072
Anal. Chem. $904, 56,672-678
Electrochemical Detector Based on a Reticulated Vitreous Carbon Working Electrode for Liquid Chromatography and Flow Injection Analysis D. J. Curran* and T. P. Tougas' Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
A new cell design for detectlon in flowing streams incorporating a reticulated vitreous carbon (RVC) worklng electrode has been Investigated. The prlnclpal feature Is geometrlcal symmetry in the radlai direction between the working eiectrode and counterelectrode. This arrangement produced weildeflned hydrodynamlc voltammograms. The reticulated structure of the worklng electrode provides a large surfacearea-to-volume ratlo whlch allows rapld coulometric conversion without slgnlflcant band broadening when the detector is used for flow injection analysis (FIA) or hlgh-performance iiquld chromatography (HPLC). The sensltivity and detection ilmlts of several test compounds were lnvestlgated In thls cell. The sensltlvlty was found to approach the theoretlcal value of 96485 CIequlv at flow rates less than 1.0 mLImln. Detection limits were close to a picoequivalent ( S I N = 2) for the compounds studied by FIA. The residual current decays In less than 30 min to a steady-state value of approximately 1 PA. Detection at the plcoequlvalent level Is possible after thls equlilbration period. Both catecholamines and amoxlcliiln were used to characterlze thls detector for use with HPLC systems.
At the present time, the most widely used controlled-potential electrolysis cells for flow streams are operated amperometrically, where the fraction of electroactive species electrolyzed is typically less than 0.1. In the case of coulometric operation, the fraction is one. The methodology of applying the perturbation signal and measuring the current response is identical for coulometric and amperometric detectors. In principle, the same cell could be operated in either mode depending on the flow rate. However, in practice, the design of coulometric cells presents some special problems, and they have not achieved the same degree of popularity as the amperometric detectors. It is generally recognized that large surface area electrodes with relatively large cell volumes are required for coulometric conversion and it is generally perceived that they are not especially well-suited for applications in areas such as liquid chromatography and flow injection analysis (FIA). Two factors, perhaps, have contributed to this secondary position for coulometric detectors. The first is the lack of widespread understanding of the nature of porous electrodes and the second is a failure to recognize that the coylometric detector is a mass flow rate sensitive device. Nevertheless, porous electrodes have been examined in excellent reviews by Newman and Tiedemann (1) and by Sioda and Keating (2). Early work on coulometric detectors was reviewed by Johnson and Larochelle (3) and some recent developments regarding both amperometric and coulametric detectors have been described by Stulik and Pacakova (4). Brunt ( 5 ) and Rucki (6) have presented general reviews of Present address: Department of Chemistry, University of Lowell, One University Ave., Lowell, MA 01854.
electrochemical detectors. Weber and Purdy (7) have discussed detector design and mass transfer coefficients. The objectives of this study were to design and characterize a coulometric detector based on a reticulated vitreous carbon (RVC) electrode and to demonstrate its use in conjunction with high-performance liquid chromatography (HPLC) and flow injection analysis (FIA).Earlier work in these laboratories produced the first reported use of RVC as an electrode for use in flow streams (B), but the electrode was massive in size and not suitable as an HPLC detector. This and subsequent work with RVC in our laboratory and others have been reviewed by Wang (9).The present work represents the first report of a coulometric RVC-based detector for HPLC. A new cell design was developed which features geometrical symmetry in the radial direction between the working electrode and counterelectrode. The conversion efficiency, selectivity, linear dynamic range, detection limits, and band broadening characteristics of this cell have been determined.
