Anal. Chem. 1990, 62, 2625-2631
In all cases studied here the carbon column resolved the isomer mixtures to a greater extent and in a shorter time despite its lower efficiency than did the ODS column. The extremely retentive and selective nature of the carbon support makes it an excellent candidate for consideration when conventional chemically bonded reversed-phase supports fail. This is true when a modest number of solutes are to be separated. As Giddings has pointed out in complex multicomponent mixtures there is really no alternative to the use of columns with large numbers of plates and concomitant high peak capacity (16).
ACKNOWLEDGMENT We thank Dr. Eric Funkenbusch and Dr. Douglas A. Hanggi of 3M Co. for many helpful conversations.
2825
(42 Knox, J. H.; Kaur, B.; Miilward, G. R. J . Chromafogr. 1986, 352, 3. (5) Colin, H.; Guiochon, 0. Carbon 1978, 76,145. (6) Leboda. R. ChromatoafaMia 1980. 73. 703. i 7 j Colin, H.; Eon, C.; Guhchon, G. J.‘C&matogr. 1976, 779, 41. (8) Smolkova, E.; Zima, J.; Dousek, F. P.; Jansta, J.; Plzak, 2. J. Chro-
mafogr. 1980, 197,61. (9) Gierak, A.; Leboda, R. J. Chromatogr. 1989, 483, 197. (IO) Weber, T. P.; Carr, P. W.; Funkenbusch. E. F. J. Chromafogr. 1990, 579, 31. (11) Rigney, M. P.; Weber, T. P.; Carr, P. W. J . Chromafogr. 1989, 484, 273. (12) Snyder, L. R.; Kirkland, K. K. An Inhoducfion to Modern LiquM Chromatography, 2nd ed.;Wiley-Interscience: New York, 1979. (13) Rekker, R. F.; devries, G.; Bijloo, G. J. J. Chromafogr. 1988, 370, 355. (14) Rekker, R. F.; deVries, G.; Bijloo, G. J. J. Liq. Chromatogr. 1989, 72, 695. (15) Sadek, P. C.; Carr, P. W.; Doherty, R. M.; Kamlet, M. J.; Taft, R. W.; Abraham, M. H. Anal. Chem. 1985, 5 7 , 2971. (16) Giddings, J. C. I n Gas Chromatography-7964;A. GoMup, A,, Ed.; The Institute of Petroleum: London, 1965.
LITERATURE CITED (1) Knox, J. H.; Unger, K. K.; Mueller, H. J. Liq. Chromatogr. 1983, 6 , 1. (2) Unger. K. K. Anal. Chem. 1983, 59, 361A. (3) Knox, J. H.; Kaur, B. I n H/gh Performance LiquM Chromatography: Brown, P. R., Hartwick, R. A,, Eds.; John Wiley and Sons: New York, 1989; Chapter 4.
RECEIVEDfor review August 9, 1990. Accepted August 24, 1990. This work was supported in part by a grant from the Institute for Advanced Studies in Biological Process Technology at the University of Minnesota and a grant from 3M.
Dynamic and Steady-State Response of Electrochemical Detectors Based on Arrays of Small Electrodes L. Joseph Magee, Jr.’ and Janet Osteryoung* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214
Conventional circular glassy carbon electrodes are compared whh reticulated vitreous carbon based and linear mlcroeiectrode arrays In a thin-layer flow cell. Noise analysis under conditions of hydrodynamlc modulation shows that the large clrcular electrode Is less susceptible to noise due to flow fhstuations. The conventional electrode has poorer sensitivity In steady-state experiments because the reactant Is depleted at the traHhg edge, but the advantage In sensitivity displayed by the microelectrode arrays Is greatly diminished under dynamlc conditions where dispersion influences the concentration profile. Nonplanar diffusion does not influence the response at the microelectrode arrays used in this study.
