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Amperometric and Fast Scan-Rate Cyclic Voltammetry Detection at a Microelectrode for Gel Permeation High-Performance Liquid Chromatography of Fullerenest BenoPt Soucaze-Guillous,Wlodzimierz Kutner,+and Karl M. Kadish’ Department of Chemistry, University of Houston, Houston, Texas 77204-5641
Amperometry and fad ecan-rate cycllc voltammetry (CV) at a l O - ~ l a m e t e platinum r microelectrode were utlllzed for detoctlon and In rttu Identlflcatlon of fullerener which were separated by gel permoatlon high-performance liquid chromatography. Tho mlcroeMrode and inlet capillary nozzle wore arranged perpendicularly. The llmitlng current8 were vktuaMy Independent of the mobile phase flow rate In the range 0.1-2.0 mL m1n-l for a distance between the mlcroelectrode and Inlet capillary nozzle 10.10 mm. A toluene extract of the laser-vaporizedroot containing low molecular maw 1uIIoro~s was rewived on two gel permeation column8 which were connected In serler. The mobile phase wa8 dlchloromdhano/cyciohexane, 90/10 (v/v), 0.01 M tetra-* butylammonlum perchlorate. Ceoand C70were the major comporwnk of the extract and were present In a 3 1 mars ratio. Other higher fullerener were a b present In lower concontration. The number and 8hapes of the peaks In chromatogramobtainedwith amperometrlcdetection at -1.3 V vo SCE wore the same as those obtained with UV-vldble rp.ctrorcoplc detection at 365 nm, thus lndlcatlng that all of tho detoctedcompound8 both absorb UV-visible light and are eiectrochemlcallyactlve. Extracdumnpeak broadening8due were aloo the to detectJon and detectablllty (83.8 ng for COO) same In both detection techniques. I n rHu fad 8caMate vottammogranw of Csoand CT0were obtained during elution, and the revonlble El12 value8 of the third and fourth electroreductlonr could be u8ed for thelr Identlflcatlon.
INTRODUCTION Since the large-scale arc-processed graphite synthesis of fullerenes has been reported,1-2 a major limiting factor in pursuing physical and chemical research in this area has been the isolation of compounds from the resulting soot. A separation of Cw and c70,the major components of the soot, is often performed in appreciable yields by liquid column chromatography at &alumina3 or graphite.4 Although this technique has been recently much improved,”7 only a ~~
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* To whom correspondence should be addressed.
+ On leave from the Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44,Ol-224 Warsaw, Poland. t Presented in partat the 182ndMeetingof the ElectrochemicalSociety; Symposium on ‘Fullerenes: Chemistry, Physics and New Directions III”, Electrochemical Society, Toronto, Canada, October 12-15, 1992. (1)Kritschmer, W.; Lamb, L. D.; Fostiropolous, K.; Huffman, D. R. Nature 1990,347, 354-358. (2) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J . Chem. Soc., Chem. Commun. 1990,1423-1425. (3) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropouloe, K.; Huffman, D. R.; Kritschmer, W.; Rubin, Y.; Schriver, K. E.;Sensharma,D.; Whetten,R. L. J.Phys. Chem. 1990,94,8630-8633. (4) Vassallo, A. M.; Palmisano, A. J.; Pang, L. S.K.; Wilson, M. A. J . Chem. SOC.,Chem. Commun.1992,6041. (5) Chatterjee, K.; Parker, D. H.; Wurz, P.; Lykker, K. R.; Gruen, D. M.; Stock, L. M. J . Org. Chem. 1992,57, 3253-3254.
0003-2700/93/0365-0669$04.00/0
separation by high-performance liquid chromatography (HPLC)‘ results in the highest purity fullerene products, particularly those of high molecular mass. Several HPLC separation modes have been exploited for the separation of fullerenes, including normal3 and reverse phases,s12 a Pirkle modified phase,13J4 and gel permeation modes.1”” UV-visible spectroscopy has been the most common detection technique used in the HPLC of fullerenes. The separated fullerenes can be identified in situ if HPLC separation is combined with on-line UV-visible spectral measurement3J5J6J8 and/or mass spectroscopic analysis.19 In the present paper we describe the first application of an electrochemical(amperometric)method for detecting fullerenes by HPLC. This simple method consists of measuring the electroreduction current at a constant potential during elution.20 Fast scan-rate cyclic voltammetry (CV) measurements were also performed at the rising portion of the HPLC peaks for in situ product identification. A gel permeation HPLC mode was utilized in our study since it offers a favorable cost-to-efficiency ratio.21 Fullerenes reveal very rich and diverse reductive electroc h e m i ~ t r y . ~ O For , ~ ~ instance, -~~ up to six one-electron reversible electrode processes have been reported for Cw and & (6) Khemani, K. C.; Prato, M.; Wudl, F. J . Org. Chem. 1992,57,32543256. (7) Scrivens, W. A.; Bedworth, P. V.; Tour, J. A. J . Am. Chem. SOC. 1992,114,7917-7919. (8) Diack, M.; Hettich, R. L.; Compton, R. N.; Guiochon, G . Anal. Chem. 1992,64, 2143-2148. (9) Diederich, F.; Whetten, R. L. Acc. Chem. Res. 1992,25,119-126. (10) Cox. D. M.: Behal. S.: Disko. M.: Goun. S. M.: Greanev. M.: Hsu. C. S.; Kollin, E. B.; Millar; J.; Robbins, J.; Robbins, W.; She&&, R. D.; 1991,113, 2940-2944. Tindall, P. J . Am. Chem. SOC. (11) Howard, J. B.; McKinnon, J. T.; Makarovsky, Y.; Lafleur, L.; Johnson, M. E. Nature 1991,352, 139-141. (12) Diederich, F.; Whetten,R. L.;Thilgen,C.;Ettl,R.;Chao,I.;Alvarez, M. M. Science 1991,254, 1768-1770. (13) Hawkins, J. M.; Lewis, T. A.; Loren, S. D.; Meyer, A.; Heath, J. R.; Shibato, Y.; Saykally, R. J. J . Org. Chem. 1990,55,62504252. (14) Pirkle, W. H.; Welch, C. J. J . Org. Chem. 1991, 56, 69734974. (15) Kikuchi, K.; Nakahara, N.; Honda, M.; Suzuki, S.; Saito, K.; Shiromaru, H.; Yamauchi,K.; Ikemeto, I.; Kuramochi,T.; Hino, S.;Achiba, Y. Chem. Lett. 1991, 1607-1610. (16) Meier, M. S.; Selegue, J. P. J . Og. Chem. 1992,57, 1924-1926. (17) Kikuchi, K.; Nakahara, N.; Wakabayashi, T.; Honda, M.; Matsumiya, H.; Moriwaki, T.; Suzuki, s.;Shiromaru,H.; Saito,K.; Yamauchi, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1992,188,177-192. (18) Diederich,F.; Ettl,R.;Rubin,Y.; Whetten,R. L.;Beck,R.;Alvarez, M.; Anz, S.; Sensharma, D.; Wudl, F.; Khenami, K. C.; Koch, A. Science 1991,252, 548-551. (19) Anacleto, J. F.; Perreault, H.; Boyd, R. K.; Pleasance, S.; Quillan, M. A.; Sim, P. G.; Howard, J. B.; Makarovsky, Y.; Lafleur, A. L. Rapid Commun. Mass Spectrom. 1992,6, 214-220. (20) Stulik, K.; Pac&kovB, V. Electroanalytical Measurements in Flowing Liquids, Ellis Horwood Ltd.: New York, 1987; Chapter 2. (21) Cotter, R. Private communication, Millipore Corp., Waters Chromatography Div., Milford, MA. 1991. (22) Jehoulet, C.; Bard, A. J.; Wudl, F. J . Am. Chem. SOC.1991,113, 5456-5457. (23) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. J. Am. Chem. SOC.1991, 113,7773-7774. (24) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J . Am. Chem. SOC. 1991,113,4364-4366. 0 1993 American Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, 1993
in aprotic solvent s y ~ t e m s . ~ The ~ , ~ Elp 8 values of the first two electroreductions are similar, but the third and fourth electroreductions are more facile (occur at most positive potentials) in the case of C70.23~24927-30 The Ell2 values for electroreduction of derivatized fullerenes31~~~ or higher fullerenes, e.g., C7e3Oand Cgq,33 are also markedly different thanthese of the low molecular mass fullerenes. Additionally, the E1/2values for electroreduction of all fullerenes may vary substantially with changes in the nature of the solvent and/ or supporting e l e c t r ~ l y t eand , ~ ~ this should be considered useful when electrochemical detection procedures are developed for HPLC of higher fullerenes. This diversity of fullerene electrochemistry is especially important in view of the small differences between UV-vis spectra of higher fullerenes15J7which makes their in situ UV-vis identification difficult. The highly hydrophobic fullerenes are insoluble in water and only slightly soluble in other highly polar s0lvents.3~They are, however, moderately soluble in aprotic solvents of low polarity.24135~36Hence, only these solvents can be used as a mobile phase for HPLC separation. The solubility of most common supporting electrolyte salts is usually small in these solvents and thus the conductivity of the resulting solvent system is also low. Therefore, a microelectrode was utilized for electrochemical detection in the present study, as has already been shown to be useful for detection in flow-through systems.37-39
i
Flgure 1. Schematic arrangementof the fbw-through electrochemloal detection: Pt disk microelectrode (10-pm dhmetw) (1); pdykrtrafluoroethylene shrouds (2); stainless steel lnlet capillary tubing (0.026mm I.D.) (3); Pt wke rlng auxiilery electrode (4). The gap between the working electrode and the nozzle of the lnlet capillary was typlcally adjustedto0.10mm wltha micrometer saew (notshown). Thedkectlon of the moblle phase Row Is lndiceted wlth arrows.
Instrumentation and Procedures. Chromatographic separation was performed with a HPLC ieocratic system (Waters, Millipore Corp., Milford, MA) composed of a U6K Universal Injector, a 6000A delivery pump with modified heads and a 440 UV-visible fixed-wavelengthabsorbance detector with a 10-mmEXPERIMENTAL SECTION long detection channel of 1.55-pL volume. The detector was equipped with a 365-nm filter. The connectin of two analytical Reagents. Soot from laser-vaporized graphite was a generous columns in series (Waters Ultrastyragel 500 ,7.8 X 300 mm) gift of Dr. R. E. Smalley. Fullerenes of low molecular mass were greatly improved the resolution of the system. Chromatograms extracted by sonication of the soot suspension in toluene (3% were recorded with a BD40 04/06(Kipp and Zonen, Holland) yield) and then separated from the remaining solid by centrifstrip chart recorder. A 90/10,v/v, dichloromethane/cyclohexane, ugation. The extract was preconcentrated by evaporation in 0.01 M in (TBA)ClOI, mixture was used as the mobile phase. It vacuum prior to injecting into the HPLC columns. Typically, was deoxygenatedwith nitrogen and then degassedby sonication the injection volume was 25 pL. before being percolated through the columns. Dichloromethane and cyclohexane (FlukaChemie AG, Buchs, Electrochemicaldetection was carried out with a flow-through, Switzerland) of the “puriss” grade (HzOcontent