EXPERIMENTAL SECTION Equipment. An EG&G Princeton Applied Research Model PARC 173 equipped with a digital coulometer (PARC 179) was used as the potentiostat throughout this work. For the linear sweep experiments experiments a PARC 175 universal programmer was connected to the potentiostat. A current offset system was constructed by connecting a voltage reference source (Health Model EU8OA) equipped with a voltage to current converter between the working electrode contact and ground. The output of the potentiostat was normally monitored on a strip chart recorder, but for experiments to measure band broadening due to the electrochemical cell, the output of the potentiostat was monitored on a Textronix storage oscilloscope (Model 564B) as well. The UV detector used to measure band broadening of FIA peaks was from a Varian 4100 HPLC system and had a cell volume of 8 pL. The IBM UV detector (LC/9522) with a cell volume of 10 pL was used for measuring band broadening in the HPLC systems. Both detectors have a fixed wavelength of 254 nm. An IBM Model LC/9533 HPLC system was used for all the chromatographic work. The electrochemical cell was connected either directly to the column effluent or downstream from the UV detector depending on the experiment. FIA System. A flow system for the FIA work was constructed with glass and Teflon tubing (Figure 1). A glass reservoir was elevated 1.5 m above the cell to provide the driving force for solvent delivery. This was done in an effort to eliminate the flow pulses present in most types of commercial pumps. The elevated reservoir provided sufficient pressure to allow flow rates up to 4 mL/min. The flow was controlled by a stopcock and measured with a Gilmont flowmeter. The flowmeter was calibrated by using the carrier stream over the entire flow rate range, and a calibration curve was constructed. Samples were introduced into the system with a Rheodyne injection valve (Model 50-20). The loop volume was calibrated by a gravimetric method. Twenty injections of a 1.217 N solution of sodium chloride were collected and precipitated with 0.1 N silver nitrate. This procedure was repeated in triplicate and the calculated loop volume was 62.8 f 0.06 pL. The entire system was placed in a Faraday cage which reduced the noise level about an order of magnitude. The cage was constructed from a steel shelving unit and was connected to a
0 1984 American Chemlcal Soclety 0003-2700/84/0356-0672$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
673
J
A
iiA
7.1 cm
Figure 1. Block diagram of flow system and electronics for coulometric electrochemicalFIA: A, solvent reservoir; B, stopcock; C, injection valve; D, flowmeter; E, RVC cell; F, voltage reference source: G, voltage to current converter; H, potentlostat; I, working electrode lead; J, Faraday cage; K, waste.
water pipe by a short length of grounding strap. The front of the cage was covered with aluminum screen. Hydrodynamic voltammograms were obtained by use of an HPLC pump (LDC Constametric 111) instead of the gravity system. Electrode and Cell. The working electrode was constructed from 2 X 5-100sRVC obtained from Normar Industries, Anaheim, CA. (Normar Industries is no longer supplying RVC and it can be obtained from Energy Research and Generation, Inc., Lowell and 57th St., Oakland, CA 94608). Electrodes were fabricated by boring a cylinder of RVC with a cork borer (size 1)as described in earlier work (6). The final dimensions of the RVC plug were 2.8 mm X 24.2 mm (d X 1). The flow-through cell (Figure 2) was machined from two pieces of 2l/, in. nylon rod (AIN Plastics). The nylon block was drilled and tapped to accept a stainless steel Swagelok fitting. This served as the counterelectrode. The RVC plug was wrapped with a membrane filter (Millipore HVLP 047 00) and fitted into the counterelectrode. The RVC plug was held firmly against a small piece of glassy carbon rod (Tokai Electrode Manufacturing Co. Ltd., Tokyo, Japan, GTC-20) by the fitting on the inlet tubing. The glassy carbon rod had been previously sealed in the nylon block with a silicone sealant (Dow Chemical Co.) and served as an electrical contact to the working electrode. On the other side of the contact a small chamber was filled with mercury and a piece of nichrome wire was inserted. This was then sealed with paraffin. The reference electrode was sealed in the block with Viton O-ring and a standard 1/2 in. plastic pipe fitting. A saturated calomel electrode (Fisher Scientific Co.) was used throughout this work and all potentials are reported relative to the SCE. The fittings used to make connections to the cell were a combination of Grippers and 1/4-28 threaded Nylon LC connectors (Rainin). The Teflon tubing used had dimensions of 1.6 mm 0.d. and 0.5 mm i.d. (FIA system) and 1.6 mm 0.d. and 0.3 mm i.d. (HPLC system). Reagents and Solutions. Reagent grade chemicals were used throughout this work unless otherwise stated. All solutions for the FIA work were prepared with distilled-deionized water. Ascorbic acid solutions were prepared immediately prior to use as they exhibited a marked loss in response in the course of a day. FIA and hydrodynamic voltammetry of ascorbic acid, dopamine,
Flgure 2. Reticulated vitreous carbon detection cell. (a) Three di-
mensional diagram of cell: A, working electrode, RVC plug wrapped with Millipore filter; B, counterelectrode, stainless steel Swaglok fitting: C, glassy carbon contact to working electrode; D, referenceelectrode compartment; E, wire lead to counterelectrode; F, fluid inlet; G, fluid outlet; H, nylon spacer: I,nylon block; J, nylon block. (b) Cross section of assembled RVC cell: A, working electrode; B, counterelectrode; C, glassy carbon contact to working electrode; D, mercury pool; E, nylon block; F, nylon block; G, reference electrode; H, counterelectrode contact; I, working electrode contact; J, inlet fitting; K, outlet fitting.
