Anal. Chem. 1987, 5 9 , 2119-2122
2119
Evaluation of Differential-Pulse Anodic Stripping Voltammetry at Mercury-Coated Carbon Fiber Electrodes. Comparison to Analogous Measurements at Rotating Disk Electrodes Joseph Wang,* Peng Tuzhi, and Javad Zadeii Department of Chemistry, New Mexico State Uniuersity, Las Cruces, New Mexico 88003
Differential-puise stripping voltammetric behavlor at uitramicroeiectrodes differs considerably from that expected at conventional macroelectrodes. The subetantlai enhancement of dMuslonai flux results in slgnmcant plating of metals during the scannlng perlod. As a result, the dependence of the stripplng peak current upon instrumental parameters such as deposltlon time and potentlal, scan rate, or pulse repetltlon tlme diverges from that common at large-area electrodes. Such effects are pronounced particularly for metals wlth relatlvely posltlve peak potentlais. The low ohmic drop permlts rellable trace metal measurements In solutions of dllute supporting electrolyte and use of a two-eiectrode conflguratlon. Although convective transport has some effect on the response, the use of qulescent solutions slmpllfies the Insfrumentation and operation of stripping voltammetry and ylelds results comparable to those obtained at conventlonal-size rotating disk electrodes. The strlpplng response Is shown to be susceptlbie to interferences common at macroelectrodes.
Anodic stripping voltammetry (ASV) is a powerful electroanalytical technique for trace metal measurements (1-3). Two basic electrode systems, the mercury film electrode (MFE) and hanging mercury drop electrode (HMDE), have gained wide acceptance in the development of ASV. As a result of its larger area-to-volume ratio the MFE yields superior sensitivity and selectivity, and hence it is preferred for ultratrace measurements. In most cases, a rotating glassy carbon disk (2-4mm diameter) is used to support the mercury film (4).Ultramicrovoltammetric electrodes, originally developed as in vivo sensors (5),have gained much attention in recent years. This attention has been driven by attractive features, including steady-state diffusional mass transport, low ohmic drop, and fast response time inherent to these electrodes (6). A number of publications have appeared recently that discuss the use of microelectrodes ASV for trace metal measurement (7-10).These studies indicate that the stripping response at microelectrodes compares favorably with that of conventional MFEs; in addition, micro-MF'Es eliminate the need of forcing the solution convection during the deposition step (8)and are suitable for measurements in very small sample volumes (9). The present work has initiated in order to characterize fully the nature of differential-pulse stripping voltammetry at ultramicroelectrodes. Differential-pulse anodic stripping voltammetry (DPASV) has been the most widely used stripping approach, primarily because of ita effective discrimination against the charging background current (11-13).Hence, the combination of DPASV with conventional MFEs has been extremely powerful for applications involving ultratrace levels of metals, e.g., studies of heavy metal speciation (14).DPASV has been widely explored both theoretically and experimentally, by many authors (1, 11, 15). T o date, however, no comprehensive study has yet been made of the DPASV response at mercury ultramicroelectrodes. The following study 0003-2700/87/0359-2 1 19$01.50/0
demonstrates that the steady-state diffusional mass transport, characterizing these electrodes, results in unique DPSAV behavior.
