852
Anal. Chem. 1989, 61,852-856
I
80
1
the scanning electron microscopy is gratefully acknowledged.
LITERATURE CITED
0
10
2.0 3.0 40 C O N C ~ N l R I T I O NUMI l
5.0
Flgure 6. Cyclic voltammetric peak currents for a PVP-2 electrode (seeTable I) as a function of ferricyanide concentration in 0.01 M HCI and 0.1 M KCI. Electrode was rotated at 2500 rpm for 10 minutes before each voltammogram was recorded at 100 rnV/s.
is limited because of poor mechanical stability of the films in hydrodynamic systems or nonpolar solvents. PVP-containing composite electrodes are not subject to these problems, and behave electrochemically very much like PVP-containing polymer films or PVP-modified carbon paste electrodes. These results are expected to be general, suggesting that composite electrodes can be used to prepare a variety of solid, renewable electrodes for use in direct electroanalysis, sensors, and detectors. The surface properties of the composite can be controlled by composition of the electrode and the surrounding solution.
ACKNOWLEDGMENT The authors wish to thank Kenneth E. Creasy for useful discussions and for the preliminary experimental results with co-poly(viny1)pyridine-containing composite electrodes. The assistance of Carol Blouin, Institute of Materials Science, with
(1) Shaw, B. R.; Creasy. K. E. J. Elechoanal. Chem. Interfacial Elechochem. 1988, 243, 209-217. (2) Shaw, 8. R.; Creasy, K. E. Anal. Chem. 1988, 60. 1241-1244. (3) Creasy, K. E.; Shaw, B. R., submitted for publicatlon in Anal. Chem. (4) Henning. T. P.; WhRe, H. S.; Bard. A. J. J. Am. Chem. Soc. 1981, 103, 3937-3938. ( 5 ) Burgmayer, P.; Murray, R. W. J. Electroanal. Chem. InferfaclalElecfrochem. 1982, 735, 335-342. (6) Doblhofer, K.; Dwr, W. J. Electrochem. Soc. 1980, 727, 1041-1044. (7) Dominey, R. N.; Lewls. N. S.; Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 467-482. (8) Liu, H. Y.; Anson, F. C. J. Elechoanal. Chem. Interfacial Elecfrochem. 1983, 158, 181-185. (9) Bartak, D. E.; Kazee, 6.; Shimazu, K.; Kuwana, T. Anal. Chem. 1988, 5 8 , 2756-2761. (IO) Coche, L.; Moutet, J. C. J. Am. Chem. Soc. 1987, 109, 6887-6889. (11) Klatt, L. N.; Connell, D. R.; Adams, R. E. Anal. Chem. 1975, 47, 2470-2472. (12) Anderson, J. E.; Tallman. D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 5 0 , 1051-1056. (13) Armentrout, D. N.; McLean, J. D.; Long, M. W. Anal. Chem. 1979, 5 1 , 1039-1045. (14) Peterson, S. L.; Tallman, D. E. Anal. Chem. 1988. 60, 82-86. (15) Anderson, J. E.; Tallman, D. E. Anal. Chem. 1976, 48, 209-212. (16) Ufframicroelecfrodes; Fleischmann, M., Pons, S.,Rollson. D. R., Schmidt, P. P., Eds.; Datatech Systems: Morgantown, NC, 1987. (17) Dickstein, H. L.; Curran, D. J., The Plttsbwgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Abstract 442, Atlantic C i i , NJ, March 9-13, 1987. (18) Akzo Chemie America, Chicago, IL, 1988, personal communicatlon. (19) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980. 127, 640-647. (20) Wang, J.; Golden, T.; Tuzhi, P. Anal. Chem. 1987, 59, 740-744. (21) Mukaida, M.; Okuno, H.; Ishimori, T. Nippon Kagaku Zasshi 1985, 86, 598-600. (22) Oyama, N.; Anson, F. C. J. Am. Chem. Soc. 1979, 101. 3450-3456. (23) Geno, P. W.; Ravichandran. K.; Baldwln, R. P. J. Electroanal. Chem. 1985, 163, 155-166. (24) Standard potentials in Aqueous Solutfon; Bard, A. J., Parsons, R., Jordan, J., Eds.; Marcel Dekker: New York 1985. (25) Guadaiupe, A. R.; Abruna, H. D. Anal. Chem. 1985, 5 7 , 142-149. (26) Gehron, M. J.; Brajter-Toth, A. Anal. Chem. 1986. 5 8 , 1488-1492. (27) Wang. J.; Lin, M. S. Anal. Chem. 1988, 6 0 , 1545-1548.