L-Dopa, epinephrine, and hydroquinone was performed in 0.28 M sulfuric acid. HPLC grade methanol (Fisher Scientific Co.) was used for preparing the solvent system for the HPLC of amoxicillin. The solvent system used for amoxicillin was 0.1 M sodium phosphate monobasic-methanol-acetic acid in a ratio of 75251 and is similar to the system used by Brooks et al. (10). The sample of amoxicillinwas obtained courtesy of Hoffmann-La Roche. The HPLC solvent system was composed of 0.1 M citric a c i d 4 1 M sodium phosphate dibasic in a ratio of 90:48 with 22 mg/L of sodium luryl sulfate added. This solvent system was a modification of one developed by Kissinger and co-workers (11).
RESULTS AND DISCUSSION Selectivity. Dc detectors operated amperometrically or coulometrically have only the selectivity produced by selection of the control potential. Selection of a potential on the limiting
674
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
t
Table I. Definition of Symbols
3.6
concentration ( mol/cm3) inlet concentration (mol/cm3) potential (V) half-wave Dotential (V\ difference-in potentialbetween O.o5il,, and 0.95il,, (mV) Faraday constant (96484.6 C/equiv) current (A) anodic limiting current (A) theoretical steady-state current (A) measured steady-state current (A) empirical constant average mass transfer coefficient (cm/s) transient mass transfer coefficient (cmis) numbers of electrons transferred per molecule (equiv/mol) FIA peak area ( C ) correlation coefficient gas constant ( 8.31441 J / (mo1.K)) conversion efficiency temperature ( K ) superficial linear velocity (cm/s) volume flow rate (cm3/s) loop volume (cm3) empirical constant standard deviation due to the RVC cell (s or PL) standard deviation of the peak (s or p L )
-3.64
I,;--
Flgure 4. Plot of In [ i / ( i l , a- I ) ] vs. potential for hydroquinone. Con-
.;i
structed with data taken from hydrodynamic voltammogram.
4
;,!
;
-;
~
~
E '
-/ bi';i ,/-A
2 310
420
530
640
T
~
I
310
750
mv
4
B
2
VI
~
c
2
420
t
t
7
420
530
640
750
~
~
D
2
i 310
530
SCE
I 640
750
310
420
530
640
750
Figure 5. Normalized hydrodynamic voltammograms of (A) ascorbic M), (C) epinephrine acid (6.66 X lo-' M), (B) dopamine (5.38X M): flow rate, 1.0 mL/min; (5.07 lo4 M), and (D) L-Dopa (5.64 X scan rate, -2 mV/s.
Table 11. Selectivity of R V C Cell Expressed as the Width of the Hydrodynamic Waves of Some Selected Compounds compound
I
360
420
480
540
600
hydroquinone L-Dopa dopamine epinephrine ascorbic acid
A E , mV 80 83 87
93 251
mV v s SCE
Flgure 3. Normalized hydrodynamic voltammogram of hydroquinone (7.12X M): flow rate, 1.0 mL/min; scan rate, -2 mV/s.
plateau of the hydrodynamic voltammogram (HV) of a particular electroactive species automatically places the control potential on the limiting plateau of all other species which react at the same or less oxidizing (or reducing) potentials, as the case may be. Only those species which are more difficult to oxidize (or reduce) are discriminated against. Clearly, the shape of the HV has an influence on selectivity: the steeper the hydrodynamic wave, the better the selectivity. Both theoretical and practical factors come into play here. For a reversible oxidation the equation for the HV is (12)
E = E l / z + (RT/nF) [In (i/il,a - i)]
(1)
Equation 1 (see Table I for the definition of these and subsequent symbols) has the same form as that for classical polarography and the larger n is, the steeper the wave. As systems depart from reversibility the wave will become less steep in most instances. Poor cell designs which produce a significant uncompensated iR drop or uneven distribution of potential along the working electrode will also lower the slope of the rising part of the HV. It is suggested that new cell
designs be tested to see if they conform to eq 1. Hydroquinone has been shown to be reversible in highly acidic medium (13) and was chosen as a test system. A HV corrected for background current and normalized to the limiting current is shown in Figure 3. A plot of In (i/il,?- i) vs. E is shown in Figure 4. The 3c intercept of 407 mV vs. SCE corresponds to the half-wave potential and the slope yielded an n value of 1.91 which is in satisfactory agreement with the theoretical value of 2.00. Normalized HVs for ascorbic acid, epinephrine, dopamine, and L-Dopa are shown in Figure 5. The In (i/i1+ i) vs. E plots for all four of these compounds deviated substantially from linearity. The catecholamines have been shown to be quasi-reversible in acidic media (14, 15) which would explain this. The ascorbic acid oxidation is known to be irreversible by virtue of an EC mechanism (16),but this does not explain the broadness of this wave. A possibility might be two overlapping one-electron waves. Ruiz et al. (17) observed two distinct waves for the oxidation of ascorbic acid at a DME in basic solution. Another way to express all of this is to quote the difference in potential between i = 0.051+, and i = 0.95il,*as shown in Table 11. For a reversible two-electron reaction, eq 1 predicts a A E of 75.6 mV. The result for hydroquinone is close to this, but the rest of the compounds
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
675
rate. The conversion efficiency is the ratio of the measured steady-state current, (iss)memd, to (iss)F (19)
Table 111. Conversion Efficiency as a Function of Flow Rate for Hydroquinone and Determination of Reffby Integration of FIA Peaks
(4)
flow rate,
mL/min
coulombsa ( X103)
0.23 0.34 0.41 0.56 0.67 0.80
