included a procedure for easily deleting selected old experiments from an historical library-the library must currently be recreated excluding undesired experiments. Also the spectrum averaging scheme makes no decisions about including ions of low abundance-all are included. Ions which occur infrequently are diminished in importance as additional spectra are averaged, but they are not rejected because we have not yet developed adequate heuristics for removing such ions. ACKNOWLEDGMENT We thank Edwin Blaisdell, Mark Stefik, Wilfred Pereira, and Alan Duffield for their contributions to initial discussions of the problem addressed by HISLIB. LITERATURE CITED (1) R:G. Ridley, in “Biochemical Applications of Mass Spectrometry”, G. R . Walier, Ed., John Wiley and Sons, New York, N.Y., 1972, p 177. (2) F. W. Karasek and J. Michnowicz, Res.lDev., 38. May 1976. (3) H. Nau and K. Biemann. Anal. Lett.. 6, 1071 (1973). (4) H. Nau and K. Biemann, Anal. Chem., 46, 426 (1974). d , S.C. Gates, J. chromatogr., (5) C. C. Sweeley, N. D. Young, J. F. ~ o ~ h nand 99, 507 (1974). (6) J. E. Biller and K. Biemann, Anal. Lett., 7 , 515 (1974). (7) R. G. Dromey, M. J. Stefik, T. C. Rindfleisch, and A. M. Duffield, Anal. Cbem., 48, 1368 (1976).
E. Jeltum, P. Helland, L. Eldjarn, U. Markwardt, and J. Marhofer, J . Cbromatogr., 112, 573 (1975). B. E. Blalsdeil. Anal. Cbem., 49, 180 (1977). W. E. Reynolds, in “Blochemlcal Applications of Mass Spectrometry”, G. R. Waller. Ed., John Wiley and Sons, New York, N.Y., 1972, p 109. S.P. Markey, W. G. Urban, and S.P. Levine, “Mass Spectra of Compounds of Biological Interest”, U.S. At. Energy Comm. Rep., No. ?‘ID-26553, National Technical Information Servlce, US. Dept. of Commerce, Sprlngfield, Va. 22161. H. S.Hertz, R. A. Hites, and K. Biemann, Anal. Cbem., 43, 661 (1971). See for example, A. N. Kolmogorov and S.V. Fomin, “Elements of the Theory of Functions and Functional Anavsis, Volume 1: Metic and Normed Spaces”, Graylock Press, Rochester, N.Y., 1957, p 16. E. W. Dijkstra, Numerische Math., 1, 269 (1959). J. A. Thompson and S . P. Markey, Anal. Cbem., 47, 1313 (1975). C. G. Hammer, B. Holmstedt, and R. Ryhage, Anal. Blochem., 25, 532 (1968). 0. Stokke, Biomed. Mass Spectrom., 3, 97 (1976). R. E. Carhart, D. H. Smith, H. Brown, and C. Djerassi, J . Am. Cbem. Soc., 9 7 , 5755 (1975). (19) D. H. Smith and R. E. Carhart, in “Chemical Applications of High Performance Spectrometry”, M. L. Gross, Ed., Proceedings In press.
RECEIVED for review April 14, 1977. Accepted June 27, 1977. Work supported by grants from the National Institutes of (No*RR-612 and GM-20832) and from the Aeronautics and Space Administration (No. NGR-05-020-632).