RECEIVED for review July 18,1988. Accepted January 5,1989. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the Research Corporation, and to support by the National Science Foundation under Grant No. CHE-8707973.
Constant Potential Amperometric Detection of Carbohydrates at a Copper-Based Chemically Modified Electrode Sunil V. Prabhu and Richard P. Baldwin* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292
A Cu-based chemically modified electrode (CME) has been developed for the constant potentlal amperometrlc detectlon of reducing and nonreduclng sugars following high-performance llquld chromatography. The CME was prepared by coatlng a glassy carbon electrode with a copper(I1) layer that catalyzes the sugar oxidation In basic solution when a potentlal sufficiently posltlve to generate Cu( I I I ) Is applled. I n flow lnjectlon and llquld chromatography usage, the electrode response was stable for more than 5 h wHh a signal loss of less than 10% over this period. With anion exchange chromatography performed In 0.15 M NaOH, mono- and disaccharides could be separated and detected at the subnanogram level at an applled potentlal of +0.48 V vs Ag/AgCI.
INTRODUCTION The development in recent years of effective high-performance liquid chromatographic procedures for the separation of carbohydrate compounds has produced a substantial need for the parallel development of sensitive and convenient methods for carbohydrate detection and quantitation (1-4). The absence of a strongly absorbing chromophore in simple carbohydrates restricts the use of direct UV-visible detection to wavelengths in the 180-220-nm range and thereby makes the usual absorption-based techniques largely unsuitable. Consequently, the approaches most commonly employed for these compounds have relied on either refractive index detection with its characteristically poor sensitivity or chemical
0003-2700/89/0361-0852$01.50/00 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989
derivatization of the parent sugar to a more readily detectable form (5). The resulting need for direct trace-level carbohydrate sensing has been partially solved by the development of electrochemical detection systems which try to make use of the electrooxidation of these compounds in highly basic solution (6-18). The principal difficulty encountered in this approach is that the carbohydrates exhibit a large overpotential for oxidation a t the carbon electrodes commonly used in conventional liquid chromatography/electrochemical detection (LC/EC). Because this serves to increase drastically the potential required for the oxidation and thereby compromises both the selectivity and the detection limits afforded by the detector, attention has naturally focused on the development of alternate approaches, usually involving the use of new electrode materials that permit the carbohydrate oxidation to take place a t potentials low enough to provide optimum detector performance. The resulting approaches can be divided into two distinct groups. The first employs platinum (6-8) or gold (9-12) electrodes on which the carbohydrates adsorb and subsequently undergo relatively facile dehydrogenation/ oxidation. In the most typical applications, these electrodes can provide quantitation a t the 100-500-ng level. However, because of the surface cleaning and regeneration steps necessary to obtain stable repetitive response, simple constant potential operation is not possible, and double or triple pulsed potential waveforms are required for acceptable long-term usage of these electrode materials. Detector systems capable of generating such waveforms and efficiently sampling the resulting currents have now become commercially available, but most LCEC detection units in use permit only constant potential operation. Furthermore, compared to the usual constant potential LCEC techniques, pulsed detection with its accompanying charging current seems inherently less sensitive for most chromatographic and flow injection analysis applications. In the second approach, catalytic electrodes employing an oxidizable metal such as nickel (13-15), copper (16),or silver (16) or containing a surface-attached electron transfer mediator such as cobalt phthalocyanine (17,18) have been developed and characterized. For these, the lowest detection limits-1 ng for glucose-have been found with Ni electrodes in distinctly alkaline solution. However, the cobalt phthalocyanine (CoPC) chemically modified electrodes (CMEs) have been shown to provide a considerable degree of flexibility in sugar detection at the cost of only slightly poorer detectability. The largest drawback of the CoPC CMEs is