2448
Anal. Chem. 1980, 52, 2448-2450
(4) Lane, R. F.; Hubbard, A. T.; Blaha. C. D. Bioeiectrochem. Bioenerg. 1978, 5, 504-527. (5) Marsden, C. A.; Conti, J.; Strope, E.; Curzon, G.; Adams, R. N. Brain Res. 1979, 777,85-99. (6) Ponchon, J.-L.; Cespuglio. R.; Gonon, F.; Jouvet, M.; Pujol, J.-F. Anal. Chem. 1979, 51, 1483-1486. (7) Huff, R.; Adams, R. N.; Rutledge, C. 0. Brain Res. 1979, 773,369-372. (8) Huff, R.; Adams, R. N. Neuropharmacology 1980, 79, 587-590. (9) Cheng, H.-Y.; Schenk, J. 0.;Huff, R. M.; Adams, R. N. J . Nectroanai. Chem. 1979. 700. 23-31. (10) See, for example, Adams, R. N. “Electrochemistry at S o l i Electrodes”; Marcel Dekker: New York, 1969; p 50. (11) Sawyer, D. T.; Roberts, J. L. “Experimental Electrochemistry for Chemists”; Wiley: New York, 1974. (12) Lennox, J. C. Anal. Chem. 1980, 52,585-586.
(13) Conti, J. Ph.D. Dissertation, University of Kansas, 1978. (14) Napp, D. T.; Johnson, D. C.; Bruckenstein, S. Anal. Chem. 1987, 39, 481-485. (15) Miller, B. J . Electrochem. SOC.1989, 716. 1117-1123. (16) Blank, C. L. J . Chromatogr. 1976, 717, 35-46. (17) Nicholson, C.; Phillips, J. M; Gardner-Medwin, A. R . Brain Res. 1979, 769,580-584. (18) Caldin, E. F. “Fast Reactions in Solution”; Wiley: New York, 1964.
RECEIVED for review May 28, 1980. Accepted September 8, 1980. T h e support of the National Institutes of Health via Grant NS08740 and a Biomedical Sciences Support Grant from the University of Kansas are gratefully acknowledged.
Rotating Ring-Disk Electrode with Demountable Disk Thomas Geiger and Fred C. Anson” Arthur Amos Noyes Laboratory, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 7 125
Rotating ring-disk electrodes provide powerful means for examining mechanisms of electrode processes ( I ) . They can be especially useful with electrodes that have had their surfaces coated or otherwise modified to achieve particular catalytic or synthetic objectives (2-5). Serious drawbacks in such applications with commercially available electrodes are difficulties associated with preparing and renewing the surface of the disk electrode from experiment to experiment: I t is almost impossible to avoid coating the ring electrode a t the same time that the disk is coated because of their close proximity. In addition, removal of disk coatings usually requires abrasive polishing and this eventually wears away the disk material with corresponding degradation in the performance of the electrode. All of these difficulties might be eliminated if the disk electrode could be easily removed from t h e ring-disk assembly for pretreatment and readily remounted. Several previous designs for easily demountable rotating ring-disk electrodes have appeared (6-8), but operating characteristics were provided for only one design (8). This electrode, which appeared to function very well, depended upon a tapered fit of the disk electrode into its holder to prevent leakage. We wished to use disk electrodes made of cylindrical graphite rods that could be cleaved repeatedly with a scalpel to expose a fresh surface so that a tapered construction was impractical. We therefore constructed a demountable ring-disk electrode of somewhat different design a n d have tested it with both basal plane and edge plane pyrolytic graphite disks combined with a platinum ring. A satisfactory disk was also fashioned from carbon paste and an “all-carbon’’ ring-disk electrode was obtained by using carbon paste for both the ring and disk electrodes in a manner similar to t h a t described by Galus e t a1 (9). Design details of t h e new electrode are described here for the case of a platinum-ring and basal-plane graphite disk, but extension to other electrode materials is straightforward.
