Electrochemically Modulated Liquid−Liquid Extraction of Ions

Anal. Chem. 2005, 77, 7310-7318

Electrochemically Modulated Liquid-Liquid Extraction of Ions Alfonso Berduque,† Amanda Sherburn,‡ Mihaela Ghita,‡ Robert A. W. Dryfe,§ and Damien W. M. Arrigan*,†

Tyndall National Institute, Lee Maltings, University College, Cork, Ireland, Department of Chemistry, University of Salford, Salford, U.K. M5 4WT, and School of Chemistry, University of Manchester, Faraday Building, P.O. Box 88, Manchester, U.K. M60 IQD

The development of ion extraction methods under electrochemical control via electrochemistry at the interface between two immiscible electrolyte solutions is discussed. A hydrodynamic flow injection system was used for the potentiostatic extraction of non-redox-active species from a flowing aqueous phase into a stationary organogel phase. The ions tetraethylammonium, 4-octylbenzenesulfonate (4-OBSA-), and p-toluenesulfonate (p-TSA-) were studied as model analytes. The extraction study comprised examination of the influence of extraction potentials, aqueous-phase flow rate, and target species concentration. The extraction process can be monitored in situ by means of the ion-transfer current, which has opposing signs for anions and cations. Hydrodynamic voltammograms were obtained from these experiments. The selective extraction of 4-OBSA-, from its mixture with p-TSA-, as well as coextraction of both anions is shown. The results demonstrate the utility of electrochemical modulation for the controlled extraction of ions from an aqueous phase into an organogel electrolyte phase. This offers potential benefits for various analytical processes including sample preparation and cleanup. Despite the improvements in sensitivity and selectivity achieved using modern analytical systems, separation techniques such as distillation, precipitation, liquid-liquid extraction, ion exchange, and dialysis are necessary in order to overcome the adverse influences of matrix components and of coexisting analytes on analytical signals. Additionally, instrumental detection capability often needs to be enhanced by use of preconcentration techniques.1 Liquid-liquid extraction (LLE) is a separation technique used to move species dissolved in a liquid phase (typically aqueous) into another phase (typically an organic phase) through the intensive contact of both phases in such a manner that an efficient transport of the species across the interface is attained due to the high interfacial area.2,3 Such techniques are usually employed * To whom correspondence should be addressed. Phone +353-21-4904079. Fax +353-21-4270271. e-mail [email protected]. † University College. ‡ University of Salford. § University of Manchester. (1) Fang, Z.; Zhu, Z.; Zhang, S.; Xu, S.; Guo, L.; Sun, L. Anal. Chim. Acta 1988, 214, 41-55.