Microelectrodes are electrodes small enough that they cannot be seen clearly with unaided normal vision. In static solutions, microelectrodes display a number of desirable properties: high current densities from nonplanar diffusional contributions to the net Faradaic current (1-1 I);steady-state currents on short time scales (12,13);low ohmic potential drop (iR drop) (14), which allows the use of more resistive solutions (14-21); very high potential scan rates (>lo00 V/s) (14,19, 22). When employed in a flow cell type of electrochemical detector, microelectrodes have the potential to display a number of advantages over larger electrodes. The first advantage is the possibility of decreasing the size of the electrochemical flow cell, thereby reducing the inherent dead volume. This is particularly important with the increased *Present address: Sterling Drug, Inc., 81 Columbia T u r n p i k e , Rensselaer, NY 12144. 0003-2700/90/0362-2625$02.50/0
popularity of microbore and capillary LC columns. A second advantage is a decrease in iR drop, which is especially beneficial in flow cell detectors where electrode placement is often mechanically limited and cell resistance is a problem. Also, lower currents may permit the use of more resistive mobile phases for liquid chromatography. Finally, we consider fluid flow in a flow cell detector. By using small electrodes, it should be possible to maintain the diffusion layer in the nonmoving layer of fluid next to the wall in which the electrode is embedded (23-25). This would give rise to higher current densities which might provide higher signal-to-noise ratios and yield currents independent of flow rate. This would make the detection scheme insensitive to fluctuations in the flow rate, or “pump noise”, to which electrochemical detectors are quite sensitive. To achieve small electrode size without introducing problems in the measurement of small currents, electrode arrays have been developed. These arrays consist of many individual, small electrodes separated by insulating regions but electrically connected. Examples include arrays based on powders of electrode material (26,27),reticulated vitreous carbon (RVC) (28,29),carbon fibers (25),and linear arrays based on gold (30-34). The general subject of response of microelectrode arrays in flowing streams has been addressed both theoretically and experimentally (e.g., ref 27). Generally microelectrode arrays have been found to yield improved signal-to-noise ratios in comparison with conventional 3-mm-diameter disk electrodes in channel flow cells. However in studies of this type typically steady-state experiments are employed to characterize a detector and the same detector is applied to a specific chromatographic problem, which provides no flexibility for investigating the properties of the detector. Thus the connection 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62,NO. 23,DECEMBER 1, 1990
between properties under steady-state conditions and those obtaining under dynamic conditions where dispersion influences the response is not well-defined. In the present work we examine the behavior of two types of microelectrode arrays in a channel flow cell under both steady-state and dynamic conditions. The main points examined are the extent to which the favorable characteristics of microelectrodes suggested above are attained with conventional conditions and the extent to which conclusions derived from steady-state measurements can be generalized to the more common dynamic conditions of chromatography.
EXPERIMENTAL SECTION Electrode arrays for use in the channel flow cell detector were fabricated from RVC, type 2 X 1, 100 (28), and from platinum foil and using a variety of insulating materials. One RVC array was fabricated with Buehler epoxide and resin (Buehler, Evanston, IL), catalog numbers 200-8130-032 and 200-8132-032,respectively, used per manufacturer’s instructions. A length of RVC with cross section 3 mm X 4.5 mm was potted in epoxy, and a block of the RVC-epoxy composite was cut with dimensions 1 in. X 1 in. X in., with the RVC in the center of the 1 in. x 1 in. faces. Electrical contact was made to the RVC on one face by using Ecobond Solder 57C (Emerson and Cuming, Canton, MA) and a pin type socket connector, and the contact was sealed in epoxy. The other face was polished with a Buehler Minimet polisher, starting with 45-pm diamond and working progressively down to 0.25-pm diamond. Final polishing was done with 0.05-pm alumina. This polished face contained the RVC working