EXPERIMENTAL SECTION Apparatus. All experiments were performed by using an EG&G Princeton Applied Research Model 264A voltammetric analyzer, in conjunction with an EG&G PAR Model 0073 X-Y recorder. The electrochemical cell was a "homemade" 100-mL Pyrex glass container. The cell admitted the working electrode, reference electrode (Ag/AgCl, Model RE-1, Bioanalytical Systems), platinum-wire auxiliary electrode, and the nitrogen delivery tube through four holes in its Plexiglas cover. Carbon fiber microelectrodes were prepared from Thornell 300 grade WYP-90 1/0 fibers (Union Carbide Corp.), having a nominal diameter of 7 pm. The fibers were soaked and washed in acetone and dried thoroughly at room temperature. A single fiber was then inserted into a 1.0-mm4.d. tip of a borosilicate glass pipet (14673-043, VWR) and sealed with epoxy resin; microcylinders of 2-mm length were used in most experiments. The pipet was back-filled with mercury, and electrical contact was established with a copper wire. A 0.75-cm-diameter glassy carbon rotating disk electrode (Model DDI 15, Pine Instruments Co., Grove City, PA) was used also as a substrate for the mercury film; the electrode was mounted on a Pine Instruments Model PIR rotating disk assembly. Reagents. Stock solutions M) of the metal ions were prepared by dissolving the pure metal or its salt in nitric acid and diluting to volume with water. The supporting electrolyte was 0.10 M acetate buffer solution (pH 4.5). All solutions were prepared with double-distilled water. Procedure. A 99-mL portion of the acetate buffer solution M mercury solution were introduced and 1 mL of the 5 X into the cell. The solution was purged with nitrogen for 8 min and was held under a nitrogen atmosphere throughout the measurement step. In situ plating of the mercury film proceeded by applying a potential of -0.9 V while using a quiescent solution. After 20 min the potential was switched to +0.1 V and held there for 2 min. Background and sample measurements were carried out successively as follows. The deposition potential was imposed on the electrode while the solution was quiescent. The metals were then stripped from the mercury film by applying a differential-pulse anodic potential ramp with a 10 mV/s scan rate, 50 mV amplitude, and 0.2 s pulse repetition. The scan was stopped at +0.1 V, and after 60 s the system was ready for the next cycle. As with other types of carbon substrates, the mercury deposit appeared to exist as a collection of droplets.
RESULTS AND DISCUSSION Figure 1 illustrates typical stripping voltammograms, for some common cations, obtained at the mercury-coated carbon fiber electrode, by employing positive-going differential pulse (a) and linear scan (b) waveforms. Both stripping modes yield well-defined peaks, with a short deposition period and a quiescent solution allowing convenient quantitation a t the 5 X lo-* M level. Good resolution of the four stripping peaks is obtained and, as expected (II), the DPASV peak potentials are shifted to more negative values, compared to those obtained with linear scan stripping. The zinc and bismuth peaks do not coalesce with the hydrogen evolution or mercury oxidation currents, respectively (Le., a wide working potential 0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL.
59, NO. 17, SEPTEMBER 1, 1987 8:
IO
nA
ibi
LL--L---L---A
0
-1.2
-0.9
-0.6
-0.3
0.0
E .V Differential pulse (a) and linear scan (b) stripping voltamM bismuth, cadmium, and lead and 1 X lo-’ mograms for 5 X M zinc in pH 4.5 acetate buffer. Three-minute deposition at -1.35 V with a quiescent solution. Scan rate, 10 mV/s (a),50 mV/s (b); pulse amplitude, 50 mV.
5
10
20 30 S C A N RATE ( m V / s )
40
50
Flgure 2. Dependence of lead (A) and copper (6) strlpping peak current on the scan rate of the differential pulse waveform. One-minute deposition at -0.9 V; cation concentratlon, 2 X lo-’ M (A) 3 X M (B). Other conditions are given in Flgure 1.
Figure 1.