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for sample pretreatment4 in order to remove interfering substances, to preconcentrate the analyte, increase the sensitivity, or both.5 However, conventional manual extraction procedures commonly used are quite problematic,5 suffering from drawbacks such as the requirement for large volumes of sample and solvents,1,4 the use of toxic organic solvents,6 time-consuming, tedious,1,6 and multistage operational processes,6 emulsion formation,7 loss of analyte,8 low sample through-put frequency,8 production of large amounts of residual solvents,8 and likelihood of sample contamination.1,8 Nevertheless, these negative aspects can be prevented or circumvented by using flow analytical extraction systems.1 Flow analytical methods, such as flow injection analysis (FIA), offer many varieties of sample pretreatment procedures that can be incorporated and carried out on-line.5 This provides scope for the automation of problematic sample manipulations.9 An emerging trend is based on the miniaturization of the conventional LLE principle by largely reducing the acceptor-to-donor phase ratio employing microfluidic devices.6,10-12 The optimization of the extraction cell design is also of great relevance, and several reports12-18 have focused on this. For instance, Kuban and coworkers13 numerically modeled the effect of various physical (2) Rieger, R.; Weiss, C.; Wigley, G.; Bart, H.-J.; Marr, R. Comput. Chem. Eng. 1996, 20, 1467-1475. (3) Ruiz-Jime´nez, J.; Luque de Castro, M. D. Anal. Chim. Acta 2003, 489, 1-11. (4) Chao, J.-B.; Liu, J.-F.; Wen, M.-J.; Kiu, J.-M.; Cai, Y.-Q.; Jiang, G.-B. J. Chromatogr., A 2002, 955, 183-189. (5) Motomizu S.; Korechika, K. Anal. Chim. Acta 1989, 220, 275-280. (6) Psillakis, E.; Kalogerakis, N. Trends Anal. Chem. 2003, 22, 565-574. (7) Priego-Capote, F.; Luque de Castro, M. D. Anal. Chim. Acta 2003, 489, 223-232. (8) Cladera, A.; Miro´, M.; Estela, J. M.; Cerda´, V. Anal. Chim. Acta 2002, 421, 155-166. (9) Nielsen, S. C.; Hansen, E. H. Anal. Chim. Acta 2000, 422, 47-62. (10) Sandahl, M.; Mathiasson, L.; Jo¨nson, J. A. J. Chromatogr., A 2002, 975, 211-217. (11) Surmeian, M.; Slyadnev, M. N.; Hisamoto, H.; Hibara, A.; Uchiyama, K.; Kitamori, T. Anal. Chem. 2002, 2014-2020. (12) Ueno, K.; Kim, H.-B.; Kitamura, N. Anal. Sci. 2003, 19, 391-394. (13) Kuban, P.; Berg, J.; Dasgupta, P. K. Anal. Chem. 2003, 75, 3549-3556. (14) K. Yunus, K.; Marks, C. B.; Fisher, A. C.; Allsopp, D. W. E.; Ryan, T. J.; Dryfe, R. A. W.; Hill, S. S.; Roberts, E. P. L.; Brennan, C. M. Electrochem. Commun. 2002, 4, 579-583. (15) Luque, M.; Luque-Pe´rez, E.; Rios, A.; Valca´rcel, M. Anal. Chim. Acta 2000, 410, 127-134. (16) Kurita, R.; Tabei, H.; Liu, Z.; Horiuchi, T.; Niwa, O. Sens. Actuators, B 2000, 71, 82-89. (17) Dehkordi, A. M. Chem. Eng. Proc. 2002, 41, 251-258. (18) Wilke, S.; Franzke, H.; Mu ¨ ller, H. Anal. Chim. Acta 1992, 268, 285-292. 10.1021/ac051029u CCC: $30.25