that, as with the Pt and Au electrodes, stable long-term operation is obtained only with use of inconvenient potential pulsing of the electrode surface. In the present work, a new electrocatalytic CME has been developed that provides stable detection at constant potential. The new CME consists of a conventional glassy carbon electrode coated with a Cu(I1) layer that serves to decrease the overpotential for carbohydrate oxidation compared to that seen a t unmodified carbon electrodes. The catalytic process, which occurs upon electrooxidation of the Cu(I1) to Cu(III), takes place at modest values of applied potential and enables detection of mono- and disaccharides at the subnanogram level. Moreover, because stable, long-term operation of the CME is possible without manipulation of the applied potential, the comparatively simple constant potential amperometric instrumentation available in most laboratories is all that is needed. EXPERIMENTAL SECTION Reagents. Stock solutions of carbohydrates, purchased from Sigma or Fisher Chemical Co., were prepared fresh daily in deionized water and, just prior to use,were adjusted to the desired
853
,. ,,
+Oh
+0:6
POTENTIAL
+0:4
+0.2
60
, Volts VS. Ag/AgCl
Flgure 1. Cyclic vottammograms of Cu CME in 0.15 M NaOH: (A) initial scan, (B) fourth scan; scan rate, 20 mV/s.
concentration and pH by dilution with the appropriate mobile phase solution. Mobile phase was prepared from carbonate-free 50% NaOH and thoroughly degassed deionized water. Electrodes. Glassy carbon electrodes used for cyclic voltammetry and flow experiments were purchased from Bioanalytical Systems (West Lafayette, IN). Modified electrodeswere prepared by placing the plain glassy carbon electrode (previously polished with alumina) in a thin-layer flow cell and passing 0.15 M NaOH over its surface for about 10 min. The electrode was then rinsed thoroughly with water and immersed face-up in an aqueous 50 mM solution of CuC12where, after about 5 min, a greenish Cu(I1) layer became visible on the glassy carbon surface. After removal from the CuC12and rinsing with deionized water, the electrode was ready for use. The copper coating remained stably attached to the glassy carbon surface under most pH conditions but could be removed by brief immersion of the surface in 0.5 M HCl. Apparatus. Cyclic voltammetry (CV) was performed with a Bioanalytical Systems Model CV-1B potentiostat with a Model MFlOOO glassy carbon working electrode (modified or unmodified), a home-made Ag/AgCl (3 M KC1) reference electrode, and a platinum wire auxiliary electrode. Flow injection and liquid chromatographyexperimentswere carried out with a Waters M-45 pump, a Rheodyne (Berkeley, CA) Model 7125 injector with a 20-fiL sample loop, an SSI Model LP-21 pulse dampener, and an IBM Model EC/320 electrochemical detector. All chromatographic separations utilized a 15 cm long, 4 mm i.d. Dionex HPIC-AS6 anion exchange column at room temperature. RESULTS AND DISCUSSION Electrochemistry. CVs obtained for the Cu-coated CME immersed in 0.15 M NaOH are shown in Figure 1. During the first scan in the anodic direction (curve A), a single, somewhat broad oxidation with a peak potential of +0.45 V vs Ag/AgCl was observed. On subsequent scans, this wave decreased in magnitude and rather rapidly disappeared (curve B). Allowing the potential to proceed to more negative values before scan reversal produced additional small redox waves, both anodic and cathodic, but did not restore the +0.45 V wave. None of these processes was seen for analogous experiments performed under the same conditions but with an unmodified glassy carbon electrode in place of the CME. Similar observations were reported previously by Miller (19) for ring-disk voltammetric experiments performed at metallic copper electrodes likewise immersed in strongly basic solution. In Miller’s work, a total of three separate oxidation waves was seen upon anodic scanning from -0.6 to +0.6 V vs SCE. These occurred at -0.43, -0.18, and +0.50 V and were shown to correspond respectively to the Cu(O)/Cu(I), Cu(O)/Cu(II), and Cu(II)/Cu(III) redox couples. In our case, only a single wave at the potential of the last process-Le., Cu(II)/Cu(III)-was seen; but, with the CME, it was cupric ion, and not metallic copper, that had been deposited onto the glassy carbon surface. Therefore, it seems most likely that the CME oxidation
854
ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989
FLC 200 PA
+O.S
+0:6
+64
+0:2
60
POTENTIAL , Volts vs. Ag/AgCI Figure 2. Cyclic voltammograms of Cu CME in 0.15 M NaOH plus (A) 1.5 mM glucose, (B) 3.0 mM glucose, and (C) 4.5 mM glucose; scan rate, 20 mV/s.
