules (6). At a time when so much research activity involves a new material, it helps to have a relatively. quick and easy analytical . tool for investigation.
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Figure 3. Chromatogram of separated fullerenesdetected at 357 nm.
Literature Cited 1. Kratschmer. W.;Lamb, L.D.: Fostiropoulos, K; Huffman. D. 354.
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R. Nofun 1990, 347,
2. laeoe, D.W.; Potter, W T.; Teeters, D. J. Chem. Educ. 1992. 69 (81,663.
Solution samples containing fullerenes are available from the authors upon request.
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3. Craig, N.C.; Gee. G. C.: Johnson, A. R. J . Chem. Edue. 1992.69 18). 665. 4. Koch. A. S.:Khernani, K. C.; Wudl, F J . Ow. Chem 1981.66.4543. 5 . Diededeh, F;Whetten, R.LAec. Chem Res. 1982.26, 119. 6. Taylor, R.; Lanr1ey.G. J.:Avent. A. G.;Dennia,T.J.S.: Kmt0.H. W ; Waltnn, n~ M. J Chem. Soe. Perkin. Danr;. 1992.2. 1029.
