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well beyond an ion of molecular weight of 400. T h e proper way to test this would be to prepare electrodes selective for an ion whose molecular weight is around 400 and to then test interferants whose molecular weights are greater and less than that of the primary ion by a t most 150. In this way, extremely large or small Kseivalues would be avoided.
CONCLUSIONS ISEs based on plasticized polymer membranes incorporating D N N S have been shown to give Kernstian electrochemical responses t o cationic species over a concentration range of a t least three orders of magnitude. Detection limits of about IO4 M are observed. T h e selectivity sequence of t h e D N N S electrode has been shown to be determined by solvent extraction principles. T h e electrode selectivity is such t h a t for electrodes responsive to large cations such as DoTA' or TBA+ essentially no interference by common inorganic cations is observed. T h a t i t is much greater than t h a t observed with electrodes based on highly cross-linked IERs, would seem to validate attributing poor selectivity to cross-linking. The study demonstrates t h a t the response characteristics of t h e CWE and conventional DoTA+ electrodes are essentially identical, with t h e CWE, however, having a slightly better dynamic range. This equivalence (and even superiority) of response of t h e C W E and t h e conventional polymer membrane electrode is important since the lower cost and easier construction of t h e CWE make it much more desirable. Because many compounds of biochemical, pharmaceutical, and clinical interest are, a t physiological p H , high molecular weight cations, DNNS-based electrodes show great promise for development of ISEs for determination of such compounds. T h e negligible interference by inorganic cations and the low detection limit would probably allow for determination of the desired compound without prior separation from its clinical matrix (Le., blood or urine). T h e selectivity characteristics of these electrodes should allow for determination of one drug in the presence of lower molecular weight (and, usually more
hydrophilic) metabolites if the difference in molecular weights is greater than about 70. As an example, a heroin (mol w t 369) electrode should be quite selective with respect to morphine (mol wt 285.4). A study of t h e response characteristics of DNNS-based electrodes to a variety of N-containing pharmaceuticals and metabolites is currently under way.
LITERATURE CITED Wyllie, M. R.; Patnode, H. W. J . Phys. Chem. 1950, 5 4 , 204. Bose, S. K. J . Indian Chem. SOC. 1960, 3 7 , 465. Basu, A. S. J . Indian Chem. SOC.1962, 39, 619. Hale, D. K.; McCauley, D. J. Trans. Faraday SOC. 1961, 57, 135. Parsons, J. S. Anal. Chem. 1958, 3 0 , 1262. Pungor, E.: Toth, K.; Havas, J . Hung. Sci. Instrum. 1965, 3 , 2. Pungor, E.; Havas, J.; Toth. K. Acta Cbim. Acad. Sci. Hung. 1964, 4 1 . 239. Pungor, E.; Havas, J. Acta Chim. Hung. 1966, 50, 77. Martin, C. R . ; Nitka, J.; Freiser, H., unpublished results, June 1979. Yeager, H. L.; Steck. A. Anal. Chem. 1979, 5 1 , 862. Danesi, P. R.; Chiarizia, R . ; Scibona, G. J . Inorg. Nucl. Chem. 1973, 3 5 , 3926. Moody, G.J.: Thomas, J. D. R . "Ion-Selective Electrodes in Analytical Chemistry", H. Freiser, Ed., Vol. I; Plenum Press: New York, 1978; pp. 288-291 Martin, C. R.; Freiser, H. Submitted to J . Chem. Educ. Martin, C. R.; Freiser, H. Anal. Chem. 1979, 51, 803. Petrucci, S. "Ionic Interactions from Dilute Solutions to Fused Salts", Voi. I ; Academic Press: New York, 1971; p 43. Srinivasan, K.; Rechnitz, G. A. Anal. Chem. 1969, 4 7 , 1203. Fujinaga, T.; Okazaki, S.; Freiser, H. Anal. Chem. 1974, 4 6 , 1842. IUPAC Comp. Anal. Nomenclature; Pergamon Press: New York, 1977; pp 168-169. James, H. J.; Carmack, G. P.; Freiser, H. Anal. Cbem. 1972, 4 4 , 853. Gustavii, K. Acta Pbarm. Suec. 1967, 4 , 233. Marinsky, J. A.; Marcus, Y. "Ion Exchange and Solvent Extraction", Voi. 6; Marcel Dekker: New York, 1974, p 5 . Scholer, R.: Simon, W. Helv. Chim. Acta 1972, 5 5 , 1801. Kina, K.; Maekawa, N.; Ishibashi, N. Bull. Chem. SOC.Jpn. 1973, 4 6 , 2772. Baum. G.; Lynn, M.; Ward, F. B. Anal. Chim. Acta 1973, 72, 385. Buck, R. P. Anal. Chim. Acta 1974, 7 3 , 321.
