Anal. Chem. 1984, 56,2263-2265
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at high mass, as the monoisotopic peak is reduced in abundance to the level of background, valuable chemical information may still be obtained from the accurate average mass determination of the molecular ion envelope at less than unit resolution, as this measurement will also reflect the mass defect. In this instance we note further that future efforts to obtain high-resolution information from more abundant peaks in a resolved spectrum will necessarily require instrumentation with resolution a t high mass superior to that currently available. LITERATURE CITED
\
I\ 5776,625
MI2
\ 5776 650 I
Flgure 2. Relative intensities of the isobars of m / r 5776.6 from the protonated molecular ion peak of Flgure 1 and redicted peak shapes at resolutions of 5 x io5, 1 x lo6,and 5 x IO . (These curves were generated by a computer program developed by J. A. Yergey in the
B
Middle Atlantic Mass Spectrometry Laboratory.) sulin. The ordinate indicates that these masses all lie between 5776.629 and 5776.648 amu. At 5 X lo5resolution the group of isobars is deteded as an asymmetric single peak. Resolution approaching 5 X lo6 is required for clear separation. It is interesting to note some of the implications of this observation. While it is certainly advantageous to operate a t the best possible resolution, we have noted before (5,6)that
(1) Yergey, J. Int. J. Mass Spectrom. Ion Phys. 1983, 52, 337. (2) Fenselau, C. Anal. Chem. 1982, 54, 105A. (3) Hill, R.; Fales, H.; McNell, C.; MacFarlane, R. Blomed. Mass Specfrom., in press. (4) Puzo, G.; Prome, J. C.; Macquet, J. P.;Lewis, I . A. S. Biomed. Mass Spectrom., in press. (5) Yergey, J.; Heller, D.;Hansen, G.; Cotter, R. J.; Fenseiau, C. Anal. Chem. 1983, 55, 353. (6) Fenselau, C.; Yergey, J.; Helier, D. Int. J. Mass Spechom. Ion W y s . lS83. 53. 5. (7) . , Barber. M.: Bordoii. R. S.: Elliot. G. J.: Sedawlck. R. D.: Tyler, A. N. Anal. Chem. 1982, 54, 645A. (8) Grotlahn, L.; Frank, R.; Blocker, H. Nuclelc Acids Res. 1982, IO, 467 1. (9) Williams, D. H.; Bradley, C.; Bojeson, G.; Santikarn, S.; Taylor, L. C. E. J. Am. Chem. SOC.1981, 703,5700.
James A. Yergey Robert J. Cotter* David Heller Catherine Fenselau Middle Atlantic Mass Spectrometry Facility Department of Pharmacology Johns Hopkins School of Medicine Baltimore, Maryland 21205
RECEIVED for review April 10,1984.
Accepted June 21,1984.
