Anal. Chem. 2004, 76, 2133-2137
Porous Electrodes Supported on Ion-Exchange Membranes as Electrochemical Detectors for Supercritical Fluid Chromatography Rosanna Toniolo,† Nicola Comisso,‡ Gilberto Schiavon,‡ and Gino Bontempelli*,†
Department of Chemical Sciences and Technology, University of Udine, Via Cotonificio 108, I-33100 Udine, Italy, and C.N.R.-IPELP, Corso Stati Uniti 4, I-35020 Padova, Italy
A conveniently assembled electrochemical cell, exploiting a porous electrode supported on a moist perfluorinated ion-exchange polymer, is proposed for profitable electrochemical detection in supercritical fluid chromatography. It consists of a porous Pt working electrode, contacted by the mobile phase from the chromatographic column, which is chemically deposited onto one side of a Nafion membrane. The rear uncoated side of this membrane, acting as a solid polymer electrolyte, is contacted by an electrolyte solution (1 M NaCl) contained in an internal compartment equipped with a Pt counter electrode and a Ag/AgCl, Cl- 1 M reference electrode. Ferrocene, eluted with supercritical carbon dioxide through a Spherisorb column installed in a supercritical fluid chromatographic system, was used as electroactive prototype analyte to test the performance of this detector, which turned out to be quite better than that provided by a conventional on-line UV absorbance detector. The recorded peaks were characterized by both a good reproducibility (4.5%) and a linear dependence of their height and area, which extended over a wide concentration range (∼3 orders of magnitude). Moreover, they were not interfered by possible solvent front, unlike peaks recorded by the UV detector. The detection limit, estimated for a signal-tonoise ratio of 3 (4.2 × 10-11 mol), was lower by ∼1 order of magnitude than that found for the UV detector. Finally, the long-term stability of this detector was satisfactory in that only a ∼6% decrease in the current responses was observed after a rather long period (2 months) of continuous use. At present, supercritical fluid chromatography (SFC) is still considered a powerful analytical tool, even though no longer so promising as it was supposed a few years ago. One of the reasons of this partial loss of interest is probably a tangible need for valuable new SFC detectors. The possibility of extending electroanalytical detection to SFC would greatly broaden the range and scope of analysis carried out by this technique. Electroanalytical measurements are, in fact, * Corresponding author. Phone: (+39) 0432-558842. Fax: (+39) 0432-558803. E-mail:
[email protected]. † University of Udine. ‡ C.N.R.-IPELP. 10.1021/ac0351421 CCC: $27.50 Published on Web 02/28/2004
© 2004 American Chemical Society
characterized by a very good sensitivity, which is expected to be further improved in supercritical fluids in which diffusion coefficients are ∼1 order of magnitude greater than those found in ordinary solvents. Unfortunately, conventional electrochemical detectors are not readily compatible with nonconductive mobile phases such as carbon dioxide, which is recognized as one of the most convenient supercritical solvents because of its solvating capabilities, as well as its easily accessible critical temperature and pressure. The possibility of performing electroanalytical measurements in supercritical carbon dioxide was the subject of several thorough investigations performed over the last two decades,1-13 the majority of which pointed out the need to add to CO2 little amounts of suitable modifiers (i.e., polar solvents or appropriate supporting electrolytes) whose impact on SFC separation processes is frequently inconvenient. Alternatively, it was proved that electroanalytical measurements became practicable in unmodified CO2 by using microelectrodes coated with thin films of suitable conductive phases, either in the form of ion-exchange polymers9 or molten salt layers.10,13 In particular, this last approach enabled an effective detector for SFC to be developed,14,15 whose performance was, however, conditioned by both some instability of the film and the partitioning of eluted analytes between the fluid and (1) Philips, M. E.; Deakin, M. R.; Novotny, M. V.; Wightman, R. M. J. Phys. Chem. 1987, 91, 3934-3936. (2) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989, 61, 272-275. (3) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989, 61, 2193-2200. (4) Niehaus, D. E.; Philips, M. E.; Michael, A. C.; Wightman, R. M. J. Phys. Chem. 1989, 93, 6232-6236. (5) Di Maso, M.; Purdy, W. C.; McClintock, S. A. J. Chromatogr. 1990, 519, 252-262. (6) Niehaus, D. E.; Wightman, R. M.; Flowers, P. A. Anal. Chem. 1991, 63, 1728-1732. (7) Dressman, S. F.; Garguilo, M. G.; Sullenberger, E. F.; Michael, A. C. J. Am. Chem. Soc. 1993, 115, 7541-7542. (8) Sullenberger, E. F.; Michael, A. C. Anal. Chem. 1993, 65, 2304-2310. (9) Sullenberger, E. F.; Michael, A. C. Anal. Chem. 1993, 65, 3417-3423. (10) Sullenberger, E. F.; Dressman, S. F.; Michael, A. C. J. Phys. Chem. 1994, 98, 5347-5354. (11) Wallenborg, S. R.; Markides, K. E.; Nyholm, L. Anal. Chem. 1997, 69, 439-445. (12) Senorans, F. J.; Markides, K. E.; Nyholm, L. J. Microcolumn Sep. 1999, 11, 385-391. (13) Lee, D.; Hutchinson, J. C.; Leone, A. M.; DeSimone, J. M.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 9310-9317. (14) Dressman, S. F.; Michael, A. C. Anal. Chem. 1995, 67, 1339-1345. (15) Dressman, S. F.; Simeone, A. M.; Michael, A. C. Anal. Chem. 1996, 68, 3121-3127.
