Anal. Chem. 19S0, 62, 2697-2702 (21) Braman, R. S.;Foreback. C. C. Science 1973, 182, 1247. (22) Andreae, M. Deep-sea Res. 1978, 2 5 , 391. (23) Maher, W.; Butler, E. A m / . Opsnomef. Chem. 1988, 2 , 191. (24) Cullen, W. R.; Rekner, K. J. Chem. Rev. 1989, 89, 713. (25) Florence. M.; Batby, 0.Chem. Aust. 1988, 5 5 , 363. (28) Andreae, M. 0.Anal. Chem. 1977, 49, 820. (27) B u m , D. S.;Kruil, 1. S.; Demko, P. R.; Smith, S. B., Jr. J . Chromafogr. 1984, 7 , 861. (28) Urasa, I. T.; Ferede. F. Anel. Chem. 1987, 59, 1563. (29) Maltani, T.; Uchlyana, S.; Salto, Y. J . Chromafogr. 1987. 397, 161. (30) Landsberger, S. Anal. Chem. 1988. 60, 1842. (31)Lown, J. A.; Johnson, D. C. And. Chlm. Acfa 1980, 176, 41. (32) Tan. L. K.; Dutrizac, J. E. Anel. Chem. 1986, 58. 1383. (33) Butler, E. C. V. J . Chromafcgr. 1988. 450, 353. (34) Williams, R. J. Anal. Chem. 1983, 5 5 , 651. (35) Kissinger, P. T.; Heineman, W. R. Lah8fory Techniques in Ekfrasnaiyflcal Chemistry; Kisslnger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984;p 289. (36) Cox, J. A.; Kulkarni. K. R. Teienta 1988, 33, 911. (37) Cox, J. A.; Kulesza. P. J. Anal. CY". 1984. 56, 1021. (38) &&wig, F. G.;Valenta, P.; Nurnberg, H. W. Fresenlus' Z . Anal. Chem. 1982, 311, 187. (39) Bond, A. M.; Luscombe, D.; Oldham, K. B.; Zoskl, C. G. J . Electroa-
.
.
nal. Chem Inferfacie1 Elecfrochem 1988, 249, 1.
(40) Thormann, W.; Bond, A. M. J . Elecffasnal. Chem. InferfacielElecfrochem. 1987, 278, 187. (41) Thormann. W.; van den Bosch, P.; Bond, A. M. Anal. Chem. 1985, 5 7 , 2764. (42) Pesavento, M. Talante 1989, 36, 1059.
2897
(43) Cabeka, T. D.; Austin, D. S.; Johnson, D. C. J . Elecfrochem. Soc. 1984, 131, 1595. (44) Austin, 0 . S.;Polta, J. A.; Pdta, T. 2.; Tang, A. P.C.; Cabelka, T. D.; Johnson, D. C. J . Ekfroanal. Chem. InferfacielEleCfrochem. 1984, 768, 227. (45) Kao, W.-H.; Kuwana, T. J . Electroanel. Chem. Interfacial Elecfroet".1984, 789, 187. (46) LouEka. T. J . Electfoanal. Chem. InfetfaclelElecfrochem. 1973, 47.
103. (47) Weber, S.G.;Purdy, W. C. Anal. Chim. Acta 1978, 100, 531. Long, J. T. Anal. Chem. 1988, 80, 903A. (48) Weber, S.0.; (49) Weber, S.G. AMI. Chem. 1988, 60, 2309. (50) Scott, R. P. W. LiquM Chromatography Defectors, 2nd ed.;Elsevier: Amsterdam, 1988;p 12. (51) Goto, M.; Shimada, K. Chromafogf8phie 1988, 2 7 , 831. (52) White, J. G.;Jorgenson, J. W. Anal. Chem. 1988, 58, 2992. (53) Yamada. J.; Matsuda, H. J . €lecfroanal. Chem. Interfaciel Elecfrochem. 1973, 44, 189. (54) Eibicki, J. M.; Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 56, 978. (55) Farmer, J. G.;Johnson, L. R. Mvk-on. Geochem. Health 1985, 4 , 124. (56) Puttemans, F.; Massart, D. L. Anal. Chim. Acta 1982, 147, 225.
RECEIVED for review June 19,1990. Accepted August 30,1990. This work was supported by a CSIRO/Deakin University Collaborative Research Award.
Anodic Stripping Voltammetry at Mercury Films Deposited on Ultrasmall Carbon-Ring Electrodes Danny K. Y. Wong and Andrew G. Ewing* Department of Chemistry, 152 Davey Laboratory, Penn State University, University Park, Pennsylvania 16802
Anodic strlpplng voltammetry of lead and cadmlum without deliberately added electrolytes has been studied at uitrasmaii carborrrlng electrodes following In sltu deposttion of mercury. The stripping of lead has been studled in detail to investigate the dependence of strlpplng peak current on experlmentai parameters such as potential scan rate, preconcentration duration, deposition potential, concentratlon of Hg' during deposition, and Concentration of Pb2+. Anodlc strlpplng voltammetry In solutions without deilberatety added supporting electrolyte avolds problems associated with knpurlties Introduced when electrolyte Is added. These Impurities appear to be highly anportant when Pb2+analysis Is carried out in dlute solutions. I n addition, a unique effect Is observed when relatlvely low concentrations of Hg' (1.0 X lod M) are used for the in situ deposition step. When low Hg+ concentrations are used, the stripping current does not decrease as rapidly as expected as the concentration of Pb2+ is reduced. The slope of the ~ps-iogplot of peak current M m2+concentration is sigrtlfkantly leils than unlty, but the callbratkn plot is ilnear, and the resuitlng enhanced peak currents at low Pb2' concentrations make strippkrg analysls possible at extremely low concentrations. Concentrations as low as 3.2 X lo-'' M of Pb2+ have been examlned.
INTRODUCTION Anodic stripping voltammetry (ASV) has always been regarded as one of the most sensitive techniques for trace-metal analysis. In general, this technique is a nondestructive method
* To whom correspondence should be addressed. 0003-2700/90/0362-2697$02.50/0
and is applicable to multielemental analysis. The part per billion detection limit (1) of stripping analysis is attributable to the preconcentration that takes place during the deposition step. ASV is often carried out with a mercury-thin-film electrode (21, which provides a large surface area to volume ratio, and this yields superior sensitivity during voltammetry. There has also been intense study in the fabrication and development of micro voltammetric electrodes (3-5) during the past decade. Microelectrodes offer a number of attractive features suitable for many electroanalytical applications over conventional-sized electrodes. Small double-layer capacitance owing to the small electrode surface area results in diminished electrode charging current. This produces a greater ratio of faradaic to nonfaradaic current and can lead to enhanced detection limits (6). Additionally, the small currents passed by microelectrodes result in negligible ohmic losses and this allows electroanalysis to be carried out in poorly conductive media. This permits the use of systems without any deliberately added supporting electrolyte. Impurities introduced with electrolyte are thus eliminated, and the range of potentials accessible for electrochemical measurements can be extended (7). The combination of small capacitive charging current and small potential drop across the uncompensated cell resistance provides a system where high scan rate cyclic voltammetry is possible. In fact, scan rates above 1000000 V s-l have been demonstrated (8). In contrast, at low scan rates (