© 2005 American Chemical Society Published on Web 10/01/2005

parameters, such as interfacial surface tension, density, viscosity, wall contact angle, and flow velocity, on the type of flow. Opposedjet (or wall-jet) configurations provide improved extraction efficiency compared to that attained using nonopposed jet (flowthrough or parallel flow) configurations, as the analyte in the aqueous phase stays in contact with the water|organic interface for a longer time.16-18 Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES)19-24 (or electrochemistry at the liquid-liquid interface) has been employed for the voltammetric and amperometric determination of ions,18,25,26 including analysis of non-redox-active species.24,26,27 The selectivity of this detection principle can be varied by the imposition of a variable potential difference between the immiscible phases employed.18 There are a number of examples of FIA and liquid chromatographic systems employing liquid-liquid electrochemical detection.18,28-33 For example, Sa´nchez-Pedren ˜o et al.28 used a flow injection system for the chronocoulometric determination of tetraethylammonium ions. In these cases, the liquid|liquid electrochemical process is employed as the detection method. The interest of the present work is the investigation into the use of the liquid-liquid electrochemical process as a selective extraction method. A drawback of electrified liquid-liquid interfaces under hydrodynamic conditions is the mechanical instability of the interface,18 but this problem can be overcome by the jellification of the organic phase27,29 with poly(vinyl chloride) (PVC)18 or by inserting a porous membrane between the two liquid immiscible phases.18,30,31,33,34 However, the good mechanical stability attained by employing organogel phases leads to an increase of the resistivity of the gellified organic phase, which can create distorted voltammetric curves due to uncompensated resistance. Nevertheless, ion transfer at water|gel interfaces can still be used for amperometric detection purposes,25,28,29 and a number of reports have shown the combined use of micro-ITIES with water|organogel interfaces both to reduce the interfacial resistance and to increase the mechanical stability of the interface.35,36 (19) Samec, Z.; Marecˇek, V.; Weber, J. J. Electroanal. Chem. 1979, 100, 841852. (20) Vany´sek, P. Anal. Chem. 1990, 62, 827A-835A. (21) Girault, H. H. Modern Aspects of Electrochemistry; Plenum Press: New York, 1993; Vol. 25, pp 1-62. (22) Vany´sek, P.; Buck, R. P. J. Electrochem. Soc. 1984, 131 (8), 1792-1796. (23) Vany´sek, P. Trends Anal. Chem. 1993, 12, 9, 357-363. (24) Ortun ˜o, J. A.; Herna´ndez, J.; Sa´nchez-Pedren ˜o, C. Electronalysis 2004, 16, 827-831. (25) Reymond, F.; Fermı´n, D.; Lee, H. J.; Girault, H. H. Electrochim. Acta 2000, 45, 2647-2662. (26) Lee, H. J.; Pereira, C. M.; Silva, A. F.; Girault, H. H. Anal. Chem. 2000, 72, 5562-5566. (27) Lee, H. J.; Girault, H. H. Anal. Chem. 1998, 70, 4280-4285. (28) Sa´nchez-Pedren ˜o, C.; Ortun ˜o, J. A.; Herna´ndez, J. Anal. Chim. Acta 2002, 459, 11-17. (29) Marecek, V.; Janchenova, H.; Colombini, M. P.; Papoff, P. J. Electroanal. Chem. 1987, 217, 213-219. (30) Hundhammer, B.; Solomon, T.; Zerihun, T.; Abegaz, M.; Bekele, A.; Graichen, K. J. Electroanal. Chem. 1994, 371, 1-11. (31) Wilke, S. Anal. Chim. Acta 1994, 295, 165-172. (32) Sawada, S.; Toril, H.; Osakai, T.; Kimoto, T. Anal. Chem. 1998, 70, 42864290. (33) Sawada, S.; Taguma, M.; Kimoto, T.; Hotta, H.; Osakai, T. Anal. Chem. 2002, 74, 1177-1181. (34) Wilke, S.; Zerihun, T. Electrochim. Acta 1998, 44, 15-22. (35) Wilke, S.; Osborne, M. D.; Girault, H. H. J. Electroanal. Chem. 1997, 436, 53-64. (36) Lee, H. J.; Beriet, C.; Girault, H. H. Anal. Sci. 1998, 14, 71-77.