AIDS FOR ANALYTICAL CHEMISTS Spectroelectrochemical Cell wlth Adjustable Solution Layer Thickness Marc D. Porter, Shaojung Dong, Yu-Peng Gui, a n d Theodore Kuwana*
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Spectroelectrochemistry (SEC) a t optically transparent electrodes has been used to investigate a wide variety of redox phenomena (1-7). In many instances, however, the application of this technique requires a specific cell design to meet the conditions of the particular measurement, viz., accessible spectral region, optical sensitivitiy, exclusion of oxygen, extent of solvent compatability, and small cell time constant for monitoring fast transient responses (1-11). It also would be advantageous if the cell could be rapidly converted between a thin-layer (TL) and a semiinfinite linear diffusion configuration (SILD). The purpose of this brief report is to describe a SEC cell design that offers easy and rapid conversion between T L and SILD configurations. Attractive features of this design are its fabrication simplicity, accessible spectral range (ultraviolet to visible), and oxygen exclusion capability. The ascribed SEC characteristics were confirmed via cyclic voltammetry (CV), cyclic voltabsorptometry (CVA), and steady-state potential step (SSPS) spectral responses for the TL configuration and 0003-2700/84/0356-2263$01.50/0
by chronoabsorptometry for the SILD configuration. The ferri/ ferrocyanide redox couple was used as the test species. EXPERIMENTAL SECTION Cell Fabrication. Front and side views of the cell in the TL confiiation are shown in Figure 1. The main body is fabricated from the barrel and plunger of a 10-mL glass syringe (American Scientific Products, Obetz, OH) which are cut to lengths of ca. 2.5 cm and 3.0 cm, respectively. The plunger is cut longer than the barrel to provide a means for manipulating the plunger. Quartz windows are sealed to one end of the barrel and plunger with silicone cement. The back window is carefully ground to a diameter slightly less than the inside diameter of the barrel prior to attachment to the plunger. Parallel alignment of the windows is accomplished by sealing the plunger inside the barrel with jewler’s wax and polishing one end to flatness on fine grit sandpaper. The barrel and plunger are separated after reheating the wax and are cleaned with acetone and water. Quartz windows allow spectral measurements in the visible and ultraviolet regions. However, windows which are fabricated from microscope slides are satisfactory for measurements in the visible region. 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56. NO. 12.
OCTOBER 1984
! A
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a W
u z U
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u Flgw 1. (A) Slde view of me cell (a) syrlnge plunger. (b) back window, (c) syrinw barrel, (d) reference electrde port. (e)auxiliary electrode port, (f) solution Inlet channel. (g) Teflon spacer, (h) minlgrki electrde and electrical wntact clip, (I) front window. (E) Simplified front view of the cell: (i)solution contact channel.
The thickness of the confined solution layer is defined by a Teflon spacer which has an adhesive film on one side (Fluorocarbon, Dielectric Division, Irving, CA). The spacer is cut to the shape as shown in the simplified cell diagram in Figure 1B with an inside diameter of 1.0 cm and an outside diameter of 1.5 em. A portion at the top of the spacer is removed to allow solution contact from the working electrode to the Ag/AgCI (saturated KCI) reference electrode and the Pt wire auxiliary electrode. The spacer thickness is between 120 and 140 pm, as measured with a micrometer. The TL cavity, which is defmed hy the spacer and windows, has an apparent solution layer thickness and optical path length of 12W140 pm. The absorbance and coulometric measurements, which more accurately define the thickness and volume of the TL cavity, are described later. The cell is transformed from a TL configuration to a SILD configuration by displacement of the plunger to a requisite distance from the front window. The working electrodes were either gold minigrids (200 lines , ' : n i BuckbeeMears Industriea, St Pad, MN) or antimony-doped tin oxide (20 sq-', PPG Industries, Pittsburgh, PA). Both electrodes were cut slightly larger than the outside diameter of the barrel to provide electrical contact. The minigrid electrode was placed between the front window and the spacer with the adhesive on the hack of the spacer sealing the electrode to the window. Due to the fragile nature of the minigrid, electrical contact was made by using a copper foil which was attached to the external portion of the minigrid with conductive epoxy. For a cell made with a tin oxide electrode (used primarily for SILD experiments), the electrode replaced the front window and the spacer was attached to the hack window. Electrical contact was made to the tin oxide electrode by using a copper ring (12). The auxiliary and reference electrodes were attached to the cell with polyearbonate Luer fittings (Value Plastics, Loveland, CO) which were sealed to the barrel with epoxy cement. A small slot was cut into the polished end of the barrel for placement of these fittings. The cell was filled with the test species through the inlet channel using either a vacuum degas/filling technique (10,13) for oxygen-sensitive measurements or hy extrusion of excesa solution through the inlet channel via displacement of the plunger. Instrumentation and Reagents. Spectral measurements were made with a DMS-!30 W-VIS spectrometer (Varian Instrument Co., Palo Alto, CA). A setscrew held the cell in an aluminum collar which was mounted in the sample chamber of the spectrometer. A conventional three-electrode potentiostat was used with data displayed on a x-y recorder (Houston Instruments Co., Bellaire, TX). The potential of the working electrode was measured vs.