EXPERIMENTAL SECTION Materials. Basal and edge plane pyrolytic graphite was obtained from Union Carbide Co. (Carbon Products Division, Chicago, IL). Basal plane rods were drilled out of the blocks supplied by the manufacturer. Edge plane graphite was supplied as cylinders. Both forms of graphite were cut into disks 7-8 mm in length that were subsequently turned on a lathe at one end (1mm) to fit the electrode holder. Carbon paste (Silicon oil base; CP-5) was obtained from Bioanalytical Systems (Lafayette, IN). Platinum ring electrodes were cut from platinum tubing supplied by Mathey Bishop Co. (Malvern, PA) in dimensions specified by us. Heat shrinkable polyolefin tubing was from Alpha Wire Co., Type FIT 300. Solutions were prepared from reagent grade 0003-2700/80/0352-2448$01 .OO/O
chemicals and water that had been distilled and passed through a commercial purification train (Barnstead Nanopure). Solutions were deaerated with prepurified argon. Apparatus. Ring-disk current potential curves were obtained by means of a commercial dual potentiostat (Pine Instrument Co., Grove City, PA) and recorded on an X-Y-Y‘ recorder (HewlettPackard Model 7046). The electrode rotator was from Pine Instrument Co., Model ASR 2. A one-compartment cell was employed with a platinum wire auxiliary electrode. The saturated calomel reference electrode was separated from the main cell compartment by immersion in a glass tube terminated by a sintered glass frit. Electrode Construction. In order to minimize problems arising from breakdown in the insulation between ring and disk electrodes, we provided both electrodes with insulating coatings. To provide as thin a gap as possible between ring and disk while avoiding stress in the insulating material, we provided mechanical stability elsewhere in the assembly. The complete electrode consisted of three parts as shown in Figure 1. An upper “holder” fits into the rotator and holds both the “disk assembly” and the “ring assembly”. The top of the stainless steel holder was machined to match the receptable and brush connections on the electrode rotator. At its lower end a hole was drilled to accept the disk assembly which is held in place with a setscrew that abuts a flattened portion of the central stainless steel shaft of the disk assembly. A t the bottom of the disk assembly a hole was drilled into which the machined end of the graphite disk fits snugly. Since it is sometimes difficult to dislodge spent disks, a small hole was drilled down the center of the disk assembly to allow the insertion of a push rod to help dislodge the disk. To hold the disk in place, we attached heat shrinkable tubing to the disk and ca. 2 cm of the holder. Since the diameters of both are the same in this region, a smooth, tight seal results when the tubing is heated. Any tubing that melts onto the face of the disk is removed by cutting with a scalpel. A t this point the assembly can be used as a simple, rotating disk electrode. We have employed such electrodes (IO) and observed good adherence to the Levich equation (11). To proceed to add the ring assembly to the disk electrode, it is necessary to remove a portion of the polyolefin tubing surrounding the disk in order to obtain a reasonably narrow gap between the ring and disk electrodes. This can be accomplished by mounting the disk electrode assembly on a lathe and carefully removing all but the last 0.34.2 mm of the coating. The hollow, cylindrical ring assembly (Figure 1) is then slid into position around the disk assembly at one end and around the Teflonsleeved portion of the holder at the other end where it is held in position with a setscrew. Electrical contact with the appropriate set of brushes on the electrode rotator is effected by means of a brass mantle on the upper end of the ring assembly and the ring is connected to the mantle by means of a platinum wire running through the Kel-F tip piece between the aluminum cylinder of the ring assembly and the platinum ring. The inner diameter of the ring electrode is large enough to permit the 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
HOLDER
DISK A S S E M B L Y
5.55Dia
A -
6.5 O I O ? 7~ ~ *
2449
RING A S S E M B L Y
--I3 --
Platinum Ring 7.2 I D 8.43 OD 2 Long
~
~
-7
6.8Dia --
Figure 1. Schematic description of the three parts of a demountable ring-disk electrode. All indicated dimensions are in millimeters but the drawings themselves are not to scale. Details of the construction are explained in the text.
polyolefin coated disk electrode to slide through comfortably. The seal between the disk coating and ring is not completely watertight so that electrolyte sometimes tends to creep into the crevice. The hole in the Kel-F tip piece is widened at its upper end to prevent any electrolyte that does creep into the crevice from making contact with either the metal portions of the disk assembly or the holder. An O-ring was also tried as a means of preventing electrolyte incursion into the crevice, but this tactic often led to damage of the thinned-down polyolefin coating on the disk during insertion of the disk assembly. The groove for the ring electrode in the Kel-F tip piece was machined to have the same inner diameter as the ring electrode but a slightly smaller outer diameter so that a tight force-fit resulted without deformation of the thin inner rim of the Kel-F insulation. For production of carbon paste ring electrode, the platinum ring was positioned ca. 0.2 mm below the rim of the groove in the Kel-F. The resulting cavity was then filled with carbon paste and smoothed by rubbing against a flat, smooth surface such as a computer card. To renew the surface of the pyrolytic graphite disk, we separated the disk assembly from the holder, and the graphite surface was polished or removed by turning on a lathe. With basal plane graphite disks, a new surface is quickly formed by cutting through the polyolefin coating and cleaving the graphite with a scalpel. After most of a graphite disk has been consumed the spent disk is removed by peeling off the polyolefin shroud. Immediate installation of a new disk is then possible and it is sealed in position with a fresh piece of polyolefin tubing.