2.144 t 2.135 t 2.131 * 2.123 ?: 2.103 f 2.082 f 2.050 k 1.975 ?:
0.005 0.007 0.003 0.002 0.01 0.005 0.002 0.002 1.881 * 0.005 1.790 f 0.006 1.718 0.004
1.11 1.52 2.00 2.50 2.70
R eff 0.995 0.990 0.988 0.985 0.975 0.966 0.951 0.916 0.873 0.830 0.797
+_
Average of three injections; injection volume, 62.8 pL; M; potenconcentration of hydroquinone, 1.780 X tial, 1.100 V vs. SCE. a
Table IV. Conversion Efficiency as a Function of Flow Rate and Determination of Reff by Measurement of the Steady-State Current R eff 0.50
1.00
2.00
compound
mL/min
mL/min
mL/min
ascorbic acid
1.05 0.97 1.13 1.05 1.04
0.90 0.92 1.01 0.91 1.00
0.71 0.68 0.83 0.74 0.94
epinephrine
dopamine L-Dopa hydroquinone
produced broader waves. It should be noted that the position of the ascorbic acid wave relative to the catecholamine waves is such that the former could be determined directly in the presence of the latter. Another way to obtain the HV is to measure the peak height at various potentials in an HPLC or FIA experiment (18). This is a time-consuming process and the potential sweep method is preferable. Conversion Efficiency. Two methods were used to determine conversion efficiency, R,ff,as a function of flow rate. In the first procedure, an FIA approach was used. The conversion efficiency was calculated from Reff
= Qmeaad/nFCVl
(2)
where Qmemdis the coulombic content of the FIA peak, C is the concentration of the sample injection (hydroquinone), and VIis the loop volume. The data and results of these experiments are shown in Table III. The second procedure involved pumping a solution of supporting electrolyte containing a fixed concentration of electroactive material through the RVC electrode and measuring the steady-state limiting current. If the conversion is complete, the theoretical steady-state current is given by Faraday's law
(iSsh = nFCinuf
(3)
where Ch is the inlet concentration and uf is the volume flow
Solutions of ascorbic acid, L-Dopa, dopamine, epinephrine, and hydroquinone were pumped through the RVC cell. For the latter, the entire HV was recorded, but for solutions of the remaining four compounds the limiting current only was measured at a potential well out on the limiting plateau. The calculated conversion efficiencies (eq 4) at several flow rates are shown in Table IV. These results together with those shown in Table I11 lead to the general conclusion that complete conversion is obtained at flow rates up to about 1 mL/min. Linearity of Response. FIA calibration curves were determined for dopamine, hydroquinone, and ferricyanide, and an HPLC calibration curve was determined for amoxicillin. The potential of the working electrode was selected on the basis of the results for the HV experiments. Values well onto the limiting current region of 900,800,200, and 1100 mV vs. SCE for dopamine, hydroquinone, ferricyanide, and amoxicillin, respectively, were used. A major factor which controls the linearity of response of electrochemical detectors is the geometrical relationship between the counterelectrode and working electrode. This has been graphically demonstrated by Lankelma and Poppe (20). When the counterelectrode is placed downstream from the working electrode, there is an uncompensated iR drop along the length of the working electrode. Our cell was designed to minimize this condition by surrounding the RVC plug with a counterelectrode. The results of the determination of these calibration curves are summarized in Table V. Of particular significance are the correlation coefficients and standard deviations of the slopes for the FIA calibration curves. These values are a clear indication of the linearity and high repeatibility of this technique. Of the four compounds studied, hydroquinone was studied over the widest concentration range M to M). Its calibration curve clearly indicated that there was no observable deviation from linearity over this concentration range. The highest concentration examined (6.3 X M) was a solution of dopamine. Since the linear dynamic range is defined as the maximum linear response divided by the background noise, a value of 1.4 X lo4 can be obtained from the dopamine data. Detection Limits. Detection limits were determined under the same conditions as the calibration curves. The current sensitivity was adjusted so a reasonable measure of the noise level could be made. Solutions of the above compounds at concentrations yielding a signal in the vicinity of the detection limit (10-'-104 equiv/L) were injected. The response and the noise level were then used to determine detection limits. The detection limit for electrochemical detectors is dependent on the number of electrons transferred, the width of the peak, and the molecular weight of the compound as well as the characteristics of the detector. In order to make a valid comparison of different electrochemical detectors, these dependencies must be eliminated or at least the comparison must be made under the same conditions. One method for elimi-
Table V. Linearity of Response of Selected Compounds and Peak Current vs. Concentration Injected compound amoxicillina~' ferricyanide b , c hydroquinoneb*' dopamine b , d
conc range, M
y int, A
slope (A/M)
4.6 x 10-7to 9 . 1 x 1 0 - 6 0.109 * 0.002 8.4 x lO-'to 8.4 x 0.512 i 0.001 1.8 x lo-' to 1.8 x l o m 5 1.502 t 0.002 1.2 x 10-7to 6.3 x 10-5 1.4921 t 0.0002 a HPLC, injection volume = 20 wL. FIA, injection volume = 62.8 pL. ' Flow mL/min. e n.s., not significant (determined by t test).