RECEIVED for review October 10, 1979. Accepted December 12, 1979. This work was conducted with financial assistance from the Office of Naval Research.
Rapid Pulsed Flow Voltammetry W. J. Blaedel" and Z . Yim Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
A pulsed flow technique (about 1 H z ) which measures the pulsed current at a flowthrough electrode is described and characterized with potassium ferrocyanide systems. Charging and other transient background currents are eliminated, permitting measurement of submicromolar concentration levels. Annular, tubular, and porous flowthrough electrodes may be used. For a small reticulated vitreous carbon electrode (5 mm diameter X 2 mm long), the sensitivity is very high, about 2 pA/pM. Applicability to flowthrough electrochemical detectors is indicated.
Hydrodynamic modulation a t solid electrodes has been shown to be a feasible technique for obtaining voltammograms a t micro- and submicromolar concentrations. Such modulation permits compensation for the high transient charging current of the electrical double layer and the sizable transient 0003-2700/80/0352-056490 1 OO/O
background current due to surface reactions of the solid electrode. In practice, these transients are: (a) allowed to die away, as in the stopped-flow technique of Blaedel and Boyer (1); or (b) subtracted out, by taking the difference current between two different rates of convective transport, as in the pulsed-rotation technique of Blaedel and Engstrom ( 2 ) or the pulsed-flow technique of Blaedel and Iverson (3);or (c) filtered out, as in the sinusoidal modulation of rotating disk electrodes by Miller and Bruckenstein ( 4 ) . Previously, Blaedel and Iverson found the pulsed-flow response time to be of the order of a quarter t o half a minute ( 3 ) . In this paper, we describe a rapid pulsed-flow technique with a response time of the order of a second. The technique is based upon the use of thin channel electrodes with widths below 0.5 mm. T h e small diameters and high linear solution flow rates reduce the distances through which the diffusion boundary layers have t o relax when the flow is pulsed ( 5 , Chapter 2 ) . At high pulse rates, the difference current is c 1980 American Chemical Society
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Figure 2. Pulsed flow current response for the oxidation of 3 p M K,Fe(CN), (Conditions: pH 7.4 phosphate buffer in 0.1 M KCI, 0.4 V vs. SSCE). (A) Wide channel tubular electrode (3-mm diameter, 4 m m long). (B) Thin channel tubular electrode (0.2 m m diameter, 1 m m long). H and L represent flow rates of 6 and :3 mL/min, respectively
Figure 1. Schematic diagram of the cell: (A) inlet port for flowing sample solution, (9) outlet port to waste, (C) port for filling reference electrolyte, (D) inlet port for flowing reference electrolyte, (E) retainer ring, (F) platinum tubular electrode. Annular and RVC electrodes not shown (see text). (G) O-ring, (H) hole for bridging contact to reference electrode, (J) junction for merging of solution streams, (K) Teflon washer, (L) anodized silver wire, (Mj retainer ring screws, (N) copper wire leads
attenuated, approaching zero a t high frequencies. At low pulse rates, the difference current approaches the difference between t h e two steady-state currents a t t h e two flow rates. T h e technique is demonstrated by application t o the measurement of micromolar concentrations of K,Fe(CN)6.