F 2 W LL LL
3 L'
1
Caroon P a s t e Ring ,
1.3
C.5 E vs SCE
3
(volts)
/
-0.5
Figure 2. Background cyclic voltammograms for rotating ring and disk electrodes in 0.2 M CF,COOH as supporting electrolyte. Scan rate was 100 mV s-'. Rotation rate was 400 rpm.
RESULTS AND DISCUSSION Figure 2 gives t h e current-potential curves obtained in a pure supporting electrolyte for the two forms of pyrolytic graphite disk electrodes. Current-potential curves for a platinum ring electrode and a carbon paste ring electrode are also shown. The low, featureless background currents at the graphite electrodes and the wide window of available potentials compare favorably with commercially available electrodes. A solution of R u ( N H & ~ +was reduced at the disk and the resulting Ru(NH,),~+was detected a t the ring by reoxidation to Ru(NH&~+. Figure 3 shows a set of disk current-potential curves and the corresponding ring current-disk potential curves with the ring potential maintained a t 0.2 V vs. SCE. Both sets of curves were essentially the same for all combinations of disk and ring electrode materials. The limiting disk currents were linearly dependent on the square root of the electrode rotation rate (Figure 4) in accord with the Levich
WNC
I
-
Figure 3. Disk current-potential curves at various rotation rates for the reduction of 1 mM R U ( N H ~ )at ~ ~a+basal plane pyrolytic graphite disk electrode and the corresponding ring current-disk potential curves for reoxidation of Ru(NH,),*+ at the platinum ring electrode which was held at +0.2 V. Disk potential scan rate was 10 mV s-'; supporting electrolyte was 0.2 M CF,COOH.
equation (1I ) . However, the corresponding ring currents fell below the expected values a t higher rotation rates (Figure 4).
Anal. Chem. 1980, 52, 2450-2451
2450
/
u 3.3 -
I
disk electrodes positioned as coplanar as possible by simple visual inspection, a collection efficiency-rotation rate calibration curve was prepared that proved t o be quite reproducible and reliable. Thus, the power and versatility of t h e rotating disk electrode for detecting and identifying intermediates formed a t the disk remain available despite the inconstant collection efficiency.
ACKNOWLEDGMENT The numerous ideas and suggestions from the personnel in the departmental instrument shop as well as their skillful machining are a pleasure t o recognize. James Swartz was another reliable source of insightful comments and encouragement.
K.;I; 1 .
9 NC. CURRE',T CLRRE',T I
4
2c 3c 4'3 43 ( R 3 T A T l C h RCTEJ'"
I3
5c
EO
(rpm)'''
Flgure 4. Levich plots of limiting ring and disk currents vs. (rotation rate)"' prepared from the current potential curves in Figure 3.