n.s.e -(4.9 n.s. -(3.1
k
0.6) X 10.'
t
0.3) X 10.'
rate = 1.0 mL/min.
corr coeff ( r ) 0.9983 0.99990 0.99994 0.99998 Flow rate = 1.1
676
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1984
Table VI. Detection Limits of Selected Compoundsa
compound hydroquinone dopamine epinephrine amoxicillin
a
n
2 2 2
technique FIA FIA HPLC HPLC
background, noise p-p, nA 6.7 6.4 25 7.0
mol wt
peak
detection limit
vol, mL
( S / N = 2), equiv
ng
9.92 x 10-13 1.39 X lo-''
0.055 189.6 0.132 183.2 4.00 X 10"' 0.366 1 365.4 3.36 X lo-'' 1.23 Injection volume, FIA = 62.8 pL, HPLC = 20 pL. Flow rate = 1.0 mL/min for all except dopamine (1.1mL/min). 110.1
0.5 0.4 1.3 0.9
J, \
H
4 rnin Flgure 6. F I A response used for the determination of the detection llmit of hydroquinone; concentration of hydroquinone Injected, 1.78 X
lo-* M.
nating some of these dependencies is to express the detection limit as the number of equivalents which when injected yield a peak height twice the magnitude of the background noise. The noise of interest here is that noise which has a frequency similar to the frequency components of the peaks being measured. Thus measuring the peak-to-peak noise level over a period of time equivalent to several peak widths should be sufficient. This is not a perfect definition since the detection limit is still a function of the peak width or volume, but it is far superior to reporting detection limits in units of mass such as picograms or nanograms. Regardless of how one decides to define detection limits, sufficient information should be provided so that other forms may be calculated. Figure 6 illustrates the FIA response used to calculate the detection limit of hydroquinone. In this case 62.8 pL of a 1.78 X M hydroquinone solution was injected into the FIA system. From a measurement of the peak height (31.1 nA) and the background noise (6.7 nA) the detection limit at SIN = 2 can be calculated. Table VI summarizes the detection limits obtained in the RVC cell for the FIA determination of hydroquinone and dopamine and the HPLC determination of epinephrine and amoxicillin. These detection limits compare quite favorably with detection limits obtained by Stulik and Pacakova (21) for epinephrine a t tubular platinum (8.2 pequiv), thin-layer glassy carbon (13.7 pequiv), thin-layer carbon paste (13.7 pequiv), wall jet glassy carbon (8.2 pequiv), and wall jet carbon paste (0.82 pequiv) based detectors, all of which were operated amperometrically. The wall jet carbon paste is the only detector with superior detection limits. However, these investigators noted that this was not a practical detector design because of the mechanical instability of the paste when subjected to the impinging stream. Band Broadening. For both the FIA system and the HPLC system, the RVC cell was connected downstream from the UV detector. With this arrangement the output of the RVC cell includes a contribution to band broadening from the tubing connecting the detectors. Hydroquinone was used as a test compound in a FIA experiment. The output of both detectors was displayed on an oscilloscope to eliminate any contribution to the measurement from the strip chart recorder (Figure 7). The average peak width a t half height was 3.1 s for the RVC cell and 2.9 s for the UV detector. The band broadening due to the RVC detector can be estimated by assuming the peaks are Gaussian. The width at half-height is equal to 2.345 UTOT where q O T is the standard deviation
,
H 2 sac
Flgure 7. Comparison of UV and RVC detector response, FIA of 1.2 X lov3M hydroquinone: (A) UV detector response; (B) RVC electrochemical detector response.