seen in Figure 1 corresponds to the formation of Cu(II1). Interestingly, the Cu(II)/Cu(III) wave described by Miller also decreased rapidly on continued scanning due apparently to a very rapid film formation and passivation process. Addition of millimolar levels of glucose and other simple carbohydratesto the NaOH solution used above produced CVs similar to those shown in Figure 2. In all cases, the Cu CME gave anodic currents, proportional in magnitude both to the carbohydrate concentration used and to the square root of the scan rate (up to 60 mV/s), at a potential similar to that given by the CME by itself in Figure 1. However, unlike the response obtained for the CME in the blank NaOH solution in Figure 2, the +0.45 V oxidation wave in the presence of carbohydrates was extremely reproducible and could be scanned nearly indefinitely with no evidence of film formation or electrode passivation. Thus, although electrochemical regeneration of the initial active CME surface was not able to be accomplished simply by application of negative potentials, the electrocatalytic process involved in the carbohydrate oxidation apparently was quite effective in returning the Cu layer to its original-presumably Cu(I1)-state. This capability was further examined by cycling a fresh CME in blank NaOH solution until the Cu oxidation wave had disappeared and then briefly immersing the electrode surface (with no potential applied) into a solution containing glucose. After removal of the CME from the glucose solution, replacing the CME back into the blank NaOH, and resuming CV scanning, an oxidation wave similar to that seen for the fresh CME surface was again observed on the initial few scans. Analogous results were obtained by exposing the CME to hydrogen peroxide and other common reducing agents in place of glucose. All of these observations are consistent with the electrocatalytic oxidation of the carbohydrate compound by a high oxidation state of Cu generated electrochemically on the CME surface. Considering the +0.45 V potential required for the oxidation, the catalytically active moiety is most likely a Cu(II1) species. Furthermore, the anodic production of the Cu(II1) appears to be an irreversible process (perhaps due to film formation); but oxidative regeneration of the starting surface by chemical redox reaction is possible. An important consequence of these observations for use of the Cu CME in carbohydrate detection is that stable, constant potential oxidation of sugars such as glucose would seem to be feasible without the need to cycle or pulse the applied potential rep-
etitively to negative values as has been required with Pt (M), Au (9-12), and cobalt phthalocyanine carbon paste (17,18) electrodes used in most previous LCEC schemes. A glucose oxidation qualitatively similar to that obtained with the CuCME could be generated at a metallic Cu electrode; but, because of high background currents and poorly reproducible carbohydrate currents, the analytical performance of the solid Cu electrode was greatly inferior to that of the Cu-coated CME. The loading of copper on the CME surface was estimated by integration of the +0.45 V wave obtained on the initial CV scan for several different electrodes in 0.15 M NaOH solution. Four independently prepared surfaces exhibited a mean coverage of 26 pg of Cu/cm2 with a standard deviation of 5.7 pg (or 22%). This degree of electrode-to-electrode variation presented no practical difficulty because the high stability of each individual CME surface made electrode regeneration or replacement a relatively infrequent occurrence. One limitation of the Cu CME, shared with all of the other electrode systems used thus far for carbohydrate oxidation, was the need for strongly basic solution conditions. Roughly the same current levels seen in Figure 2 for 0.15 M NaOH persisted down to approximately pH 12. But for lower pH values, the catalytic currents decreased drastically, disappearing completely for hydroxide concentrations below 0.001 M. LC/EC.On the basis of the CV results described above, it seemed likely that amperometricdetection of carbohydrates via flow injection or LC/EC might be able to be carried out at the Cu CME a t potentials on the order of +0.5 V vs Ag/ AgC1. This potential is easily low enough to provide the high sensitivity and selectivity for which LC/EC is noted. This was confirmed by the series of glucose injections shown in Figure 3 where the CME response obtained at +0.48 V is illustrated. The results shown represent the current obtained under flow injection conditions for 80 injections of a 10 pM glucose solution over a period of more than an hour. After the 80th injection, the peak response was still more than 90% of its