Anal. Chem. 2004, 76, 2415-2418
Correspondence
Anodic Stripping Voltammetry Enhancement by Redox Magnetohydrodynamics Emily A. Clark and Ingrid Fritsch*
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701
The effect of an external magnetic field on linear scan anodic stripping voltammetry (ASV) in solutions of 10-610-7 M concentrations of lead, cadmium, and copper at mercury films on glassy carbon electrodes has been investigated. A high concentration of Hg2+ was added to the analyte solution to induce a large cathodic current during the deposition step. Therefore, a large Lorentz force from the net flux of charge through the magnetic field resulted in convection due to magnetohydrodynamics. The faster delivery of analytes to the mercury film electrode during deposition caused an increase in the anodic stripping peaks. The effect of varying Hg2+ concentrations (0-60 mM) and magnetic field strengths (0-1.77 T) on the enhancement of the stripping peaks was investigated. Enhancements as large as 129% for peak currents and 167% for peak areas were observed. An enhancement of ∼100% was observed when 60 mM Fe3+ replaced high concentrations of Hg2+. This method of convection exhibits promise for small-volume ASV analysis with possible improved limits of detection and decreased preconcentration times. Anodic stripping voltammetry (ASV) is commonly used to analyze metals (e.g., lead, cadmium, copper, zinc, indium, etc.) in streams and lakes1-4 and biological fluids.5-7 It has also been used to detect DNA hybridization.8,9 In ASV, a deposition (preconcentration) step reduces chemical species of interest at an electrode and a second step applies a potential function that reoxidizes (or strips) the species.10 The magnitude of the resulting * To whom correspondence should be addressed. Tel: (479) 575-6499. Fax: (479) 575-4049. E-mail:
[email protected]. (1) Richter, P.; Toral, M. I.; Abbott, B. Electroanalysis 2002, 14, 1288-1293. (2) Prakash, R.; Srivastava, R. C.; Seth, P. K. Electroanalysis 2001, 14, 303308. (3) Emons, H.; Baade, A.; Schoning, M. J. Electroanalysis 2000, 12, 11711176. (4) Petrie, L. M.; Baier, R. W. Anal. Chem. 1978, 50, 351-357. (5) Liu, T. Z.; Lai, D.; Osterloh, J. D. Anal. Chem. 1997, 69, 3539-3543. (6) Hardcastle, J. L.; Compton, R. G. Electroanalysis 2002, 14, 753-759. (7) West, C. E.; Hardcastle, J. L.; Compton, R. G. Electroanalysis 2002, 14, 1470-1478. (8) Authier, L.; Grossiord, C.; Brossier, P. Anal. Chem. 2001, 73, 4450-4456. (9) Wang, J.; Liu, G.; Polsky, R.; Arben, M. Electrochem. Comm. 2002, 4, 722726. (10) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. 10.1021/ac0354490 CCC: $27.50 Published on Web 03/20/2004
© 2004 American Chemical Society
anodic peak current is proportional to the amount of analyte reduced in the preconcentration step and, therefore, the concentration of analyte in the sample.4,10,11 Mercury is often the electrode material of choice because it has a higher hydrogen overvoltage than solid electrodes, allowing a larger cathodic potential window, and its surface is highly reproducible and easy to renew.11 Dynamic mercury film formation was employed in this work, in which the preconcentration step involves a mercury film formed at a solid electrode by codeposition of mercury (added to the sample solution) with the analytes.4,12,13 Stirring the analyte solution during the deposition step delivers analytes to the electrode faster and improves detection limits or shortens deposition times to achieve a desired detection limit. However, reproducible mechanical stirring of small samples and stirring in portable devices is challenging. We report the use of redox magnetohydrodynamics (MHD) to induce convection in the analyte solution during the deposition step. In the work described here, the codeposition of analytes with mercury (or other redox species) at a much higher concentration than the analyte generates a large cathodic current, J (A/m2), in the presence of the magnetic field, B (T), of an electromagnet to exert a Lorentz force, FL (N/m3), on the charge-carrying ions, whose magnitude and direction are governed by the right-hand rule:14,15
FL ) J × B The resulting MHD convection delivers species to the electrode at a faster rate, yielding higher quantities of analytes in the mercury thin film and, therefore, larger anodic stripping peaks. Thus, sensitivity improves. Although the use of MHD to control deposition of metals onto solid electrodes (e.g., electroplating) has been investigated previously,16,17 magnetic field effects on analytical methods, such as ASV, have not. MHD convection exhibits several advantages. It requires no moving parts (e.g., no stirrer or rotating disk electrode). The (11) (12) (13) (14)
Wang, J. Analytical Electrochemistry, 2nd ed.; Wiley-VCH: New York, 2000. Florence, T. M. J. Electroanal. Chem. 1970, 27, 273-281. Kumar, V.; Heineman, W. R. Anal. Chem. 1987, 59, 842-846. Grant, K. M.; Hemmert, J. W.; White, H. S. J. Am. Chem. Soc. 2002, 124, 462-467. (15) Leventis, N.; Gao, X. J. Phys. Chem. B 1999, 103, 5832-5840. (16) Tacken, R. A.; Janssen, L. J. J. J. Appl. Electrochem. 1995, 25, 1-5. (17) Fahidy, T. Z. J. Appl. Electrochem. 1983, 13, 553-563.