EXPERIMENTAL Design a n d Construction of the T h i n Channel Electrode Cell. A schematic diagram of the cell is shown in Figure 1. The cell is a modification of the flow-through cell with open liquid junction reported earlier (6). The modular design with exchangeable electrodes permits use of three types of thin channel electrodes: (a) annular type, (b) open tubular type, and (c) porous plug type with reticulated vitreous carbon (RVC)-a newly introduced electrode material ( 7 , 8). For the annular design. the channel A J (3-mm diameter) was extended by drilling to A', and a stainless steel rod (2-mm diameter) was inserted from A' until it extended a few millimeters beyond the platinum tubular electrode F (3-mm i.d., 4 mm long). The rod was guided and centered by a slightly oversized wrapping of the end with Parafilm, which was removed after the rod was permanently glued in place to the cell body with a fast-setting epoxy (Hardman, Inc., Belleville, N.J. 07109). The cell resistance between the annular and reference electrodes was measured a t 60 Hz with a conductance bridge (Model l5B1, A. H. Thomas Co., Philadelphia, Pa.), and was found to be 4 K!2. A Plexiglas rod instead of the stainless steel rod has also been used with a similar measured cell resistance. For the thin channel open tubular electrode, a platinum tubing (0.2-mm id., 1 mm long) was potted inside the Plexiglas electrode holder in the same way as described previously (6). Care was exercised to align the tubing with the flow channel via a centering hole, which was subsequently drilled through to be part of the inlet channel. The cell resistance was 4 KQ. The porous plug electrode was made from RVC (Fluorocarbon Co., Anaheim, Calif. 92803). A hole was drilled through the Plexiglas holder to accommodate snugly a thin RVC disk (5-mm diameter, 2 mm long, 100 pores per inch). Electrical contact was made by pressure to one end of a glassy carbon rod (2-mm diameter), the other end of which was connected with conducting epoxy (Epo-Tek 415G, Epoxy Technology Inc., Billerica, Mass. 01821) to a copper wire lead that extended t o the outside. The cell resistance was 3 KQ. The reference electrode was a silver-silver chloride electrode in 0.1 M KC1 (SSCE, 0.282 V vs. the normal hydrogen electrode; all potentials in this paper are referred to the SSCE). In contrast to previous practice (6j, the reference electrode solution did not flow. The channel, between the reference compartment and the junction, J, served as a buffer zone to prevent contamination of
the reference solution by the flowing sample solution. T o be on the safe side, the reference solution in -the buffer zone was flushed between experiments. Methods of Flow Modulation. A three-way solenoid valve (Allied Control, Plantsville, Conn. 06479) was used to alternate between two gravity-fed flow lines with different hydrostatic heads. The valve was located downstream from the flowcell. The on/off switching of the solenoid valve was controlled by a variable speed cam-actuated microswitch. To reduce electronic noise, the solenoid was battery operated, and was equipped with normal arc suppression circuitry. The natural pulsation of a single piston reciprocating pump (Minipump, Milton Roy Co., Riviera Beach, Fla. 33404) provided another method of flow modulation. The pump was modified with a variable speed motor. Instrumentation. For controlled potential amperometry, the potential was applied either with a Sargent XV polarograph or a house-built two-electrode potentiostat. Current was measured with a pico-ammeter (Model 414e,, Keithley Instruments, Cleveland, Ohio 44139). For linear scan voltammetry, the potentiostat and pico-ammeter were used in conjunction with a house-built active narrow bandpass filter/amplifier (tuned to the appropriate frequency). The current output was displayed with a strip-chart recorder (Omniscribe, Series 5000, single channel; Houston Instruments, Bellaire, Texas). For peak-to-peak measurements a t 2.5 Hz, a transient recorder (Biomation, Cupertino, Calif. 94014) was used to avoid attenuation caused by lag of the recorder. In a few cases, the current pulsed output was rectified and averaged. Reagents. Solutions were prepared fresh before use with reagent grade chemicals and deionized water (Culligan Cartridge Water Treatment Systems, Culligan International, Northbrook, Ill.). All solutions were prepared with 0.1 M KC1 and 0.05 M phosphate buffer a t pH 7.4 (1:4 molar ratio of KH2P0, and K2HP0,). All measurements were made inside a faraday cage.