electrode and acted as a channel wall of the flow cell. The section area was 0.135 cm2, carbon area fraction 0.056, carbon surface area 0.0076 cm2,and carbon boundary length 4.35 cm. Because of the nature of the above epoxy, this electrode was limited to use in the hydrodynamic studies which employed only aqueous solutions. For high-performanceliquid chromatography (HPLC) use, the epoxy chosen was manufactured by Emerson and Cuming and consisted of Stycast 2850 FT resin and Catalyst 11. This epoxy contains a high percentage of inorganic filler (proprietary composition) which made it much more compatible with the organic solvents used. Construction and preparation of the electrode were similar to the above.procedure. These electrodes are referred to as A and B, respectively, in the following sections. Photographs of a typical electrode surface are presented in ref 28. Each electrode contains about lo00 individual elements, and the resulting diffusional properties have been characterized statistically in static solution (28). Two different platinum line electrodes were constructed from 0.1 mm thick platinum foil, one piece approximately 0.5 cm X 1 cm, the other approximately 0.5 cm X 0.4 cm. Each piece was sandwiched between two 1/4 in. thick pieces of Plexiglas plate and placed in a vice on a milling machine. The desired lengths of foil in. end mill, were then machined about 2 mm deep using a with the Plexiglas plates used to keep the foils flat. The first electrode had a surface length of 2 mm. The second one had three separate lengths, each 2 / 3 mm separated by 2 mm gaps. A platinum wire, 0.5 mm diameter, was then attached to the platinum foil with silver epoxy (Johnson Matthey Chemicals, silver epoxy A500 R resin and A500 H hardener). These were then potted in Maraglas epoxy (Acme Chemical and Insulation Co., New Haven, CT, no. 658 resin and 558 hardener). Final electrode assemblies were prepared as described above for the RVC arrays. The geometry of the electrodes in a thin-layer flow cell is shown in Figure 1. Equipment and Reagents. For hydrodynamic studies three different pumps were used depending on the type of data required. Flow rate dependence and band spreading experiments were performed with an LKB Model 2150 pump, because it was the quietest and most accurate of the three available. Noise analysis in HPLC was done with an IBM (manufactured by LDC/Milton Roy) dual piston pump, the performance of which is typical of an average HPLC pump. Hydrodynamic modulation studies were done with an Altex Model llOA single piston pump without a pulse dampener. This gave an easily isolated and reproducible fluctuation in flow with each piston cycle. Hydrodynamic modulation (or pump noise) was measured with a Kipp & Zonen strip
e -
4
FLOW
-
_.*
N
FLOW
-
1
1
w
1
FLOW
_+
I
d
FLOW ~~
~
Figure 1. Arrangement of electrodes in the flow cell: (1) disk, (2) parallel line, (3)segmented parallel line, (4) perpendicular line (cf. Table
I).
chart recorder and a Hewlett-Packard Model 3582A spectrum analyzer with a Hewlett-Packard Model 7034A x/y recorder. The electrochemical detector was a BAS Model LC-4A amperometric detector with a Model LC-17 flow cell consisting of the working electrode assembly, a Ag/AgC1(3 M NaC1) reference electrode, and a stainless steel auxiliary electrode. The working electrodes used were the 3 mm (A = 0.071 cm2)glassy carbon disk supplied with the detector, RVC array A, and the platinum line electrodes described earlier. The Teflon spacer provided a channel thickness of 127 pm. Aqueous mobile phases for flow injection analysis, steady state, and hydrodynamic modulation studies were either 0.05 M H2S04 or 0.1 M KC1. The analyte used was potassium ferrocyanide, K4Fe(CN),.3H20 (Baker reagent grade). The HPLC system consisted of the LKB Model 2150 dual piston pump, a Rheodyne Model 7125 injector with a %pL sample loop, a 15-cm Hamilton Resin reverse-phase HPLC column, the BAS electrochemical detector described above, and the Kipp & Zonen Model BD 40 strip chart recorder. The two working electrodes used were the BAS disk electrode and RVC array B. The mobile phase was a 5050 mixture of acetonitrile (Baker HPLC grade) and 0.05 M H2S04(Fisher reagent grade sulfuric acid in Millipore grade water) filtered through a 0.45-pm filter prior to use. Standards were phenol (Fisher purified grade) and p-chlorophenol (Aldrich 99+%) dissolved in mobile phase. An applied potential of +1.25 V vs Ag/AgCl (on the limiting current plateau for both) was used for detection of these compounds.