Analytical Chemistry, Vol. 76, No. 8, April 15, 2004 2415
voltage requirements remain low; the reducing potential of the metal analyte is sufficient. The redox MHD produces continuous convection; the Lorentz force exists for the duration of the deposition. To determine the effectiveness of MHD convection on ASV, we compared results from experiments performed in the absence and presence of a magnetic field. Analytes (2 µM) of Pb2+, Cd2+, and Cu2+ were analyzed by ASV at a glassy carbon electrode with concentrations of Hg2+ from 0 to 60 mM. We also determined the effect of magnetic field strength up to 1.77 T with solutions containing 0.8 µM Pb2+ in 40 mM Hg2+. To demonstrate that alternative redox species can be used to induce MHD convection for ASV, we added ∼60 mM Fe3+ to a solution of 2 µM Pb2+ and a low concentration (1 mM) of Hg2+. Although several other electrochemical methods to strip metals from the mercury film electrode10 have demonstrated 10-10-10-12 M detection limits (e.g., differential pulse voltammetry18-20 and square wave voltammetry21,22), we chose linear scan ASV (with somewhat higher detection limits) for our initial MHD studies because of its simplicity. EXPERIMENTAL SECTION Chemicals and Materials. All chemicals were reagent grade and used as received. Aqueous solutions were prepared with highpurity deionized (DI) water (Milli-Q, model RG). Nitric acid, potassium nitrate, copper(II) nitrate pentahemihydrate (all certified ACS grade), and ferric nitrate nonahydrate (Fe(NO3)3‚9H2O) were obtained from Fisher Scientific (Fair Lawn, NJ). Mercury(II) nitrate monohydrate (99.99+%) and cadmium nitrate tetrahydrate were purchased from Aldrich Chemical Co. (St. Louis, MO). Lead(II) nitrate (99.2%) was acquired from J.T. Baker Chemical Co. (Phillipsburg, NJ). The 3-mm-diameter glassy carbon working electrode was purchased from CH Instruments, Inc. (Austin, TX). A platinum flag counter electrode was constructed from a Pt sheet (99.99%, 0.1 mm thick, Alfa Aesar, Ward Hill, MA) spot welded to a Pt wire, the free end of which was sealed in a glass tube where connection was made to a copper wire. Electrochemical Measurements. An Electrochemical Workstation model 750A (CH Instruments, Inc.) was controlled by a PC computer running CHI-750A software. A Ag/AgCl (saturated KCl) electrode served as a reference. Before each experiment, the potential at the working electrode was held at 1 V for 60 s in electrolyte (30 mM HNO3/0.1 M KNO3) to oxidize mercury remaining from the previous experiment. Then, the working electrode was polished sequentially with 1-µm diamond (Bioanalytical Systems, Inc., West Lafayette, IN) and 0.05-µm alumina (Buehler, Lake Bluff, IL) polish, followed by sonication (Branson 2210, Branson Ultrasonic Corp.) for 30 s in DI water. The electrodes were placed in a round-bottom culture tube located between the poles of an electromagnet (New England Techni-Coil). The electromagnet was powered by an EMS 100-50 (18) Cushman, M. R.; Bennett, B. G.; Anderson, C. W. Anal. Chim. Acta 1981, 130, 323-327. (19) Mart, L.; Nuernberg, H. W.; Valenta, P. Fresenius Z. Anal. Chem. 1980, 300, 350-362. (20) Lukaszewski, Z.; Karbowska, B.; Zembrzuske, W. Electroanalysis 2002, 15, 480-483. (21) Lu, T. H.; Yang, H. Y.; Sun, I. W. Talanta 1999, 49, 59-68. (22) Parthasarathy, N.; Pelletier, M.; Tercier-Waeber, M. L.; Buffle, J. Electroanalysis 2001, 13, 1305-1314.