I
I
0.4
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0.2
P O T E N T I A L , VOLT
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I
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2. Cyclic voitabsapmmefic absabanca-potential (a)and cy& voltammetric current-potential (b) responses fa 1.0 mM ferrkyanlde in t .O M KCI with the cell in the thln-layer conflguratlon. The sweep rate was 1 mV s-'.
a Ag/AgCl (saturated KCI) reference electrode. Solutions were prepared daily from reagent grade K,Fe(CN), and KCI (Fisher Scientific, Inc., Chicago, IL). The fem/ferrmyanide redox couple was used as the test species for confirmation of the SEC characteristics in order to provide a consistant reference system as with previous reports (1fF12).Double distilled water was used for all preparations. RESULTS AND DISCUSSION Verification of the Properties of the Thin-Layer Configuration of the Cell. The CV current-potential (i-E) and CVA absorbance-potential (A-E) responses for 1.0 mM Kpe(CN), in 1.0 M KCI with the cell in the T L confwation are shown in Figure 2. A gold miniiid is used as the working electrode and the sweep rate is 1 mV 8-l. The shape of the i-E curve is characteristic of an exhaustive electrolysis in a T L cell which has a large uncompensated solution resistance (14). The peak separation between the cathodic and anodic peak current potentials is ca. 55 mV. The background-corrected integrated charges under the cathodic and anodic sweeps are equal (+1.2%). The return of the absorbance to its initial value at the end of the reverse sweep for the A-E response in Figure 2a also confirms retention of the electroactive species within the restricted volume (10). The wavelength for optically monitoring the change in Fe(CN)," concentration was 420 nm. Spectral responses for SSPS experiments were obtained for the determination of the formal reduction potential, E O ' , and the stoichiometry, n,for the Fe(CN),"/' redox couple. The equilibration time between each change in applied potential was ca. 60 s. Analysis of the data at 420 nm using a Nemst plot gave a value for Eo' of 0.253 V vs. Ag/AgCl (saturated KCI) and a value for n of 1.02. These values are in agreement with previously reported data (10,11, 14). The precision of these measurements was *3% for four separate trials. The reproducibility of the cell volume during repetitive removal and reinsertion of the plunger was evaluated by the quantity of charge required for the exhaustive electrolysis of 1.0 mM K,Fe(CN), in 1.0 M KCI. The average value of the charge was 1350 f 20 pC for five trials. The cell volume was
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
13.9 f 0.2 pL, a reproducibility of f1.5% for these trials. The thickness of the solution layer, calculated from absorbance measurements, was found to be 160 f 3 pm. A value of 1020 M-’ cm-’ was used for the molar absorptivity of ferricyanide (15).