RESULTS AND DISCUSSION A steady-state experiment, in which a constant concentration of analyte in a mobile phase is pumped through the flow cell under laminar conditions, provides the most simple convective diffusion regime. Steady-state experiments were used initially to characterize the electrode arrays. Under conditions typical of our experiments the maximum average linear velocities are about 9 cm/s, which yield Reynolds numbers of about 11, well below the limiting value of 2000 for laminar flow in a channel (35).The analyte was Fe(CN),4-. The detector potential was set a t +0.60 V (on the limiting current plateau for oxidation of Fe(CN)64-) and the output filter on the detector was set to 5 s to eliminate current fluctuations arising from pump noise. Background currents were constant and independent of flow rate for both RVC and disk electrodes. The flow rate dependence for the disk and RVC array B over a range of 0.1-3.0 mL/min is shown in Figure 2. A log current vs log flow rate plot for each electrode is linear over the range studied with a slope of 0.36 for the
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
2627
I
z’el
-
a
5
c Z W
(I:
3 U
0 J
L O G FLOW R R T E
P O T E N T I R L V 5 RG/RGCL
ICC/SECl
Figure 2. Flow rate dependence for the disk electrode (1) and the RVC array electrode (2). 11 pM K,Fe(CN),.3H20 in 0.05 M H2S0,.
disk and 0.32 for the RVC array. These slopes agree well with the power dependence on flow rate established for the channel flow cell (36,37). The current densities for the RVC array average about 4 times those for the disk, with a value of 4.39 at the lowest flow rate and 3.88 at the highest. Similar results are obtained for RVC array A. Over the range 0.2-2 mL/min, the slope of the log current vs log flow rate plot is the same, but the current density is only about 3 times that for the disk. This difference is not due to the RVC material itself, which is the same. (Parenthetically, the same experiment carried out with the Altex pump, used without calibration, yielded for the exponent of the flow rate dependence the value 0.25, a value reported previously for this type of detector (29). We attribute this low exponent to inaccurate flow rates.) It has been observed that microelectrodes are less susceptible to pump noise than are conventional electrodes (25,27, 29,38). This has been attributed to lateral diffusion. In the present case the steady-state experiments show the same flow rate dependence over several orders of magnitude change in flow rate. Furthermore, previous results show that lateral diffusion is unimportant for our conditions (25,27). One might anticipate, then, that these microelectrode arrays would be just as sensitive to pump noise as a large disk. It is also not clear if conclusions about the importance of diffusion in the steady state can be applied to typical chromatographic conditions. With these points in mind we undertook experiments under conditions of pulsed flow. Pulsed Flow. Under laminar conditions, the net linear velocity resulting from low frequency pulsations superposed on a continuous flow can be described by the equation u / u = u,/u U’/U (1)
+
where u, represents the steady laminar flow, u’represents the frequency-dependent pulsed flow, and U is the average flow (39). The equations describing this pulsed flow are quite complex (39-41), but for our purposes the effect can be described easily. At low frequencies (below 10 Hz) the pulsations change the relative velocity uniformly; that is, the pulsations cause the same relative change in velocity over the entire steady-state parabolic flow profile. Only at higher frequencies is the original parabolic velocity profile distorted, with the major portion of the pulsation concentrated closer to the walls of the tube or channel and moving increasingly closer with increasing frequency. An HPLC pump generates a continuous flow with periodic rather than continuous pulsations. The signals observed with a spectrum analyzer correspond to the frequency of the piston refill action and are lower in frequency than if this action were carried out in a symmetric sinusoidal fashion. However, the net effects of the continuous model apply to the periodic situation as well.
IV1
Figure 3. Hydrodynamic voltammograms from the disk (1) and RVC array (2) electrodes: (0)direct amperometry, (0)difference current at 0.12 Hz; flow rate, 1.O mumin, Altex pump; 10 pm Fe(CNhC In 0.1 M KCI.