From the above overview, it can be ascertained that electrochemistry at the ITIES can be successfully employed for the elucidation of ion-transfer processes and for the detection of ions. However, the use of electrochemistry at ITIES to control analytical extraction processes has not been addressed. Although there is some interest in electroassisted extractions for metal recovery applications,37 that is based on batch extraction processes and with chemical control of the interfacial potential difference. The aim of the work described here was to investigate the feasibility of using a LLE cell with electrochemical control for the selective extraction of ions and is a contribution to development of new sample pretreatment procedures rather than an analytical detection procedure. The cell is based on a polarized gel-supported liquid-liquid interface, where the receptor phase is a stationary PVC-stabilized organic gel and the donor aqueous mobile phase flows over it. When a potential is applied that causes the stationary phase (organogel phase) to be more positive relative to the mobile aqueous phase, anions in the aqueous phase will be extracted and retained. Conversely, an applied potential causing the stationary phase to be more negative with respect to the aqueous phase will induce the extraction of cations. Furthermore, the different transfer potentials of ions mean that the applied potential can be tailored to the extraction of a particular ion in the presence of other ions, imparting selectivity to the extraction system. Therefore, separation of different ionic species is possible. In the following sections, study of a hydrodynamic liquid-liquid electrochemical extraction system is presented using three model analytes. The potential-modulated selective extraction of aromatic sulfonate anions is demonstrated. EXPERIMENTAL SECTION Reagents. The aqueous-phase electrolytes, lithium chloride and lithium sulfate, the model analyte species studied, i.e., sodium 4-octylbenzenesulfonate (4-OBSA-), tetraethylammonium chloride (TEA+), and sodium p-toluenesulfonate (p-TSA-), the organic solvents used, nitrobenzene (NB) and 1,2-dichloroethane (DCE), and low molecular weight poly(vinyl chloride) (PVC) were purchased from Sigma-Aldrich. The aqueous phase was either LiCl (10 mM) or Li2SO4 (10 mM) in deionized water (18 MΩ cm-1 purity, from a UHQ-PS system. Elga Ltd.). The organogel stationary phase contained either bis(triphenylphosphoranylidine)ammonium tetrakis(4-chlorophenylborate) (BTPPATPBCl) or bis(triphenylphosphoranylidine)ammonium tetraphenylborate (BTPPATPB) at 10 mM, dissolved in NB or DCE. Model analyte solutions of 4-OBSA-, TEA+, and p-TSA- at different concentrations were prepared in LiCl (10 mM) or Li2SO4 (10 mM). The organic-phase electrolytes, BTPPATPBCl and BTPPATPB, were prepared by metathesis36,38 of bis(triphenylphosphoranylidene)ammonium chloride (BTPPA+Cl-; Aldrich, 97% grade) and potassium tetrakis(4-chlorophenylborate) (K+TPBCl-; Fluka, g98% grade) or sodium tetraphenylborate) (Na+TPB-; g99.5%). The organogel phase was prepared as described in the literature.39,40 BTPPATPBCl or BTPPATPB (10 mM) was added (37) Bustero, I.; Cheng, Y.; Mugica, J. C.; Fernandez-Otero, T.; Silva, A. F.; Schiffrin, D. J. Electrochim. Acta 1998, 44, 29-38. (38) Ulmeanu, S.; Lee, H. J.; Fermin, D. J.; Girault, H, H. Electrochem. Commun. 2001, 3, 219-223. (39) Murtoma¨ki, L.; Barker, M. H.; Manzanares, J. A.; Kontturi, K. J. Electroanal. Chem. 2003, 560, 95-103.

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Figure 1. PTFE electrochemical cell and general composition. (A, D) Platinum mesh counter electrodes for the aqueous and organic phases; (B) Ag|AgCl or Ag|Ag2SO4 reference electrode for the aqueous phase’ (C) Ag|AgCl or Ag|Ag2SO4 pseudo-reference electrode. Aqueous phase: LiCl or Li2SO4 (10 mM) in deionized water. Organic phase: BTPPATPBCl or BTPPATPB (10 mM) in the organic solvent Os (NB or DCE) stabilized with PVC (10% w/v).