RESULTS AND D I S C U S S I O N Figure 2 is a chart record of current responses t o pulsed flow for a wide channel tubular electrode previously used (6) a n d the thin channel electrode used in this study. T h e response time is much faster for the thin channel electrode. T h e log-log plots of current vs. volume flow rate (not shown) for all three types of thin channel electrodes are linear with slopes around 0.3, indicating t h a t the flow regimes are laminar (9). Figure 3 shows the dependence of the pulsed current upon frequency of modulation over t h e range 0.1 to 2 Hz. At low frequencies, the pulsed current amplitude corresponds t o the difference between the steady-state currents at the two flow rates (3 a n d 6 mL/min). As t h e frequency is increased, t h e decay to the low current state is cut off, and the pulsed current amplitude decreases. Figure 4 shows graphically the dependence of the pulsed current signal upon the frequency of flow modulation for t h e three different types of electrodes. Figure 5 shows t h e linearity between t h e pulsed current amplitude and concentration for different flow modulation
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Figure 3. Dependence of pulsed current amplitude upon frequency of flow modulation. Conditions: thin channel tubular electrode, as in Figure 2
I
CONCENTRATION OF Kq FeCCN)e, p M Figure 5. Dependence of pulsed current amplitude upon concentration of K,Fe(CN), for the thin channel tubular electrode. Conditions as in Figure 2 OlpA
05
IO
I5
__
FREQUENCY OF FLOW MODULATION, H z Figure 4. Dependence of pulsed current amplitude upon frequency of flow modulation. (A) Annular electrode. (B) Thin channel tubular electrode. (C) Porous plug (RVC) electrode. Conditions as in Figure
2
frequencies, with the thin channel tubular electrode. Operated in t h e steady state, the sensitivity is 0.007 pa/pM. Data for t h e annular electrode similar to t h a t in Figure 5, for the concentration ranges 0-20 pM and (tl pM, indicate a greater sensitivity under similar conditions-0.022 p A / p M . The standard deviation for the (tl pM calibration plot is 0.03 fiM, indicating t h a t concentrations well below 0.1 pM are easily detectable. For t h e porous RVC electrode, the sensitivity is very high, 2.4 p A / p M . Differences in geometry and dimensions among t h e three types of electrodes do not permit meaningful comparisons of sensitivities a t equal electrode areas. Figure 6 shows representative linear scan voltammograms (plots of pulsed current like that in Figure 3, as the potential is scanned anodically). Flow modulation is a t 0.5 Hz, with the pulsed current signal being processed by bandpass filtering. Such a high frequency permits recording of currentpotential curves within a few minutes. T h e voltammogram of Figure 6A for the thin channel platinum tubular electrode shows a sharp rise in the potential limited region, and a respectable plateau region. T h e RVC porous plug electrode is
Figure 6. Linear scan voltammogram with flow modulation at 0.5 Hz after bandpass filtering. (A) Thin channel tubular electrode. (B) Porous plug (RVC) electrode. Conditions as in Figure 2, 10 p M K,Fe(CN),
characterized by high current levels, with t h e total current being about 20-fold higher than the modulated current. The high currents cause a serious ir drop and broadening of the wave (Figure 6B), as well as a shift of the half-wave potential t o more anodic values. At lower concentrations and lower current levels, the shape of the voltammograms approaches t h a t of Figure 6A. The high sensitivity and fast response of rapid pulsed flow electrodes indicate great promise and applicability to flowthrough detectors.
LITERATURE CITED (1) W . J. Blaedel and S. L. Boyer, Anal. Chem., 43, 1538 (1971). (2) W. J. Blaedel and R. C. Engstrom. Anal. Chem., 50, 476 (1978). (3) W . J. Blaedel and D. G. Iverson, Anal. Chem.. 49, 1563 (1977). (4) B. Miller and S. Bruckenstein. Anal. Chem., 46, 2026 (1974). (5) V. G. Levich. "Physicochemical Hydrodynamics", Prentice Hall, Engle-
wood Cliffs, N.J., 1962. (6) W . J. Blaedel and 2. Yim, Anal. Chem.. 5 0 , 1722 (1978). ( 7 ) A. N. Strohl and D. J. Curran. Anal. Chem., 5 1 , 353 (1979). (8) W. J. Blaedel and J. Wang, Anal. Chem., 5 1 , 799 (1979). (9) W . J. Blaedel and L. N. Klatt, Anal. Chem., 38, 879 (1966)
RECEIVED for review May 9,1979. Accepted December 3,1979. This work has been supported in part by a grant (No. CHE76-15128) from the National Science Foundation.