Table I. Dependence of the Cadmium DPASV Response on Deposition Time at Mercury-Coated Carbon Fiber and Rotating Glassy Carbon Electrodes” conventional (rotating) MFE scan rate, mV/s
2
10
ultramicro-MFE 2
10
intercept*
0.06 i 0.36 0.13 f 0.65 5.43 f 0.08 2.84 i 0.25
slopeC c o n coeff
7.78 f 0.07
12 i 0.13
0.999
0.999
1.07 f 0.01 4.50 f 0.05 0.999
0.999
nDepositionat -1.1 V; cadmium concentration, 1 X low7M; rotation speed (conventional MFE), 1600 rpm. Other conditions are given in Figure la. *Expressed in nA and FA for the micro- and conventional electrodes, respectively. Expressed in nA/min and pA/min for the microand conventional electrodes, respectively. ~ _ _ window). The minimization of charging current in the differential pulse mode results in improved detectability, as indicated from the signal-to-background characteristics of the stripping peaks. (Substantially lower peaks, and overall inferior signal-to-background characteristics, were observed when a slower (10 mV/s) linear scan was employed.) Despite the reduction in electrode dimensions and the very low concentrations, trace measurements a t microelectrodes yield currents at the nanoampere range due to the preconcentration step; hence, stripping measurements can be performed with commercially available instruments. The voltammograms shown in Figure 1 were recorded while using diffusion as the sole mode of transporting the metal ions toward the mercury surface during the deposition. Such use of quiescent solutions (associated with the steady-state diffusional flux) simplifies the methodology and instrumentation for stripping voltammetry and may improve its reproducibility. In the following sections we will demonstrate that the unique diffusional profiles of ultramicroelectrodes also yield unusual DPASV response characteristics, compared to those observed a t prevalent macroelectrodes. The various factors leading to this unique behavior will be systematically appraised. For example, Table I examines the dependence of the peak current on the deposition time. The response for cadmium at the micro- and macro-MFEs is compared at different scan rates. With the larger electrode (rotating during deposition, stationary while stripping), the expected linear dependence with zero intercept is observed, as the amount of cadmium deposited is directly proportional to the deposition
period. In contrast, the ultramicroelectrode (stationary during both deposition and stripping steps) exhibits a linear current-time dependence, but with significant nonzero intercepts. The latter are attributed to substantial plating of cadmium during the long scanning period, from the deposition potential to the peak potential, associated with the nonlinear diffusional mass-transport. The longer this scanning time (i.e., the slower the scan rate), the larger is the contribution of the metal deposited while scanning, and the larger is the intercept. Similarly, the “continued plating” effect becomes more pronounced for larger differences between the deposition and stripping potentials, e.g., in measurements of copper (not shown). For example, with scan rate of 2 mV/s, the response following 0-min deposition (the intercept) was approximately 50% of that following 9-min deposition (as expected from 8min scanning period). Hence, with slow scan rates-common to DPASV measurements-there is no distinct “borderline” between the deposition and stripping steps when ultramicroelectrodes are concerned. (Obviously, this obviates the need for a rest period between the two steps.) Faraday’s law, commonly used to calculate the concentration of the metal in the amalgam, C,, can thus be written as
c, =
‘
(1)
nFV
where t d and E d are the deposition time and potential, respectively, Epis the peak potential, v is the scan rate, and V is the volume of mercury electrode. The limiting current for the deposition of the metal, iL, may depend upon the exact geometry of the microelectrode, whether it is a hemisphere (8),microcylinder (16),or microband (17). (Equation 1is valid only for steady-state limiting currents.) Overall, the correction term in eq 1, ( E d - Ep)/v, represents the scanning period during which plating continues. Hence, the interrelationship of parameters such as td, v, or Ed, affecting the “actual” plating time, must be considered. Further substituting iL and V in eq 1, using for example the common case of hemispherical electrode (B), we obtain a relatively simple result for C,
c,
=
I
(2) r2 where Cb is the bulk concentration of the metal ion. The significant plating of the analyte while scanning the potential has pronounced effect on the dependence of the DPASV response upon other parameters. Figure 2 examines
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1,
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I,
C
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/;,
20
i/ 10
- _ -
B - - - - I
30
I,
nA 20
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rnv
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0.5
1.0
R EPET IT ION TIME, S
Dependence of differential Stripping peak current on the pulse amplitude (A) and repetition time (e) while wing carbon flber (a,b) and rotating glassy carbon (c,d) mercury-coated electrodes. Onemlnute deposition at -1.1 V; electrode rotation (c,d), 1600 rpm. Cadmium (a,c)and lead (b,d) present at the 2 X lo-' M level. Other conditions are given in Figure 1. Flgure 3.