to the organic solvent; this solution was then heated at ∼80 °C for 5 min. Once the solution was sufficiently hot, low molecular weight PVC was added, to a concentration of 10% w/v, and the temperature held at 80 °C for a further 5 min. The temperature was then increased to 100 °C (5 min), 120 °C (10 min), and a few more minutes at a temperature no higher than 150 °C. The solution was stirred throughout this procedure. The gel obtained (after cooling) was then placed in the LLE cell cavity for the organogel phase, after inserting the electrodes in the machined PTFE device. The gel was refrigerated and protected from light. Apparatus. A CHI660B electrochemical analyzer (CH Instruments) was used for all electrochemical experiments. The electrochemical cell employed was machined from Teflon (poly(tetrafluoroethylene), PTFE), to support a four-electrode configuration allowing flow of the aqueous phase over the organogel phase, as shown in Figure 1. This device consists of two pieces that are held together using PTFE nuts and bolts and an o-ring (Viton O-ring, 15.6-mm internal diameter and 1.78-mm wall thickness) to prevent leakage of the aqueous phase. The crosssectional interface area of the cell was 1.13 cm2. The electrodes consisted of two platinum mesh counter electrodes (one in each phase), a Ag|AgCl reference electrode for the aqueous phase, and a Ag|AgCl pseudo-reference electrode for the organogel phase. The Ag|AgCl electrodes were prepared by the potentiostatic oxidation of silver wires in a solution of KCl (3 M). In experiments involving Li2SO4 (10 mM) as aqueous-phase electrolyte, Ag|Ag2SO4 electrodes were used as reference and pseudo-reference electrodes, which were prepared by the potentiostatic oxidation of silver wires in K2SO4 (1 M). Figure 1 shows the cell composition used in this work, where the double bar indicates the interface between the aqueous and organogel phases, a is the identity of the model ionic analyte studied (tetraethylammonium, 4-octylbenzenesulfonate or p-toluenesulfonate), and j represents its molar concentration. (40) Lee, H. J.; Beattie, P. D.; Seddon, B. J.; Osborne, M. D.; Girault, H. H. J. Electroanal. Chem. 1997, 440, 73-82.

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The aqueous phase was propelled into the PTFE cell using a syringe pump (KDScientific KDS200 series syringe pump) with controllable flow rates. Between the pump and the electrochemical cell, a six-port valve (C22-3186 valve, from Carl Stuart Ltd.) was connected using polyetheretherketone (PEEK) fittings and tubing in order to allow flow injection analysis with a 100-µL PEEK injection loop. All potentials reported throughout were corrected to the Galvani scale (∆wo φ) by assuming a standard ion-transfer potential (∆wo φTEA+0) of -0.059 V for TEA+ between water and NB41,42 and ∆wo φTEA+0 ) 0.044 V for TEA+ between water and DCE. 41,43 Methodology. Initially, the electrochemical cell was connected to the flow system (syringe pump and six-port valve with a 100µL sample loop), and the aqueous solution was set to flow at a rate of 1 mL min-1 until the electrochemical cell was completely filled, signified by commencement of collection of the exit solution in the waste receptacle. The solution flow was then halted, and a stationary solution cyclic voltammogram (CV) of the blank solution (using, e.g., BTPPATPBCl 10 mM as organic electrolyte salt in NB-PVC organogel) was then performed, as a test of the proper setup of the cell and of the electrochemical parameters chosen. Afterward, the potential was set to a specific value (using amperometry) and the flow was restarted at the same flow rate (1 mL min-1). After the stabilization of the background current, the analyte studied was injected using the 100-µL volume sample loop and the amperometric response studied as a function of the applied interfacial potential difference, aqueous-phase flow rate, analyte concentration, and composition of analyte mixture solutions. RESULTS AND DISCUSSION Potentiostatic Extraction of Ions. The injections of TEA+ (1 mM in 10 mM LiCl, 100-µL injection) into the flow cell resulted (41) Samec, Z. Pure Appl. Chem. 2004, 76, 2147-2180. (42) Tatsumi, H.; Katano, H. Anal. Sci. 2004, 20, 1613-1615. (43) Johans, C.; Clohessy, J.; Fantini, S.; Kontturi, K.; Cunnane, V. J. Electrochem. Commun. 2001, 3, 219-223.