2416 Analytical Chemistry, Vol. 76, No. 8, April 15, 2004
Figure 1. Effect of different concentrations of Hg(NO3)2 on ASV at 0 (dashed curve) and 1.77 T (solid curve). Linear sweep (2 V/s) was performed after 60-s deposition in solutions containing Hg(NO3)2 and 2 µM Cu(NO3)2, 2 µM Pb(NO3)2, 2 µM Cd(NO3)2, 30 mM HNO3, and 0.1 M KNO3 and after rinsing with electrolyte three times over 30 s.
power supply (Lambda EMI) and cooled by an Affinity F-series chiller (Lydall). The magnet poles, with 7.5-cm diameter, were set to a 2.8-cm gap. A hole was drilled in the bottom of the culture tube to which Tygon tubing was epoxied. A pinch clamp was secured on the tubing to control flow from the culture tube that results from the pull of gravity. The working electrode faced downward, so that its surface normal was perpendicular to the magnetic field to achieve a maximum Lorentz force, inducing a MHD flow parallel to the electrode surface. The tip of the reference electrode was positioned beside the working electrode with the counter electrode residing on the bottom of the culture tube, ∼1 cm below the working and reference electrodes.23 The current in the magnet power supply was adjusted to reach the desired magnetic field strength: from 0 T at 0 A to 1.77 T at 50 A. The magnetic field strength was measured with a GM1A gaussmeter (Applied Magnetics Laboratory, Inc., Baltimore, MD). At the beginning of the analysis, the culture tube contained 1 mL of deposition solution consisting of analytes (Pb2+, Cd2+, Cu2+), Hg2+ or a combination of Hg2+ and Fe3+, and electrolyte (30 mM HNO3 and 0.1 M KNO3). After 60 s of deposition at -0.8 V, 3 mL of electrolyte was added to the tube and the pinch clamp was opened to drain the solution from the bottom of the tube back to the 1-mL level. This procedure was repeated at 70 and 80 s (with the working electrode still held at -0.8 V) to lower the concentration of mercury (or iron) in solution and thereby lower the cathodic background current that results from reduction of the high concentration of mercury (or iron). The stripping step (2 V/s) commenced at 90 s in the remaining electrolyte solution. RESULTS AND DISCUSSION The anodic stripping peaks that result from linear scan ASV of solutions containing 2 µM concentrations of Cu2+, Pb2+, and Cd2+ with various concentrations of Hg2+ and at 0 and 1.77 T are shown in Figure 1. The analytes are discriminated by their different redox potentials. At 0 mM Hg2+, the stripping peaks are small, and those for Pb and Cd are unresolved, presumably due to the presence of bare glassy carbon. The analyte peaks become slightly larger with increasing Hg2+ concentration at 0 T, because of a small contamination of copper, lead, and cadmium in the Hg(NO3)2. (23) Leventis, N.; Chen, M.; Gao, X.; Canalas, M.; Zhang, P. J. Phys. Chem. B 1998, 102, 3512-3522.
Figure 2. Effect of different magnetic field strengths on ASV. Linear sweep (2 V/s) was performed after 60-s deposition in a solution containing 0.8 µM Pb(NO3)2, 40 mM Hg(NO3)2, 30 mM HNO3, and 0.1 M KNO3 and after rinsing with electrolyte three times over 30 s. (The peak at -0.55 V vs Ag/AgCl (saturated KCl) is from Cd contamination in the mercury.)
The peaks exhibit a much larger dependence on Hg2+ concentration at 1.77 T than at 0 T. This is expected because of the greater MHD convection with increasing cathodic current from increasing Hg2+ concentration. (The contaminants do not affect the percentage increase or enhancement of peak properties in the magnetic field.) ASV peak enhancements are due to MHD during the preconcentration step and not the stripping step, based on experiments where the magnetic field was absent during the stripping step (results not shown). In addition, when the magnetic field was present for the stripping step and not the preconcentration step, there was no peak enhancement. No MHD effect is observed at 1.77 T when the Hg2+ concentration is 0.1 mM, indicating that a minimal force is required to overcome the resistance of solution to motion.24 However, at 1 mM Hg2+, the anodic peak currents and peak areas for all three analytes increase in the magnetic field on average by 10 ( 6 and 15 ( 2%, respectively. (Averages and standard deviations were calculated by combining the measurements of Cu, Pb, and Cd peaks within a single ASV experiment.) At 10 mM Hg2+, the average increases in peak current and area were 61 ( 1 and 73 ( 6%, respectively. Those for 40 mM Hg2+ were 129 ( 7 and 167 ( 11%, respectively. At 60 mM Hg2+ (a saturated solution), the enhancements do not increase further: 120 ( 1% for peak current and 164 ( 1% for peak area. This can be explained by the mercury film becoming thick enough to exhibit more semiinfinite behavior. At the semi-infinite extreme, the current becomes diffusion-limited and depends on the concentration of analyte in the mercury (which remains unchanged from 0 to 1.77 T). The discrepancy between enhancements in peak current and peak area for experiments involving 10, 40, and 60 mM Hg2+ is due to peak broadening (resulting in a lower than expected peak height), because of thicker films formed in the deposition step with MHD. The enhancement in peak width at half-height, b1/2, is 7, 21, and 25%, respectively (error