the effect of the potential scan rate on the lead (A) and copper (B) peak heights. The lead response increases with the scan rate, until it levels off at 20 mV/s. Similar behavior is commonly observed at conventional-size MFEs (15). In contrast, the largest copper peak is obtained at the lowest scan rate (1 mV/s); a gradual decrease in the response is observed upon increasing the scan rate from 1to 10 mV/s, following which the peak increases in a manner similar to that observed for lead. Hence, simultaneous measurement of metals with different peak potentials yields differences in the scan rate dependence, when significant plating occurs during the stripping step (i.e., at scan rates lower than 10 mV/s). Unusual effects of such continued plating are particularly pronounced when measuring metals with relatively positive peak potentials. Similar differences in the scan rate dependence were observed when the same metal (Pb) was measured following deposition at different potentials (-0.8 vs. -1.2 V for 90 s and other conditions as in Figure 2; trace not shown). Obviously, such scan rate effects depend also upon the deposition period, i.e., upon the relative fractions of the metal deposited during the deposition and stripping steps. Once again, the data shown in Figure 2 reflect the fact that several variables (scan rate and deposition time and potential) have an interactive effect on the DPASV response of ultramicroelectrodes. The effect of the deposition potential was examined by using a 1 x M lead solution, 2-min deposition periods at potentials ranging from -0.4 to -1.3 V, and a scan rate of 10 mV/s (not shown). The resulting i, vs. Ed plot exhibited a continuous increase of the current with increasing Ed. In contrast, with analogous linear scan measurements at a faster scan rate (50 mV/s), the peak current increased, as expected, from zero to a limiting value (at potentials more negative than -0.65 V). Such DPASV E d dependence is attributed to the continuous plating, associated with the longer and longer scanning periods (eq 1).
Figure 3 examines the effects of the pulse amplitude (A) and pulse repetition time (B) upon the response of the micro(a,b) and conventional-size (c,d) MFEs. At both electrodes the current increases upon increasing the pulse amplitude, resulting in a curved relationship (similar to that reported for conventional MFEs (11)). The upper practical limit for the
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amplitude is 100 mV; due to resolution problems associated with peak broadening (bl/z of 44-53 mV for amplitudes of 25-100 mV, respectively), an amplitude of 50 mV is recommended as a reasonable compromise between sensitivity and selectivity. Differences in the behavior of the conventional and microelectrodes are observed when the effect of the pulse repetition time is examined (Figure 3B). The large electrode exhibits the expected (11) dependence, i.e., slightly higher response for shorter repetition times. In contrast, a nonlinear increase of the peak height at longer times is observed at the microelectrode. It is well-known ( I ) that when the differential pulse waveform is used, some of the metal stripped from the electrode during the pulse is replated onto the electrode in the waiting period between pulses. The enhanced mass transport associated with the nonplanar diffusion at ultramicroelectrodes substantially magnifies this replating effect. The above gain in sensitivity is compensated by a progressive degradation of the peak resolution at longer pulse repetition times. A mixture of 5 X lo-* M cadmium and lead was used for comparing the DPASV response at the micro- and conventional-size MFEs (3-min deposition at -1.1 V). Different modes of mass transport were employed during the deposition step: quiescent and stirred solutions (for the microelectrode) and electrode rotation (for the conventional-size one). Both electrodes allowed convenient quantification at the 5 X loW8 M level. The resolution as well as the signal-to-noise characteristics were also similar. It is noteworthy that the response of the micro-MFE was affected by convective mass transport; for example, solution stirring (-300 rpm) resulted in doubling of the DPASV peaks compared to those observed in a quiescent solution. Nevertheless, reliable trace measurements with well-defined peaks are obtainable with quiescent solutions; these were employed in all subsequent