I CORRESPONDENCE Radiofrequency Oxygen Plasma Treatment of Pyrolytic Graphite Electrode Surfaces Sir: The formation of oxygen-containing functional groups such as carboxyl, hydroxyl, carbonyl, lactone and quinone-like groups by electrochemical (1-5) or air oxidation (6-8) on the surface of high density carbon or pyrolytic graphite has been suggested. More recently, there has been considerable interest in these groups as a means of chemical attachment of molecular species for the purpose of “chemically modifying” electrode surfaces, as exemplified by the early work of Miller and co-workers on the fabrication of a chiral electrode (IO). They employed high temperature air oxidation for the formation of carboxyl groups for use in binding of a stereospecific species to high density, microcrystalline graphite surfaces. Other oxidations of graphite involving the use of chemical agents (permanganate, chromate, etc.) have been reported (8, 9). In this communication, we wish to discuss the use of a radiofrequency (rf) plasma treatment of pyrolytic graphite (PG) in an oxygen atmosphere for the formation and enhancement of the surface population of oxygen-containing functionalities believed to primarily be carboxyl and hydroquinone/quinone groups. Surface analysis by scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (ESCA), and electrochemical characterization by cyclic voltammetry and differential pulse polarography (DPP) of the PG surfaces prior to and following plasma treatment are presented as supporting evidence. The nature of these plasma oxidized PG surfaces is probed further by chemical modifications which are specific to certain oxygen functionalities followed by surface and electrochemical analysis of these reacted surfaces. EXPERIMENTAL Materials. Isotropic, vapor deposited pyrolytic graphite (PG) electrodes in the form of disks (0.750 or 0.375 in. diameter X 0.125 1632
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
in. thick) were obtained from Ultra-Carbon Corporation (Bay City, Mich.). According to the manufacturer the ultra-pure substrate material (UT-6 grade graphite) was ground flat prior to deposition of the pyrolytic (PT-101)graphite coating (ca. 1000-pm thickness). Dimethyl sulfate (Eastman, practical grade), N,N’-dicyclohexylcarbodiimide (CDI; Pierce Chemical) and benzidine (BZ; Sigma Chemical) were used as received. Methylene chloride (MCB,reagent grade) was freshly distilled from anhydrous calcium chloride; the fraction boiling at 39.0 “C was retained. Water was deionized and triply distilled. Buffer compositionswere as follows: pH 2.50 and pH 2.35, Sorensen’s glycine I (0.1 M glycine + 0.1 N NaC1) and modified Sorensen’s glycine I (0.1 M glycine + 0.1 N KClO,); pH 5.10, Sorensen’s citrate I1 (0.1 M sodium citrate); pH 7.00,phosphate (EM “titrisol” brand concentrate). All other chemicals were reagent grade or equivalent. Apparatus. All electrochemical measurements were made in a two-compartment cell machined from Lucite (11). The carbon disk electrodes were held by compression against a neoprene O-ring to provide a leak-proof seal. Electrical contact with the working electrode was accomplished by means of a brass screw embedded in the back side of the disk. Cyclic voltammetric studies were conducted using a conventional three-electrode potentiostat with circuitry for compensation of solution resistance (12). A Princeton Applied Research model 174 polarograph was employed for differential pulse polarographic determinations. All electrode potentials were measured vs. a Ag/AgCl (1.00 M KC1) reference electrode. Scanning electron micrographs were recorded with a Cambridge model S4-10 Stereoscan. A Physical Electronics Industries, Inc., model 548 electron spectrometer was used to obtain ESCA spectra. Spectra were recorded at a system pressure 58 X lo-’ Torr. Plasma etching of the PG electrodes was carried out using a rf generater manufactured by Harrick Scientific (Ossining, N.Y.). Electrodes were centrally positioned on a quartz plate inside the quartz discharge vessel. Oxygen (anhydrous) was admitted to the discharge cell via a needle valve, and differentially pumped
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Figure 2. Low resolution ESCA spectra of PG surfaces: (A) untreated; (B) rf plasma etched (0.5-h exposure); (C) as B, wtth subsequent coupling of BZ. O/C atomic ratios: (A) 0.04, ( 8 ) 0.12, (C) 0.10; N / C atomic ratios: (A) nil, (B) nil, (C) 0.04
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by means of a liquid nitrogen trapped mechanical vacuum pump. The constant pressure of oxygen (monitored via a thermocouple gauge) was maintained by adjustment of the needle valve during etching. Procedures. The PG electrodes were initially extracted for 24 h in a Soxhlet apparatus using anhydrous methanol. Following extraction, residual solvent was removed by application of vacuum (