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the film. In fact, electrochemical detection occurs after that analytes permeate the conductive film and reach the underlying electrode. Consequently, the design and development of further devices allowing electroanalytical detection in SFC appear desirable in order to introduce alternatives to the presently employed methods. In the past decade, promising results in the monitoring of electroactive analytes in nonconducting media were gained by a quite different approach based on porous electrodes supported on moist ion-exchange membranes.16-40 These sensors were usually prepared by coating the side of an ion-exchange membrane facing the analyte sample with a porous conductive film (working electrode), whereas the rear side contacted an internal electrolyte solution containing the counter and reference electrodes. In these devices, any membrane-permeation step is avoided since the membrane separating the sample from the internal electrolyte does not act as a filter for detected analytes, but serves to ensure the transfer of charged species from the working to counter electrode, thus playing the role usually guaranteed by supporting electrolytes.30 In other words, these ion-exchange membranes act as insoluble solid polymer electrolytes (SPEs) confined to close contact with the electrode surface. They are thus able to work as ion pumps releasing or taking up ions during the transfer of positive or negative charges from the working electrode to the detected species, which occurs as soon as the analyte reaches (16) Kaaret, T. W.; Evans, D. H. Anal. Chem. 1988, 60, 657-662. (17) De Wulf, D. W.; Bard, A. J. J. Electrochem. Soc. 1988, 135, 1977-1985. (18) Schiavon, G.; Zotti, G.; Bontempelli, G. Anal. Chim. Acta 1989, 221, 2741. (19) Harth, R.; Mor, U.; Ozer, D.; Bettelheim, A. J. Electrochem. Soc. 1989, 136, 3863-3867. (20) Schiavon, G.; Zotti, G.; Bontempelli, G.; Farnia, G.; Sandona`, G. Anal. Chem. 1990, 62, 293-298. (21) Xing, X. K.; Liu, C. C. Electroanalysis 1991, 3, 111-117. (22) Schiavon, G.; Zotti, G.; Toniolo, R.; Bontempelli, G. Electroanalysis 1991, 3, 527-534. (23) Weisshaar, D. E.; Lamp, B.; Merrick, P.; Lichty, S. Anal. Chem. 1991, 63, 2383-2386. (24) Bettelheim, A.; Harth, R.; Ozer, D.; Mor, U.; Segl, B. Anal. Chem. 1991, 63, 2724-2727. (25) Schiavon, G.; Zotti, G.; Toniolo, R.; Bontempelli, G. Analyst 1991, 116, 797-801. (26) Loub, L.; Opekar, F.; Pacakova, V.; Stulik, K. Electroanalysis 1992, 4, 447451. (27) Schiavon, G.; Zotti, G.; Toniolo, R.; Bontempelli, G. Anal. Chem. 1995, 67, 318-323. (28) Mayo, N.; Harth, R.; Mor, U.; Marouani, D.; Hajon, J.; Bettelheim, A. Anal. Chim. Acta 1995, 310, 139-144. (29) Schiavon, G.; Comisso, N.; Toniolo, R.; Bontempelli, G. Electroanalysis 1996, 8, 544-548. (30) Bontempelli, G.; Comisso, N.; Toniolo, R.; Schiavon, G. Electroanalysis 1997, 9, 433-443. (31) Jordan, L. R.; Hauser, P. C.; Dawson, G. A. Anal. Chem. 1997, 69, 558562. (32) Jordan, L. R.; Hauser, P. C.; Dawson, G. A. Analyst 1997, 122, 811-814. (33) Jordan, L. R.; Hauser, P. C.; Dawson, G. A. Electroanalysis 1997, 9, 11591162. (34) Jordan, L. R.; Hauser, P. C. Anal. Chem. 1997, 69, 2669-2672. (35) Toniolo, R.; Comisso, N.; Bontempelli, G.; Schiavon, G.; Sitran, S. Electroanalysis 1998, 10, 942-947. (36) Jacquinot, P.; Hodgson, A. W. E.; Muller, B.; Werhli, B.; Hauser, P. C. Analyst 1999, 124, 871-876. (37) Toniolo, R.; Geatti, P.; Bontempelli, G.; Schiavon, G. J. Electroanal. Chem. 2001, 514, 123-128. (38) Jacquinot, P.; Hodgson, A. W. E.; Hauser, P. C. Anal. Chim. Acta 2001, 443, 53-61. (39) Knake, R.; Jacquinot, P.; Hauser, P. C. Electroanalysis 2001, 13, 631-634. (40) Jacquinot, P.; Muller, B.; Werhli, B.; Hauser, P. C. Anal. Chim. Acta 2001, 432, 1-10.
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the three-phase active sites where the electrode, the polyelectrolyte, and the sample meet. In this article, we suggest a convenient design for this type of sensor, enabling profitable electrochemical detection in SFC to be achieved. Preliminary results point out that the proposed sensor assembly provides very satisfactory results, particularly in view of its both good sensitivity and quite extended long-term stability. This paper gives a brief illustration of its performance evaluated on suitable synthetic samples and compares it with that provided by a conventional on-line UV absorbance detector. EXPERIMENTAL SECTION Chemicals and Instrumentation. All chemicals used were of reagent grade quality and were employed without further purification. Stock solutions (1 mM) of ferrocene and n-butylferrocene (Aldrich Chemical Co.) were prepared by dissolving weighed amounts of these compounds in n-hexane. When required, these solutions were diluted to the desired concentration with n-hexane. SFC-grade CO2 which, according to the supplier (SIAD, Bergamo, Italy), contained