T h e collection efficiency of t h e ring-disk electrode (12) was therefore not independent of rotation rate a t values above ca. 900 rpm. At lower rotation rates the collection efficiency became constant and very close to the value of 0.3 calculated from the geometrical dimensions of the disk, ring, and intervening gap (12). The decline in collection efficiency a t high rotation rates appeared not to result from lack of exact coplanarity of the disk and ring electrodes because intentional introduction of noncoplanarity produced only small changes in the collection efficiency. I t seems possible that the inevitable ridge present in the gap region where the two insulating materials abut each other may introduce perturbations in the hydrodynamic flow at the higher rotation rates that result in variations in collection efficiency. Nevertheless, with ring and
LITERATURE CITED
(1) Aibery, W. J.; Hitchman, M. L. "Ring-Disk Electrodes";Cbrendon Press: Oxford, 1971. (2) Coliman, J. P.; Marrocco, M.;Denisevich, P.; Koval, C.; Anson, F. C. J. Electroanal. Chem. 1979, 101, 117-122. (3) Collman, J. P.; Denisevich, P.; Konai, Y.; Manocco, M.;Koval, C.; Anson, F. C. J. Am. Chem. Soc., in press. (4) Yeager, E.; Zagal, J.; Nikolic, G. 2.; Adzic, R. R. Proc. Symp. Electrode Processes. 3rd 1979, 436. ( 5 ) Zagal, J.: Bindra, P.; Yeager, E. J . Electrochem. SOC. 1980, 727, 1506- 15 17. (6) Zhutaeva, G. V.; Shumiiova, N. A. Elektrokhimiya 1966, 2, 606-607. (7) Doronin, A. N. Elektrokhimlya 1968, 4 , 1193-1 194. (8) Harrington, G. W.; Laitinen, H. A.; Trendafilov, V. Anal. Chem. 1973, 45, 433-434. (9) Galus. 2.; Olson, C.; Lee, H. Y.; Adarns, R. N. Anal. Chem. 1962, 3 4 , 164- 166. (10) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192-1198. (1 1) Levich. V . G. "Physicochemical Hydrodynamics"; Prentice-Hall: Engle. . wood Cliffs, NJ, 1962. (12) Albery, W. J.; Bruckenstein, S. Trans. Faraday SOC. 1966, 62, 1920-1 93 1.
RECEIVED for review July 25, 1980. Accepted September 22, 1980. Contribution No. 6274. This work was supported by the National Science Foundation.
Coupling of Capillary Gas Chromatograph and Fourier Transform Mass Spectrometer Edward B. Ledford, Jr., Robert
L. White, Sahba Ghaderi, Charles L. Wilkins," and Michael L. Gross*
Department of Chemistty, University of Nebraska -Lincoln, Lincoln, Nebraska 68588
We report here the first demonstration that, with the fast scanning ability of Fourier transform mass spectrometry (FT/MS), mass spectral data can be acquired during a typical SCOT capillary column gas chromatography peak elution (5-10 s). Preliminary studies in our laboratory have shown that is is possible to scan fast enough to obtain the GC peak profiles of eluting components. I n addition, high-resolution mass spectral data over narrow mass ranges have been acquired during capillary column peak elutions. Thus, the F T / M S instrument can perform in a rapid lower resolution spectral scan mode or in a high-resolution multiple ion monitoring mode.
EXPERIMENTAL SECTION The FT/MS instrument used for this study was specially designed with a high gas conductance vacuum chamber. This allows the mass analyzer to operate at low pressure, which is necessary for high mass resolution. The forward end of the vacuum chamber, which contains a cubic trapped ion analyzer cell (plate separation 0.0254 m), has a nominally rectangular shape and is constructed of 304 stainless steel. The width (internal 0.0667 m) is sufficient to permit insertion into the 0.0762-m air gap of a Varian V-7300 0.305-m electromagnet. That volume expands into a 0.152-m i.d. stainless steel tube which is outfitted with a 0.152-m Thermionics Model GS 6000 gate valve. The tube terminates at a Balzers TPU 0003-2700/80/0352-2450$01 .OO/O
500 turbomolecular pump which is backed by a Welch Model 1397 mechanical pump. Pressures were measured by using a Veeco Model RG-840 ionization gauge and controller. The pumping speed at the cell is 360 L/s which was determined by calculation and verified experimentally by introducing a known flow rate of helium and observing the pressure. The electromagnet (Varian, Model 7300) is equipped with one pair of 0.305-m cylindrical ring-shim pole caps with iron-cobalt ring for high homogeneity. Its maximum field strength is 1.37 T at the center of the air gap, and it was operated at 1.2 T for these experiments. It is outfitted with a Varian V-7800 13-kW basic power supply and a V-7550 current regulator. The vacuum system is interfaced to a Perkin-Elmer Sigma I1 gas chromatograph containing an OVlOl 15.2 m X 0.5 mm SCOT capillary column at 150 "C (isothermal). The interface consisted of either a direct coupled transfer line (0.5 mm i.d., 1.6 mm 0.d. glass-lined stainless steel obtained from Scientific Glass and Engineering) or a standard glass jet separator coupled to the transfer line. Both were held at 200 "C. When the separator was used, 20 mL/min of make-up helium gas was introduced prior to the jet separator. Ion formation and mass analysis were accomplished as previously described ( I ) . The ion excitation was provided by a Rockland System 5110 programmable frequency synthesizer under the control of a 40K word Nicolet 1180 minicomputer, equipped with a high-speed buffered digitizer capable of data acquisition 63 1980 American Chemical Society