work. Poorly defined peaks were observed when the conventional-size disk electrode was operated in a quiescent solution, as expected from the square-root dependence of the current upon electrode rotation (1). The microelectrode (immersed in a quiescent solution) yielded lead and cadmium peaks with signal-to-noise characteristics identical to those observed at the conventional-size disk rotated at 400 rpm during the deposition step. The former also yielded superior bismuth response characteristics but an inferior zinc response. Reduced sensitivity to external convective transport should be obtained upon increasing the perimeter-to-area ratio. The dependence of the DPASV response upon the length of the mercury-coated microcylinder electrode was evaluated in another experiment (2-min deposition at -1.1 V, using cylinders of 0.5, 2.5, 5.0, and 10 mm long). A least-squares treatment of the current vs. length data yielded slopes of 0.229 f 0.006 (cadmium) and 0.279 f 0.004 (lead) pA/mm (correlation coefficients, 0.999). The detectability is also improved as the length increases because the DPASV mode provides effective discrimination against the increased charging current. A series of eight concentration increments from 1 X to 8X M cadmium yielded a linear calibration plot, with a slope of 778 nA/pM, intercept of 20 nA and correlation coefficient of 0.998 (2-min deposition at -1.1 V). The precision was estimated from 10 succeeive measurements of 1 x M lead. The mean peak current found was 378 nA with a range of 371-384 nA and a relative standard deviation of 1.3% (2-min deposition at -0.9 V). Detection limits were estimated from measurements of 1X lo4 M cadmium and lead, following 10-min deposition at -1.1 V. Detection limits of about 1.5 X M lead and 2.2 X 10-l' M cadmium were estimated based on a signal-to-noise ratio of 3. Traditionally, DPASV applications have employed high concentrations of supporting electrolyte. Copeland et al. (18)
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
1 1 0 nA
1 . -0.9 -0.7 -0.5
A
J--.
-0.3
i
- 0.9
1
1
-0.7
-0.5
-0.3
E ,v Figure 4. Effect of supporting electrolyte concentration on stripping voltammograms for 5 X 10" M cadmium and lead at carbon fiber (A) and rotating glassy carbon (B) mercury-coatedelectrodes. Electrolyte concentration, 0.001 M (a) and 0.10 M (b). Three-minute preconcentration at -1.1 V. Rotation speed (B), 1600 rpm. Other conditions are given in Figure 1.
demonstrated that the DPASV response of conventional mercury-coated rotating disk electrodes exhibits significant peak current diminutions as the supporting electrolyte concentration decreases. One of the very attractive characteristics of ultramicroelectrodes is their low ohmic drop. This property of microelectrodes has been exploited recently for cyclic voltammetry (19,20) and amperometric flow detection (21) in solutions of dilute electrolyte. Analogous advantages of microelectrodes for stripping measurements of trace metals have not been explored. Figure 4 compares stripping voltammograms at the micro- (A) and macro- (B) MFEs for a solution of 5 X M in both cadmium and lead, containing low (0.001 M (a)) and high (0.1 M (b) supporting electrolyte concentrations. Note that a t the large electrode the peaks are greatly diminished at the lower supporting electrolyte concentration (B(a)), coupled with serious broadening and shifts of the peaks; meaningful stripping results could not be obtained. In contrast, virtually undistorted voltammogrms are observed a t the micro-MFE when using the dilute electrolyte solution (A(a)); the peak potentials and half-widths are essentially the same as those obtained at the high electrolyte level (A(b)). The minimization of resistance effects can be exploited for simplifying the instrumentation for stripping voltammetry. For example, the DPASV response for a solution containing 2.5 x M cadmium and lead at the micro- and macro-MFEs were compared while using twoand three-electrode configurations (90-s deposition at -1.1 V). The increased cell resistance, associated with the two-electrode operation, resulted in a severe distortion of the macroelectrode voltammogram. No such distortion was observed at the microelectrode, where an identical response was obtained for two- and three-electrode configurations. The practical implications of these data for performing reliable stripping measurements of trace metals in resistive media are obvious.