Since the relative change in linear velocity across the channel from pump pulsations is uniform, for electrode response to be unaffected by pump pulsations, it would be necessary for the electrochemical diffusion layer to be contained in the stagnant layer of fluid next to the electrode surface. Under these conditions, the electrode should behave much as it would in a conventional static solution. The phenomenon of pump noise or periodic fluctuations in flow velocity is in fact a type of hydrodynamic modulation, which is an experimentally useful technique in itself (42). For our system, the signal monitored is the difference between the maximum and minimum currents detected at the electrode at the primary frequency of the pump. The signal appears as a peak in the output of the spectrum analyzer, the magnitude of which is proportional to the magnitude of this difference. The applicability of hydrodynamic modulation in an electrochemical flow cell detector as an analytical technique has been demonstrated (43,44). It will be used here to characterize the RVC array and disk under conditions of hydrodynamic modulation. These experiments were done with array A and the Altex l l O A pump. Hydrodynamic voltammograms for the oxidation of Fe(CN)64-a t the disk and the RVC array are shown in Figure 3. The currents measured at 0.12 Hz, the primary frequency of the pump at 1.0 mL/min, were only about 10% of the currents measured by the total pen deflection on the strip chart recorder. The pen deflection reflects not only the magnitude at the primary modulation frequency but also the magnitude at the harmonic frequencies. In addition to hydrodynamic voltammograms, calibration curves using the two measurement techniques were obtained over the range 2-10 pM. All were linear ( R 2 0.99). The sensitivities expressed in nA/(pM cm2) were 454 and 1224 by direct measurement and 7.37 and 32.5 by differential measurement at 0.12 Hz for the disk and RVC array, respectively. Thus the RVC array (A) is 2.7 times more sensitive than the disk in the steady-state mode at 1.0 mL/min, which is consistent with the steady-state results described above. However the RVC electrode is 4.4 times more sensitive when hydrodynamically modulated. Two points are evident from these experiments. First, both steady-state and hydrodynamically modulated techniques can be used to generate analytical data (i.e. hydrodynamic voltammograms and calibration curves), which was expected. Second, the relatively large currents obtained at the RVC with hydrodynamic modulation at 0.12 Hz show that it is more susceptible to flow fluctuations, or pump noise, than the large disk. The RVC array electrode used in these experiments, when characterized statistically in static solution, behaves very nearly as a collection of disks of radius 34 pm (28). One would
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
Table I. Steady-State Response to Electrodes of Various Geometries" segmented perpen-
parallel disk RVC linec area, mm* boundary density,
7.1 0.76 1.33 57
0.2 21
parallel lined
dicular line'
0.2 23
0.2 21
mm-'
current density sensitivity,
4.54 11.76b
4.50
5.50
11.0
nA/(pM mm2)
* 5 pM Fe(CN)$- in 0.1 M KC1, 1.0 mL/min. *Data of Figure 2. cFieure 1-2. dFigure 1-3. eFigure 1-4.
expect that diffusional effects characteristic of small size (nonplanar diffusion) would only manifest themselves through increased current density and flow independence for channel thicknesses above ca. 3.4 mm (4% whereas the present channel is only 127 pm thick. The normal flow dependence power) at steady state and greater susceptibility to hydrodynamic modulation demonstrate clearly that the enhanced current density with respect to the conventional disk is due to factors other than nonplanar diffusion. To explore this point further, and for easier characterization of electrode behavior under flowing conditions, simple linear electrodes with a critical dimension similar to that of the RVC (line width = 100 pm vs equivalent disk diameter = 68 pm) were used. The platinum line electrodes described in the Experimental Section (Figure 1) yielded results for steadystate experiments presented in Table I. The line electrode, when oriented with the direction of flow, displays current densities nearly identical with those of the disk. Even though the line has a much higher boundary density than the disk, it displays no current enhancement from nonplanar diffusion. The broken line electrode does show an enhanced current density, although the active electrode surface area and boundary density remain constant (approximately). The single line electrode, when oriented perpendicular to the direction of flow, displays a current density nearly equal to that observed with the RVC array. Transport in the flow channel is primarily by convection. Because the flow is laminar, there is no convective mixing. Reaction a t the electrode surface produces a concentration gradient that is steepest at the leading edge. The depletion region extends further away from the electrode at points further downstream. A simple diffusion layer model predicts that the depletion layer at the trailing edge is 22 pm thick for the disk and 6 pm thick for an element of the array (at 1 mL/min) (37). The corresponding mean