S E
""t "
O0O0
, o ~ " @ o ~0 Q
25
50
75
100
,
125
SECONDS
Flgure 8. Comparison of UV and RVC detector response: HPLC of epinephrine; Injection volume, 20 pL; potential, 800 mV; flow rate, 1.0 mL/min. Detector responses were normalized and the time of maximum response was superimposed.
of the detected peak. It is commonly accepted that contributions to band broadening (variances) are additive (22). The difference between the variances computed from the above data is the variance of the contribution due to the RVC cell (qvC2) and the tubing connecting the cells. Taking the worst case and assuming the contribution due to the tubing is negligible, a value for URVC can be estimated to be a maximum of 0.47 s, and, using the flow rate of 1.0 mL/min, this can be expressed as 7.8 wL. If peak distortion is to be kept to less than 5%, uRVcmust be less than 0.3qTOT (23). With this criterion, the minimum peak width which can be tolerated in this detector is about 100 pL. The same type of experiment was performed with the HPLC system using epinephrine as a sample. The major difference between these experiments is that HPLC peaks are typically much wider than FIA peaks. Virtually no difference in the band width of the HPLC peaks was observed (Figure 8), despite the fact that the cell volume of the UV detector was 10 pL while the void volume of the RVC electrode is estimated to be about 150 pL. The effective volume of the RVC cell is much less than its void volume. This apparent discrepancy is related to the mass flow rate sensitive nature
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
/I
1
i-----f--t---i 0
2
4
6
SEC
Flgure 9. Current-time response of RVC cell to potential step experiment: initial potential, 250 mV; final potential, 600 mV; flow rate, 1.00 mL/mln; (A) current-time response to hydroquinone: (B) current-time response to supporting electrolyte (0.28 M sulfuric acid).
of the coulometric detector as opposed to the concentration sensitive nature of the UV detector. If the UV cell is viewed as an exponential diluter, then it requires five to ten cell volumes to completely flush the cell. The previous experiments indicate that it takes a period of time corresponding to a similar volume passing through the electrochemical cell to achieve complete electrolysis. A second type of experiment provided a more direct method of measuring the time response characteristics of the RVC cell. The FIA flow system was used with the RVC cell connected directly to the injection valve, but instead of injecting a sample, a solution of hydroquinone (1.34 X M) was pumped continuously through the cell. The potential was initially set at a value where no oxidation of hydroquinone occurs and then stepped to a value in the limiting current region. The current-time response was monitored on the oscilloscope, as shown in Figure 9. The average time to reach steady-state current was 2.0 s. The current-time behavior between the end of the rapid fall in current and the steady state was treated as exponential. A plot of In i vs. t was linear (correlation coefficient of 0.9995) and had a slope of 1.95 s-'. This slope can be interpreted as the first-order rate (decay) constant for bulk electrolysis in this flow cell. The value of 1.95 8-I represents an improvement of almost 2 orders of magnitude over the value of 0.028 s-l reported by Strohl and Curran (24)for an earlier flow-through RVC cell. Ideally, the time response of a flow stream detector is best determined by applying a step concentration function and measuring the output response as a function of time. A step change in concentration is difficult to achieve. Takata and Muto (25)used an indirect method by studying peak width as a function of flow rate and extrapolating to a peak width at infinite flow rate. Hanekamp et al. (26) did use a concentration step approach. In their experiment, 1 mL of a solution containing nitrobenzene was introduced with a loop injector directly into various detector cells which they assumed behaved as ideal mixing chambers. The time response of the output should therefore be exponential. They measured the time required to reach 63.2% of the steady-state signal and identified this as the time constant. From this quantity and the volume flow rate they calculated a response volume. The results at 1 mL/min varied from 5 p L for a wall jet electrode to 40 pL for one of the DME electrode detectors studied. Stulik and Pacakova (21) performed the same experiment with tubular, thin-layer, and wall jet electrodes and obtained response volumes ranging from 3.75 to 8.5 pL. The nature of the time response of coulometric porous electrodes is also exponential (2,277. In this case, the exponential response is related to the nature of mass transport to the electrode surface rather than to flushing of the cell as in the case of the UV detector. The response volume was ob-
677