initial value, and the relative standard deviation for the series of injections was less than 5%. This result compares quite favorably with our earlier work (17,18) with cobalt phthalocyanine CMEs where the response decreased by nearly two-thirds after only four injections at constant potential. The long-term stability of the Cu CMEs was such that, despite this slow decrease in response, the electrodes’ useful lifetime in the flow system was at least 2-3 days. Even after continuous exposure to the 0.15 M NaOH stream for this length of time, quantitation of the carbohydratesat submicromolar levels was still easily accomplished. The reason for the slow decrease in electrode response is not known for certain. Most likely, partial leaching of copper from the CME surface may be taking place under the flow conditions; alternatively, adsorption of oxidation products onto the electrode surface might also be occurring. The hydrodynamic voltammogram (HDV, Figure 4) for glucose obtained at the Cu CME under flow injection conditions was identical in shape with the corresponding glucose CV, exhibiting maximum electrode response at slightly less than +0.6 V. Unlike HDVs observed for uncomplicated diffusion limited electrode processes, the HDV obtained here did not have a simple plateau shape. Rather, the HDV was distinctly peak shaped showing a decrease in current for injections made at higher potentials. The requirement of the carbohydrate electrocatalysis for very basic solution conditions presented no chromatographic difficulty as these mobile phase conditions have been shown to be effective for the ion exchange separation of carbohydrates (3, 15). In fact, as shown in Figure 5,O.lO M NaOH proved to be a very effective mobile phase for both carbohydrate
ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989 10 20
30
40 50
53 56
60
63
66 70
855
80
T
'I"
nA
1
Figure 3. Flow injection response obtained at the Cu CME for injection of 10 pM glucose: E, +0.48 V vs Ag/AgCI; mobile phase, 0.15 M NaOH; flow rate, 1.0 mL/min. 14
fE2
A Table I. LC/EC of Carbohydrates at the Cu CME
n
2
compound
d +0:1
+0:2
+0:3
+&
retention time, min
+0.5
+0.6
+0.7
Potential ,V vs Ag/ AgCl
Figure 4. Hydrodynamic voltammogram for 0.15 mM glucose at the Cu CME. Flow conditions were the same as in Figure 3.
separation and detection. Using +0.48 V vs Ag/AgCl as the working electrode potential, the detection limit (signal/noise = 3) for glucose was 0.22 ng (or 1.2 pmol), and the linear dynamic range extended from 6.2 X to 5.0 X lo4 M for the 20-pL sample size employed. Similar performance, summarized in Table I, was obtained for all the other sugars examined. Numerous aspects of the Cu CME and its catalysis of carbohydrate oxidation still remain to be fully characterized. However, even at this early stage, the analytical performance of this electrode in the LC/EC determination of carbohydrates appears to be clearly superior in detection limits and linear range to that reported previously for metallic electrodes. A glucose detection limit of 0.22 ng with the Cu CME compares favorably with limits reported for platinum (3, gold (12), nickel (15),and cobalt phthalocyanine (18)electrodes. For example, Neuberger and Johnson (20) have recently described a coulometric variation of the commercially available pulsed amperometric/gold electrode approach which improved glucose detection capabilities from 315 to 9 ng (for nonchromatographic flow injection determinations). This corresponds to a figure of merit 40-fold poorer than that obtained here using simple constant potential amperometry at the Cu CME in actual chromatography experiments. At the same time, the CME is quite stable, and its operating requirements are fully compatible with the constant potential LCEC instrumentation currently in widest use. Furthermore, the CME itself is easily prepared by simply depositing a Cu film onto an ordinary
detection limit" ng pmol
glucose D-ribose
3.7
0.22
1.2
4.5
fructose
4.8 6.3 6.1 10.2 2.9
0.22 0.27
0.85
1.5 1.5 2.5
0.48 0.90 0.67
1.4 2.5 5.0
sucrose lactose maltose 2-deoxy-~-ribose
linear range: M (20 p L injection) 6.2 X 6.8 X 6.8 X 1.2 X 6.8 X 1.2 X 2.5 X
10% X 10-'-5 X 10-"-5 X 10-'-5 X 10% X
10-'-5 10-I-8
X X
lo-' lo-' lo-' lo-'
lo-' lo-' lo-'
"Using constant potential amperometric detection at +0.48 V vs Ag/AgCl. bFrom least-squares analysis of at least three concentrations; correlation coefficients were always greater than 0.99. glassy carbon substrate. We anticipate that further improvements may be achieved for the CMEs by more systematic study of the Cu deposition process and that extension of the work for the detection of numerous compounds related to the simple carbohydrates considered here will be straightforward.