Verification of Properties of the Semiinfinite Linear Diffusion Configuration of the Cell. Verification of the SILD condition for the cell was performed using chronoabsorptometry (I). In this configuration a cell with a tin oxide electrode was filled with the back window displaced ca. 1 cm from the electrode. The potential was stepped from +0.600 V to -0.125 V vs. Ag/AgCI (saturated KC1). A plot of the absorbance vs. time (A-t) responses as A vs. t’l2 was linear. From the slope of the plot the diffusion coefficient for Fe(CN):- in 1.0 M KCI was calculated to be 0.76 X lo4 cm2 with a precision of f2% for three trials. The agreement of the diffusion coefficient with previously reported values confirms SILD mass transport of the test species (16, 17). Oxygen Exclusion Capability of the Cell. The oxygen exclusion capability of the cell in a T L configuration with a Au minigrid electrode was estimated by using CV at a sweep rate of 2 mV s-’ with potential limits from 0.0 V to -0.8 V vs. Ag/AgCl (saturated). The cell was evacuated for 15 min before injection of 1.0 M KCI solution which had been degassed with a nitrogen purge for ca. 30 min. The cathodic i-E response obtained immediately after solution introduction indicated no observable oxygen (at these conditions, an air-saturated solution exhibits a broad, poorly defined i-E wave for oxygen reduction with an anodic peak current maximum at ca. -0.35 V vs. Ag/AgCl (saturated)) and was identical with a trace obtained 1h later. Due to the exhaustive electrolysis capability of the TL cell a t this sweep rate, a detection limit for the concentration of oxygen in the cell, calculated by assuming a four-electron reduction for oxygen, was ca. 5 pM. This calculation assumed a maximum value for the observable charge for oxygen electrolysis at a signal-to-noise ratio equal to 2. Thus, the maximum loss of a species present at 1 mM due to a redox reaction with oxygen with a 1-to-4stochiometry for electron transfer would be ca. 2% in a 1-h experiment. CONCLUSIONS The simplicity of fabrication, accessible spectral region, oxygen exclusion capability, and application in either a
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thin-layer or semiinfinite linear diffusion configuration make this cell an attractive alternative to some of the existing SEC designs. The application of this cell for IR spectral measurements and its compatibility with nonaqueous solvents is currently under investigation.
ACKNOWLEDGMENT Discussions with Duane E. Weisshaar and S. Bart Jones are appreciated. Registry No. K,Fe(CN)G,13746-66-2. LITERATURE CITED Kuwana, Theodore Ber. Bunsenges. fhys. Chem. 1973, 77, 656-871, Kuwana, Theodore; Winograd, Nicholas I n “Electroanalytical Chemistry”; Bard, Alien, J., Ed.; Marcel Dekker: New York, 1974 Vol. 7. Kuwana, Theodore; Heineman, William R. A m . Chem. Res. 1976, 9 , 241-248. Heineman, William R. Anal. Chem. 1977, 50, 390A-402A. Bard, Allen J.; Faulkner, Larry R. “Electrochemical Methods: Fundumentals and Applications”; Wiley: New York, 1980; Chapters 10 and 14. McCreery, Richard L. I n ”Physical Methods in Chemistry, Vol. 11”; RossRer, Bryant, Ed.; Wiley: New York, 1984. Heineman, William R.; Hawkridge, Fred M.; Blount, Henry N. I n ” Electroanalytical Chemlstry”; Bard, Alien J., Ed.; Marcel Dekker: New York, 1984; VoI. 13. DuBois, Daniel L.; Turner John A. J . Am. Chem. Soc. 1962, 704, 4989-4990. Finkiea, Harry 0.; Boggess, Robert K.; Trogdon, Johnny W.; Schuitz, Franklin A. Anal. Chem. 1983, 55, 1177-1179. Zak, Jerzy; Porter, Marc D.; Kuwana, Theodore Anal. Chem. 1963, 5 5 , 2219-2222. Porter, Marc D.; Kuwana, Theodore Anal. Chem. 1984, 56, 529-534. Strojek. Jerzy; Kuwana, Theodore J . Electroanal. Chem. 1968, 76, 471-483. Hawkridge, Fred W.; Kuwana, Theodore Anal. Chem. 1973, 45, 1021-1027. DeAngeiis, Thomas P.; Heineman William R. J . Chem. fduc. 1876, 5 3 , 594-597. Winograd, Nlcholas; Blount, Henry N.; Kuwana, Theodore J . fhys. Chem. 1968, 73. 3456-3462. Adams, Ralph, N. “Electrochemistryat Solid Electrodes”; Marcel Dekker: New York, 1969; Chapter 8. Gerhardt, Greg; Adams, Ralph N. Anal. Chem. 1982, 54, 2618-2620.
RECEIVED for review March 12,1984. Accepted May 1,1984. This work was supported by grants from the National Science Foundation (Grant No. 8110013) and The Ohio State University Materials Research Laboratory.