tained for the RVC cell from the results of the potential step experiment. Using the slope of the In i vs. t plot, a time constant of 0.51 s was calculated which yields a response volume of 8.5 pL. Unlike concentration sensitive detectors, the coulometric detector is a mass flow rate sensitive detector and is not adequately described by either of the common classifications of flow stream detectors: bulk or solute property. The coulometric detector is totally destructive, unlike amperometric or UV detectors and there is an important consequence of this difference with regard to detector design. For a UV detector a particular molecular contributes to the signal during the whole period of time it is in the light path. Thus the shorter the residence time in the cell, the faster the response time of the detector assuming no electronic limitations. In a coulometric detector, a species contributes to the signal only when it reacts at the electrode surface. Once it has reacted it makes no further contribution. Therefore, the true time response of the RVC detector is related to the length of time it takes to get the analyte to the electrode surface. The less time it takes to do this, the more efficient the electrochemical conversion is at higher flow rates. Clearly, faster response times yield smaller response volumes which, in turn, mean less band broadening. This also permits the use of smaller electrodes with less noise and background current. Mass Transfer Coefficient. Both the sensitivity (conversion efficiency) and the time response of an electrochemical detector are related to the magnitude of the mass transfer coefficient. Newman and Tiedemann (I) have clarified usage of this term and have discussed the functional dependencies of this parameter on the hydrodynamic conditions. Sioda and Keating (2) have defined the average mass transfer coefficient, km,and developed a method for evaluating it is a function of flow rate assuming that the dependency is of the form
R, = jua
(5) where u is the superficial linear velocity and j and a are empirically determined constants. By use of the data from the RVC conversion efficiency study, values of j = 0.0089 and a = 0.42 were obtained from a plot of log log (1/(1- R ) ) vs. log u (correlation coefficient, 0.994) using 65.6 cm2/cm3for the specific surface area (s) and 0.97 for the porosity (E). These results are in agreement with those suggested by Sioda and Keating (2) (a= 0.4 and j = 0.008) for a porous electrode with a length of 0.5 cm and a specific surface area of 100 cm2/cm3. Blaedel and Wang (27) reported values of a between 0.32 and 0.73 for an RVC based cell which was not operated coulometrically. With our experimental values of cy and j , a value of 5.3 X cm/s was calculated for E, at a volume flow rate of 1 mllmin. The potential step experiment used here to measure the time response of the cell is identical in form with the experiments of Chu et al. (28) for determining the mass transfer coefficient. The overall rate constant for a porous bed electrode is given by k,s/c. By use of the measured overall rate constant of 1.95 s-l, the calculated mass transfer coefficient is 2.9 X cm/s. Newman and Tiedemann ( I ) have discussed the differences between these methods for obtaining the mass transfer coefficient and have concluded that there is an inherent difference in the transient value of k and the average value, km. In addition, they have derived the following relationship: km,transient/kn = 1.58 (6) for an electrode with straight pores and laminar flow. Experimentally we obtained a value of 5.6 for this ratio which is much closer to the theoretical value than the value of 30 obtained by Chu et al. (28). While the use of km,transient is inappropriate for predicting the conversion efficiency, it is useful for describing the time response of the detector cell.
678
Anal. Chem. 1984, 56,678-681
CONCLUSIONS On the basis of this work, the RVC coulometric detector is very competitive with state-of-the-art amperometric detectors available for FIA and HPLC. The large mass transfer coefficient, coupled with complete electrolysis and the nature of the decrease in concentration along the length of the electrode in the direction of flow, is such that the response volume of the electrode is only 5 to 6% of its void volume. Operationally, this RVC detector is equivalent to a 1 0 - ~ L UV cell in terms of band broadening, and the response time is quite adequate for FIA work. Detection limits are also as good as most amperometric detectors suggesting that the larger noise associated with the large surface area of the RVC electrode is compensated by the larger Faradaic signal obtained by complete conversion. There are other advantages of the RVC detector. The pressure drop across the electrode is negligible. The electrode surface requires no mechanical preparation such as polishing prior to being placed in service. The electrodes are readily replaced, and the electrode material is inexpensive. Like other coulometric detectors, the flow dependence of the measured current can be eliminated by measuring the current-time integral. Because of larger currents, hydrodynamic voltammograms of rather dilute solutions are easily obtained by the scanning technique. This provides a rapid and convenient way to select the control potential. Registry No. Carbon, 7440-44-0. LITERATURE CITED (1) Newman, J. S.;Tledemann, W. I n "Advances in Electrochemistry and Electrochemical Englneerlng, Vol. 1 l", Tobias, C. W., Gerlscher, H., Eds.; Wlley-Interscience: New York, 1977;pp 355-438. (2) Siode, R. E.; Keating, K. B. I n "Electroanalytical Chemistry, Vol. 12"; Bard, A. J. Ed.; Marcel Dekker: New York, 1982;pp 19-27.