LITERATURE CITED (1) (2)
(3)
Honda, S. Anal. Blochem. 1984, 140, 1-47. Robards, K.; Whitelaw, M. J . Chromatogr. 1988, 373, 81-110. Olechno. J. D.; Carter, S. R.; Edwards, W. T.; Gillen. D. 0.Am. B b technd. Lab. 1987. (Sept-Oct),38-50.
(4) Gross, A. F.; Wen, P. S., Jr.; Athnaslos, A. K. Anal. Chem. 1987, 5 9 , 212R-252R. (5) Chapiin. M. F. In Carbohyhte Analysis; Chaplin, M. F., Kennedy, J. F., Eds.; IRL Press: Oxford, 1986; Chapter 1. (8) Hughes, S.; Johnson, D. C. Anal. Chim. Acta 1981, 132, 11-22. (7) Hughes, S.; Johnson, D. C. J . Agric. Food Chem. 1982, 30, 712-714. (8) Hughes, S.; Johnson, D. C. Anal. Chlm. Acta 1983, 149, 1-10, (9) Neuburger, 0.0.;Johnson, D. C. Anal. Chem. 1987, 59, 203-204. (IO) Edwards, P.; Haak, K. K. Am. Lab. 1983, (April), 78-84. (11) Rockiin, R. D.; Pohl, C. A. J . Llq. Chromafogr. 1983, 6 , 1577-1590. (12) Neuburger. G. G.; Johnson, D. C. Anal. Chem. 1987, 59, 150-154. (13) Schlck, K. G.; Magearu. V. G.: Huber, C. 0. Clin. Chem. 1978, 2 4 , 448-450.
858
Anal. Chem. 1989, 61, 856-860
(14) Buchberger, W.; Wlnseuer, K.; Breltwieser, C. H. Fresenlus' Z . Anal. Chem. 1989. 375, 518-520. (15) Reim. R. E.: Van Effen, R. M. Anal. Chem. 1988, 58, 3203-3207. (16)Vassilev. Y. 6.: Kharova. 0. K.: Nlkolaeva. N. N. J . E/ectroana/.
RECEIVED for review October 24,1988. Accepted ~ a n u a r y23,
(17) Chem. Santos,1985, L. M.;196. Baldwin, 127-144. R. P. Anal. Chem. 1987. 59, 1766-1770. e 206, 85-98. (18)Santos, L. M.; Baldwin, R. P. Anel. Chim. ~ c t 1988. (19) Mlller, B. J . Elecfrochem. SOC. 1989, 716, 1675-1880.
1989. This work was supported by the National Science F~undationthrough EPSCoR Grant 86-10671-01 and by Burdick & Jackson Research Grant BJ8807.
(20) Neuburger, 0.G.;Johnson, D. C. Anel. Chim. Acta 1987, 192, 205-213.