values from the data of Table I are 14 and 5.6 pm, respectively. There is no way by which the depleted layer can be regenerated at the disk. At the array, the molecular transit time across the mean diffusion layer is 23 ms. However the mean spacing is 263 pm. At the mean distance from the wall, 2.8 pm, the linear flow rate is 0.377 cm/s, and the residence time in the inactive space is ca. 70 ms. This example, estimated for a flow rate in the channel of 1mL/min, indicates that reactant concentration can be replenished by radial diffusion for this array. Dividing an electrode into shorter lengths separated by insulating regions has two effects. First, the insulating regions allow material to diffuse to the depleted region without being lost through reaction at the surface. This allows the concentrations by the wall in which the electrode is embedded to be replenished from the bulk. Second, shorter electrodes (in the direction of flow) behave more efficiently because electrode efficiency decreases with increasing length, leading to a decrease in current density, or response per unit area. The design of the above experiment makes it possible to
distinguish unambiguously between effects of depletion (45-49) and nonplanar diffusion under steady-state hydrodynamic conditions. Chromatographic and Flow Injection Conditions. In the steady-state experiments there is a uniform concentration of analyte in the flow system. The only concentration gradients are those near the electrodes due to electrochemical depletion of the analyte. Under non-steady-state conditions, those typically encountered in chromatography and flow injection analysis, a plug of material is introduced into the flow system upstream of the detector and is carried to the detector by the flow of mobile phase. The radial velocity gradient in the flow system creates dispersion, and the resulting concentration gradients give rise to both radial and axial diffusion. This complex problem has been described for flow in open tubes (50, 51). In that case at shorter times dispersion is mainly by Poiseuille flow (51);a t longer times the most important diffusion is that of material in slowly moving laminae near the walls into more rapidly moving laminae nearer the center of the tube. The question of amperometric response is more complicated still (52-54), and much less investigated, because the signal depends on the radial as well as the axial distribution of material. In particular, Meschi and Johnson (54) have shown that for tubular electrodes under conditions such that depletion is unimportant, amperometric response depends on d 6where , u is the volume flow rate, at high dispersion. A second complication arising in amperometric detection is the lack of conformity of most systems to a simple geometry. In the present case, the simple model of laminar flow through a straight tube does not apply, because the real system incorporates bent tubing, unions, a channel of variable width, and an inlet perpendicular to the channel. Thus theoretical results for a straight tube only can serve as a qualitative guide to interpretation of results. The next set of experiments addresses the influence of band spreading on the perceived efficiency of electrode response. In order to simplify the interpretation of results, flow injection was used and band size was controlled by injection volume. This experiment mimics the differing band sizes obtained in chromatographic separations. The experimental setup consisted of the LKB pump, the Rheodyne injector, and the BAS detector. A mobile phase of 0.1 M KC1 was pumped through the system at a series of flow rates from 0.25 to 2.0 mL/min. Samples of 5 pM Fe(CN):- were injected into the flow system using six sample loops (5-200 pL) at each flow rate and detected downstream. The injector/detector connection consisted of a 5 cm piece of stainless steel tubing 0.010 in. i.d., an adapter (Rainin catalog no. 200-42) and a 5 cm piece of Teflon tubing 0.031 in. i.d. This was identical with the arrangement used for the column/detector connection used for HPLC. Dispersion of the bolus of sample can be described by the simple expression (55) -In (1 - C,/Co) = kV, where C, is the maximum concentration in the effective detector volume, C" is the initial concentration, and V , is the volume of sample. In the present case the signal depends not on the concentration of analyte in the detector but rather on the flux of analyte a t the electrode surface, and thus the amperometric behavior is more complicated than the response for a detector of concentration (e.g., spectrophotometry). However eq 2 serves as a useful reference point from which to examine the results. Figures 4 and 5 display the response for the disk and array, respectively, a t various flow rates and injection volumes according to eq 2. The concentration ratio, C,/Co, is replaced by the current ratio, ip/iss, where i, is the peak current. At
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
2629
Table 11. Detector Response under Dynamic Conditions"
-a In (1ip/iM)/aV,: /LL-1 disk array
flow rate, cm3/min 0.25
0
0
0
0
0
0
0
0
V,,pL
Figure 4. Dependence of peak response on injection volume at various flow rates (mL/min) as indicated: disk electrode, 5 p M Fe(CN),+ in 0.1 M KCI.