(3) Johnson, D. C.; Larochelle, J. Talanfa 1973,2 0 , 959. (4) Stullk, K.; Pacakova, V. J. Necfroanal. Chem. 1981, 129, 1. (5) Brunt, K. Trace Anal. 1981, 1, 47. (6) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979,5 1 , 353. (7) Ruckl, R. J. Talanta 1980,2 7 , 147. (8) Weber, S.(3.; Purdy, W. C. Ind. Eng. Chem. Prod. Res. Dev. 1981, 2 0 , 593. (9) Wang, J. Electrochlm. Acta 1981,2 6 , 1721. (10) Brooks, M. A.; Hackman, M. R.; Mazzo, D. J. J . Chromatogr. 1981, 210, 531. (11) Klssinger, P. T.; Bruntlett, C. S.; Davis, G. C.; Felice, L. J.; Rlgglns, R. M.; Shoup, R. E. Clln. Chem. (Winston-Salem, N . C . ) 1977,23, 1449. (12) Van Zee, J.; Newman, J. J. Nectrochem. SOC. 1977, 724, 708. (13) Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker:
New York, 1989;pp 365-367. (14)Hawley, M. D.; Tatawawada, S. V.; Plekarskl, S.;Adams, R. N. J . Am. Chem . SOC. 1967,8Q,447. (15) Sternson, A. W.; McCreedy, R.; Felnberg, 6.; Adams, R. N. J . Nectroanal. Chem. 1973,46, 313. (16) Perone, S.: Kretlow, W. J. Anal. Chem. 1986,38, 1760. (17) Rulz, J. J.; Aldaz, A.; Dominguez, M. Can. J . Chem. 1977,5 5 , 2799. (18) Anderson, J. L.; Welsshaar, D. E.; Tallman, D. E. Anal. Chem. 1981, 53, 908. (19) Sloda, R. E. Nectrochlm. Acta 1988, 13, 375. (20) Lankelma, J.; Poppe, H. J. Chromatogr. 1978, 125, 375. (21) Stullk, K.; Pacakova, V. J . Chromatogr. 1981,208, 289. (22) Johnson, E. L.; Stevenson, R. "Basic Liquid Chromatography"; Varlan Associates: Palo Alto, CA, 1978;p 274. (23) Poppe, H. Anal. Chlm. Acta 1980, 774, 59. (24) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979,5 1 , 1050. (25) Takata, Y.; Muto, G. Anal. Chem. 1973,45, 1864. (26) Hanekamp, H. 6.; Bos, P.; Brlnkman, U. A. Th.; Frie, R. W. Fresenlus' 2.Anal. Chem. 1979,297, 404. (27) Blaedei, W. J.; Wang, J. Anal. Chem. 1979,5 1 , 799. (28) Chu, A. K. P.; Flelschmann, M.; Hills, 0.J. J . Appl. Necfrochem. 1974,4 , 323.
RECEIVED for review August 25, 1983. Accepted December 20, 1983. This paper was taken in part from the Ph.D. dissertation of T. P. Tougas and was presented in part at the 186th National Meeting of the American Chemical Society, Washington, DC, Aug 1983.
Background Compensation in Fast Scan Square Wave Voltammetry and Other Pulse Techniques at the Dropping Mercury Electrode Adina Lavy-Feder and Chaim Yarnitzky*
Department of Chemistry, Technion-Israel Institute of Technology, Haifa, Israel 32000
The calculatlon of the charge Injected Into the double layer, after a potentlal Is applied to the worklng electrode, has led to the lntroductlon of a new background compensatlon method. Thls method Is useful for all pulse polarographlc technlques such as fast scan square wave voltammetry and pulse polarography, normal or dlfferentlal, when the common dropplng mercury electrode Is used. The deslgn of slmple electronlc unlts whlch can be attached to a one drop square wave analyzer (ODSWA) and the PAR Model 174 polarographic analyzer Is descrlbed. The performance of the unlts Is demonstrated by recording voltammograms at low current sensltlvltles. The Influence of the compensation on the Faradalc peak currents Is also glven.
The use of polarographic instruments in analytical and research laboratories has increased considerably in the last
decade. However, problems concerning electronic noise and background current due to drop growth still exist and limit the use of pulse voltammetric techniques to concentrations above lo-' M. In normal or reverse pulse methods the situation is even worse due to the gradually increasing potential steps applied. The use of a simple averager for signal to noise enhancement has been demonstrated (1). While the averager was shown to yield distinctive peaks, measurable with higher accuracy, the sensitivity of the method remains unaffected; these peaks are superimposed on an unconstant, nonlinear background current, which must be compensated before averaging can take place. Ultimate compensation may be achieved by adopting the alternate drop method (2) which, unfortuantely, suffers from three inherent disadvantages: (a) the recording time is doubled; (b) the electronic noise is increased; (c) the peak current is decreased due to the subtraction of the Faradaic current measured in the staircase mode from the current
0003-2700/84/0356-0678$01.50/00 1984 Amerlcan Chemical Society
