Anal. Chem. 2000, 72, 943-948
Voltammetric and Reference Microelectrodes with Integrated Microchannels for Flow through Microvoltammetry. 2. Coupling the Microcell to a Supported Liquid Membrane Preconcentration Technique O. C. Keller and J. Buffle*
CABE, Department of Inorganic, Analytical and Applied Chemistry, Sciences II, Geneva University, 30 Quai E. Ansermet, 1211 Geneva 4, Switzerland
The paper describes the assembly and functioning of the microvoltammetric cell described in Part 1, with a hollow Fiber Supported Liquid Membrane (HFSLM), for trace metal analysis. Membrane stability, working-electrode behavior, mercury-film lifetime inside the HFSLM, hydrodynamic conditions, as well as transport kinetics of the metal through the SLM have been studied in detail. System calibrations have been performed in the range 5-120 nM Pb(II). The reproducibility and sensitivity of the whole microsystem is discussed as well as limitations and possible improvements. The aim of the analytical microsystem described here is to measure the concentrations of heavy metals such as Pb(II), Cu(II), Zn(II), and Cd(II) at trace level with minimum or even no sample handling. To achieve this goal, a hollow fiber based supported liquid membrane (SLM) is coupled to the voltammetric flowthrough microvoltammetric cell discussed in Part 1.1,2 The SLM has been used mostly for industrial separation and recovery of target elements: gases,4 amino acids,5 organic acids,6 anions,7 metal complexes,8 and cations9,10-12. Until now, few analytical applications of the SLM, and very few trace metal analytical applications, have been reported,9,13-18 possibly due to the difficulty * To whom correspondence should be addressed: (phone) 41 22 702. (fax) 41 22 702 6069; (e-mail)
[email protected]. (1) Keller, O. C. De´veloppement d′un microsysteme analytique integrant membrane liquide supporte´e et de´tection e´lectrochimique pour l’analyse de me´taux lourds. No. 2957. Ph.D. Thesis. University of Geneva, Switzerland, 1997. (2) Keller, O. C.;Buffle, J. Anal.Chem. 1999, 72, 936-942. (3) Parthasarathy, N.; Buffle, J. Anal. Chim. Acta 1994, 284, 649. (4) Ward, W. J.; Robb, W. L. Science (Washington, D.C.) 1967, 156, 1481-84. (5) Molinari, R.; De Bartolo, L.; Drioli, E. J. Membr. Sci. 1992, 73, 203. (6) Kiani, A.; Bhave, R. R.; Sirkar, K. K. J. Membr. Sci. 1984, 20, 125-45. (7) Visser, H. C.; Rudkevich, D. M.; Verboom, W.; Jong, F.; Reinhoud, D. N. Angew. Chem., Int. Ed. Engl. 1994, 33, 467-68. (8) Nijenhuis, W. F. V. S.; deJong, F.; Reinhoudt, D. N. Recl. Trav. Chim. PaysBas 1993, 112, 317. (9) Danesi, P. R. Sep. Sci. Technol. 1984-85, 19, 857. (10) Barnes, D. E.; Marshall, G. D. Sep. Sci. Technol. 1995, 30, 751. (11) Izatt, S. R.; Hawkins, R. T.; Christensen, J. J.; Izatt, R. M. J. Am. Chem. Soc. 1985, 107, 63. (12) Danesi, P. R.; Chiarizia, R.; Castagnola, A. J. Membr. Sci. 1983, 14, 161. (13) Parthasarathy, N.; Buffle, J. Anal. Chim. Acta 1991, 254, 1. 10.1021/ac990507w CCC: $19.00 Published on Web 02/02/2000
© 2000 American Chemical Society
of developing stable membrane systems. A SLM consists of a microporous hydrophobic membrane impregnated with a hydrophobic organic solvent containing a liposoluble complexant of the test metal as a carrier.4-15,19-22 This membrane is placed between the sample (source phase) and receiver (strip phase) aqueous solutions. At the sample/membrane interface, the metal ion, M, reacts with the carrier, C, dissolved in the membrane, to form a complex, MC, soluble in the membrane. The complex diffuses across the membrane, and M exchanges at the membrane/strip interface by complexing with a hydrophilic ligand, L, present in the strip solution. If the volume of the strip phase is less than that of the source phase, or if the strip phase is strongly complexing, then the test metal ions can be concentrated.21 The mechanism of metal transport in SLM systems containing noncyclic and macrocyclic extractants has been reported in refs 19, 20, 22. A SLM coupled to voltammetry presents remarkable potential advantages: high selectivity toward the metal ions of interest, the possibility to achieve high preconcentration factors and therefore high sensitivity, without sample handling, and the capability of measuring selectively the free metal ion3 in complex systems. Modulated anodic stripping voltammetries are among the most sensitive techniques for metal analysis with detection limits close to 10-10 M. By combining such methods with HFSLM, detection limits of 10-12-10-13 M can be foreseeable; such highly sensitive techniques are needed to measure free metal ion concentrations in natural waters. To get high preconcentration factors in short (14) Parthasarathy, N.; Buffle, J. Anal. Chim. Acta 1991, 254, 9. (15) Parthasarathy, N.; Buffle, J. Anal. Chim. Acta 1994, 284, 649. (16) Papantoni, M.; Dyane, N.; Ndung’u, K.; Jo¨nsson, J. A.; Mathiasson, L. Analyst (Cambridge, U.K.) 1995, 120, 1471. (17) Uto, M.; Toshida, H.; Sugawara, M.; Umezawa, Y. Anal. Chem. 1986, 58, 1798. (18) Guyon, F.; Parthasarathy, N.; Buffle, J. Anal. Chem. 1999, 71, 819. (19) Ward, W. J. AIChE J. 1970, 16, 405. (20) Fyles, T. M. Can. J. Chem. 1987, 65, 884. (21) Keller, O. C.; Poitry, S.; Buffle, J. J. Electroanal. Chem. 1994, 378, 165. (22) Izatt, R. M.; Bruening, R. L.; Bruening, M. L.; Lindh, G. C.; Christensen, J. J. Anal. Chem. 1989, 61, 1140.
Analytical Chemistry, Vol. 72, No. 5, March 1, 2000 943
Figure 1. Complete System. 1, reference electrode; 2, auxiliary electrode; 3, channel; 4, strip-solution inlet; 5, channel; 6, working electrode (Ir); 7, gel of the Hg-plated Ir microelectrode; 8, epoxy; 9, silicone tubing; 10, O-ring; 11, seal screw; 12, HFSLM; 13, source cell; 14, stripsolution pump; 15, source-solution pump; 16, polypropylene heat-shrinkable tubing; 17, gel of ref + auxiliary electrode.
times, the volume of the strip solution (in which the test metal accumulates) must be very small, and it has been shown that hollow fibers with internal diameter