Deposition of Trace Metals on Solid Electrodes: Experimental Verification of Limits of Electrochemical Preconcentration Aleksander Ciszewski,' Judith R. Fish, and Tadeusz Malinski* Department of Chemistry, Oakland University, Rochester, Michigan 48309-4401
Roman E. Soda2 Department of Chemistry, The University of Rhode Island, Kingston, Rhode Island 02881-0801
Experimental .data obtained by dlfferentlal pulse anodlc strlpplng voltammetry and electrothermal vllporization Inductlvely coupled plasma atomic emlsslon spectroscopy determlnatlons of amounts of copper and lead deporlted on glassy carbon electrodes ersentlally valldate a recently published model for the process of preconcentratlon of metal Ions by electrodepooltion. Data elucidating the related phenomena of Bmoctnt d8podt* equmrhm concenkatbn of U n r ~ V e r e d Ions, and percent recovery have been determlned and are discussed. For solution concentrations that allow mrinHayer electrock c?versge (usually greater than lo-' M), ec@brh concentrations of cations remalnlng In solullon are ladependent d Inttlal concentratlons. For concentratknsfdevant to trace metal analyols, less than lo-' M, and monolayer or less electrode ooverage, data show Inltlakoncentratlon-dependent liinlts to electrochemlcal preconcentratlon, which decrease percent recovery. Electrodepodtion parameters that mlnhnlre equlllbrlum concentration levels are dlocussed. The analytical Importance of the verlfled model b that, for a set of experlmental condltlons, It allows calculation of predlcted recoverbs from data collected at macrocorlcentratbn levels to concentratlm levels 3 magnltudes lower.
INTRODUCTION Preconcentration techniques, which separate trace metals from interfering matrix components, can improve detection limits, enhance accuracy, ease calibration, and, also, effect more representative results due to the increased sample size. Electrolytic preconcentration (EP) requires a single controlled potential electrolysis (CPE) to concentrate traces of reducible metals as a deposit on an inert electrode or amalgam in mercury, leaving electrochemically inactive interfering elements in solution. This technique is frequently used for improving detection limits in many instrumental methods of analysis (1-3) and has been used in flameless atomic ab-
'Permanent address: Technical University of Poznan, Department of Analytical Chemistry, 60-695Poznan, Poland. 2Presentaddress: Industrial Chemistry Institute,01-793Warsaw, Poland.
sorption spectroscopy (ETA-AAS) (4-8), neutron activation analysis (NAA) (9-11), and inductively coupled plasma atomic emission spectroscopy (ICP-ES) using a variety of electrodes and sample introduction methods (12-19). Detection limits in the range of parts per billion (ppb) for multielemental analysis of environmentally important metals in natural waters and biological fluids have been achieved. The method of EP-ICP-ES using electrothermal vaporization (ETV) has been optimized to afford sufficient preconcentration to allow accurate multielemental determinations of cations approaching parts per trillon (pptr) levels (18, 19). Also, anodic stripping voltammetry (ASV), a method that intrinsically combines E P with voltammetric determination, has achieved, under many variations, detection limits in the range of 0.1-0.01 ppb (20). The concept of absolute detection limit for an instrumental technique refers to the minimum mass of analyte that must be introduced to the instrumentation as sample in order to achieve a signal to noise ratio of 3 to 1. Electrolytic preconcentration extends the lower limits of solution concentrations from which this minimum amount may be obtained. Thus, it would seem that in any combined method, the limit of detectable concentration will ultimately depend upon the efficiency of the electrodeposition process, which in turn depends on the applied potential and absence of reactions that interfere with deposition. A t ultratrace metal ion concentrations, electrodeposition half-lives have been observed to much longer than at moderate concentrationsand stationary solution concentrationsof metal ions have been reached during electrolysis (10, 18, 21). A model that ansumed simultaneous metal electrodeposition and chemical dissolution of deposit had proposed the establishment of different equilibrium concentrations for different ranges of initial solution concentration of metal ion being deposited (22,23). Most recently, a more general model has been developed, which leads to a single equation for the equilibrium concentration of metal ions in solution and unites two limiting approaches of the earlier model (24). The purpose of the present work is to verify this most recent model by experimental determinations of equilibrium metal ion concentration levels following deposition on glassy carbon electrodes by differential pulse ASV and ETV-ICP-ES. Experimental data obtained essentially validate the model. This
0003-2700/89/0361-0856$01.50/0 0 1989 American Chemical Society