0.75 1.00 1.25 1.50 1.75 2.00
array
3 3 3 4 4 4 5 5
4 5 5 5 5
0.066 0.060
0.112 0.083 0.059
0.50
nb disk
0.043 0.040 0.041 0.035 0.038 0.031
0.060 0.049 0.038 0.037 0.037
6 6 6
Data of Figures 4 and 5. Number of points used beginning with (0,O). 'Equals k, cf. eq 2.
200 n l
Figure 5. Dependence of peak response on injection volume at various flow rates (mL/min) as indicated: RVC array electrode A; other conditions as Figure 4.
each flow rate ,i is taken to be the response at V, = 200 pL, for which the current-time profile displays a plateau. For points with -In (1 - ip/i,) > 4 (ip/iB > 0.981, the,uncertainty is too large to yield a meaningful result, and these points are omitted. For the array, and for the disk with significant dispersion, the model of eq 2 is well obeyed. This suggests that for both the disk and the array the decrease in normalized response with increasing flow rate and decreasing injection volume is due to dispersion. The slopes of the response function are given in Table 11. From these, and by inspection of Figures 3 and 4, it is seen that the effect of dispersion is more pronounced for the array and that the difference is larger, the larger the dispersion. For example, the normalized response (ip/iss) a t 0.25 mL/min, 5-pL injection, is 0.433 for the disk and 0.281 for the array, whereas a t 1.25 mL/min and 20-pL injection the corresponding values are 0.620 and 0.573. The dispersion model also predicts that the slope ( k of eq 2) depends on flow rate as k = aUP (55). For the data of Figures 3 and 4, using the slopes of Table 11, this relation is obeyed with p = -0.57 for the disk and p = -0.35 for the array. Thus decreasing flow rate decreases dispersion less at the array than a t the disk. For these experimental conditions, the steady-state current density of the array is about 3 times that of the disk. However, with large dispersion this ratio drops to about 2. The signal a t the array is more sensitive to dispersion because the small size of the individual elements confines the diffusion layer closer to the wall. Results from straight tubes (51,56) suggest that these experimental conditions correspond to the intermediate range of dispersion, in which material near the wall at the trailing edge diffuses toward the center. The phenomenon of depletion also affects the results of these experiments. This is evident from Figure 3, in which points for the disk a t lower dispersion deviate sharply from the straight lines predicted by eq 2. These deviations all occur
1
$
4 nine
4 mine
Figure 6. Comparison of the chromatographicpeak responses at two
different electrodes: peak I corresponds to 1 ppm phenol, peak I1 corresponds to 1 ppm p-chlorophenol; E = +1.25 V vs Ag/AgCI; U = 1.50 mL/min. for ip/iSs> 0.8. This behavior also suggests that depletion is unimportant in those regimes of flow rate and sample volume for which eq 2 is obeyed. Note that there is no evidence of depletion for the array (Figure 4). Two general conclusions can be drawn from these experiments. The first is that a microelectrode may give up a significant fraction of its steady-state advantage with respect to a large electrode if it operates in a regime where dispersion is important. The second is that depletion depends on dispersion and, therefore, can be a confounding factor in interpretation of results. Chromatography. The importance of the sensitivity of the array to dispersion was assessed by means of the chromatographic response of mixtures of phenol and p-chlorophenol under conventional HPLC conditions. Figure 6 compares typical chromatograms for the detectors, from which one sees that the sensitivity ( & / A )of the array is about twice that of the disk. Calibration curves were obtained in the following manner: three injections of the 1 ppm standard mixture were made to ensure the system was functioning properly. Then there injections of the next lowest concentration standard (500 ppb) were made, and successively the other standards were run from high to low. After the lowest concentration standard was run, the 1ppm standard was run again and compared to the original response to check the effects of electrode fouling. The disk response had decreased about 3.5% while the array response had decreased about 7%. Calibration curves for the two phenolic compounds for the two different electrodes are shown in Figure 7. The RVC array gave slightly lower detection limits (phenol, 2 ppb; p-chlorophenol, 5 ppb) than the disk (phenol, 10 ppb; p chlorophenol, 10 ppb). The disk was limited a t the high end