Because of the unique behavior of ultramicroelectrodes, we decided to examine their susceptibility to interferneces common in DPASV measurements at electrodes of conventional size. Measurements of zinc in the presence of copper were used to test interferences due to intermetallic compounds. The 2 X lo-' M zinc peak was diminished by 33% and 66% upon adding 1 X and 2 X lo-' M copper, respectively (2-min deposition at -1.35 V). Analogous experiments at the macro-MFE yielded 48% and 82% diminutions of the zinc peak upon similar increments of the copper concentration. Another common interference, the adsorption of surface-activeorganic compounds, was tested by adding gelatin to a 2.5 x M lead solution (90-s deposition at -1.1 V). Peak current depressions of 34%) 55%, 68%) and 69% were observed in the presence of 4, 8, 12, and 16 ppm gelatin. A nondeaerated sample containing 3 X M copper and 2 x lo-' M lead was used to evaluate the dissolved-oxygen interference (1-min deposition at -0.9 V). A large oxygen reduction peak was observed over the -0.4-0.0 V range; this peak obscured almost completely the copper response, without affecting the lead peak. Similarly, the use of ultramicroelectrodes does not alleviate the problem of overlapping peaks in DPASV. Overall, it can be concluded from the above experiments that DPASV measurements at ultramicroelectrodes are prone to the main types of interferences observed at electrodes of conventional size. Registry No. Mercury, 7439-97-6.
LITERATURE CITED (1) Wang, J. Stripping Analysis: Principles, Instrumentation, and Applications ; VCH Publishers: Deerfleld BeachlWeInheim, 1985. (2) Florence, T. M. J. Electroanal. Chem. Interfacial Electrochem. 1984, 168, 207. (3) Peterson, W. M.; Wong, R. W. Am. Lab. (FairfleU, Conn.) 1981, 73(11), 116. (4) Florence, T. M. J. Electroanal. Cbem. Interfacial Chem. 1970, 2 7 , 273. (5) Gonon. F. G.; Formbarlet, C. M.; Buda, M. J.; PuJol,J. F. Anal. Chem. 1981, 5 3 , 1386. (6) Wightman, R. M. Anal. Chem. 1981, 5 3 , 1125A. (7) Cushman, M. R.; Bennett, B. G.; Anderson, C. W. Anal. Chim. Acta W81, 130, 323. (8) Wehmeyer, K. R.; Wightman, R. M. Anal. Chem. 1985, 5 7 , 1989. (9) Baranski, A. S.; Quon, H. Anal. Chem. 1988, 5 8 , 407. (10) Schulze, G.; Frenzel, W. Anal. Cblm. Acta 1984, 159, 95. (11) Lund, W.; Onshus, D. Anal. Cbim. Acta 1976, 8 6 , 109. (12) Florence, T. M. Anal. CMm. Acta 1980, 119, 217. (13) Nurnberg. H. W. Nectrochlm. Acta 1977, 2 2 , 935. (14) Nurnberg, H. W. Fresenlus' Z . Anal. Chem. 1983, 316, 557. (15) Copeland, T. r.; Christie, J. H.; Osteryoung, R. A.; Skogerboe, R. K. Anal. Chem. 1973, 45, 2171. (IS) Rius, A.; Polo, S.; LloDis, J. An. R. SOC. E m . Fls. Oulm., Ser. 6 1949, 45, 1029. Kovach, P. M.; Caudill. W. L.; Wightman, R. M. J. Electroanal. Chem. Interfacial Electrocbem. 1985, 185, 285. Copeland, T. r.; Christie, J. H.; Skogerboe, R. K.; Osteryoung, R. A. Anal. Chem. 1973, 45, 995. Howell, J. 0.; Wightman, R. M. Anal. Chem. 1984, 5 6 , 524. Bond, A. M.; Fleiscmann, M.; Robinson, J. J. Electroanal. Chem. Interfacial Electrochem. 1984, 768, 299. Bixler, J. W.; Bond, A. M. Anal. Chem. 1986, 58. 2859.
RECEIVED for review December 16, 1986. Accepted May 22, 1987. This work was supported in part by the National Institutes of Health, Grant No. GM 30913-04.