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ANALYTICAL CHEMISTRY, VOL. 62,NO. 23,DECEMBER
I, 1990
-60
a
c
5 t-
I
,-
w
I L
w
-bo
0 -80
J
dl"
-2
J
7
30
-104
-110
0
0.5
1.0
nrPUlYCI
1.5
1.0
2.5
I "ERT2 1
F W 8. Comparison of background noise at dkk and RVC electrode
for pulsed flow: a, b, no flow: c, d, 1 mL/min; a, c, disk electrode; b, d, RVC array electrode; frequency domain, spectrum analyzer response (average of 4); time domain, strip chart recorder tracing under the same conditions; applied potential, 1.25 V vs Ag/AgCI; mobile phase composition is 1:l MeCN/0.05 M H,SO,. Detector sensitivity was 5X for b and d: dBV = 20 log V, where Vis the voltage output of the detector. Time constant for output of potentiostat was 0.5 s.
+
well-known, and there are many compounds (e.g. the phenols examined here) that require operating in these ranges. The lower detection limits for the RVC array are due mainly to the increased electrode response. The HPLC pump used (LKB Model 2150) was reasonably pulse free and pump noise was not a problem with either electrode. Background noise was that typical for any transducer and was a function of electrode surface area (57). On the basis of the increased response/unit surface area of about 2 times, the improvement in detection limits is about what one would predict.
CONCLUSIONS This work deals with several aspects of electrochemical detection in flowing streams. The most important result is the distinction between steady-state and dynamic conditions. Previous investigations into the behavior of electrode arrays for electrochemical flow cells have dealt exclusively with one of these cases. Because of the differences between them, and the complexity of the dynamic case, it is not possible to predict the behavior in either one of these cases, given the behavior in the other. This leads to misconceptions and incorrect generalizations. The most notable distinction is that of electrode efficiency under the two sets of conditions. For example, RVC array A displayed an increase in current density of about 3 times over the single disk electrode under steady-state conditions over a range of flow rates. This increase was considerably less under dynamic conditions and depended on flow rate as well as injection volume. Similarly, RVC array B displayed an increase in current density over the disk electrode of about 4 times under steady-state conditions, but under the dynamic conditions of HPLC, an increase of only about 2 times was realized. The arrays investigated in this work displayed enhanced current, not from nonplanar diffusion under conditions of flow but rather from increased efficiency from the regeneration of analyte in the insulating regions between active elements of the electrode array. Without a thorough investigation, this enhanced current could be mistaken as the result of nonplanar diffusion. This misconception could be reinforced if one also considered the noise analysis performed with the HPLC. Analysis of the background noise shows the disk electrode to be more susceptible to pump pulsations than the array electrode, which is predicted for microelectrode behavior in a flowing stream. However, this result is associated with
ANALYTICAL CHEMISTRY, VOL. 62, NO.
electrolytic decomposition of the solvent. More thorough investigation into response under conditions Of hydrodynamic modulation indicates that electrochemical respOnse at an array of s m d electrodes is, in fact, more sensitive to pump pulsations than is the response at a sing1e large electrode. These observations should be useful those with design - of improved electrochemical detectors.
ACKNOWLEDGMENT The authors thank Neal Sleszynski for applying the RVC array electrodes. This work is based in part on the doctoral dissertation of L. Joseph Magee, Jr.
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RECEIVED for review November 22,1989. Revised manuscript received August 23,1990. Accepted August 28,1990. This work was supported in part by the National Science Foundation under Grant CHE8521200.