Figure 2. Potentiostatic extraction response of TEA+ (A), 4-OBSA(B), and p-TSA- (C) as a function of the ∆wo φ applied. Note: the amperometric peak currents were normalized with respect to the time at which each single injection was performed.

in positive current peaks whose magnitude was dependent on the potential difference applied across the interface (∆wo φ). The positive current, by convention,41 signifies the transfer of a cationic species from the aqueous phase to the organic phase. Figure 2A shows the peak responses obtained at different applied potential settings; the time axis is normalized with respect to the time at which each injection was made. The peak is due to the potentialmodulated transfer, or extraction, of the cationic analyte from the aqueous phase into the organic phase. The current is an in situ measurement of the extraction process, thus enabling both electrochemical manipulation and scrutiny of the process. The peak current increased when ∆wo φ was more positive, and no peak current response was observed for ∆wo φ < -0.238 V. At positive potentials, i.e., at ∆wo φ . -0.014 V, the amperometric peaks were of similar magnitude, independent of the applied potential. CV analysis (under stationary conditions) after the potentiostatic extraction of TEA+ at very positive potentials showed the TEA+ extracted into the organogel phase (see later), with the forward peak maximum at ∼-0.014 V. The applied potentialindependent currents obtained represent the maximum rate at which TEA+ can be extracted into the organogel phase, all other conditions being constant. Hence, this explains the similar magnitude of amperometric peak currents at potentials more positive of the CV peak potential of TEA+. The extraction of 4-OBSA- (1 mM in 10 mM LiCl, 100 µL) displayed behavior opposite to that observed of TEA+. Injections of this model analyte produced potential-dependent negative current peaks in the flow cell system. The negative current, by convention,41 results from the transfer of an anionic species from the aqueous phase into the organic phase. The magnitude of the 4-OBSA- transfer peak was observed to depend on ∆wo φ: the magnitude of these negative peaks increased (i.e., a more negative current) when the potential applied was decreased (made more negative, Figure 2B). Sufficiently negative potentials provided the amperometric peak response, while positive potentials did not give any response. In other words, the application of suitable negative potentials causes the extraction of 4-OBSA- into the organogel phase, while positive potentials do not induce extraction. The same procedure was also applied to extract p-TSA-. Figure 2C shows the amperometric response of p-TSA- (1 mM in 10 mM LiCl, 100 µL), which is similar to that already seen for 4-OBSAbut with the important difference that the extraction of p-TSA-

Figure 3. Hydrodynamic voltammograms for TEA+ (b), 4-OBSA- (O), and p-TSA- (9). Experimental conditions as in Figure 2.

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Figure 4. Potentiostatic extraction response as a function of the concentration of 4-OBSA-, and calibration curve. Concentration of 4-OBSA-: 1, 5 × 10-4 M; 2, 2 × 10-4 M; 3, 1 × 10-4 M; 4, 7.5 × 10-5 M; 5, 5 × 10-5 M; 6, 2 × 10-5 M; 7, 1 × 10-5 M; 8, 1 × 10-6 M. Note: the peak currents were normalized with respect to the time at which each single injection was performed. Other experimental conditions as in Figure 2.

Figure 5. (A) Amperometric response of 4-OBSA- vs flow rate. ∆wo φ ) -0.188 V, Sample injection, 100 µL of 1 mM 4-OBSA-. Three measurements-injections per flow rate test. Inset: the linear increase of the peak current with the cube root of the flow rate; error bars indicate the standard deviation of the triplicate measurements. (B) Increase of the amperometric response of TEA+ when the flow rate is increased. ∆wo φ ) 0.063 V. Sample injections, 100 µL of 1 mM TEA+.

requires more negative extraction potentials; i.e., it is more difficult to extract into the organogel phase than 4-OBSA-. Hydrodynamic Voltammograms. The data obtained from the constant potential extraction curves displayed in Figure 2 enables 7314 Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

the comparative responses of the different model analytes to be studied. Hydrodynamic voltammmograms (HDVs) of these ions are easily constructed (Figure 3) by plotting the amperometric peak current for the extraction of the given ion at each applied

potential versus ∆wo φ. This provides a simple means for comparing the extraction capability of a chosen analyte at a given applied potential as well as determining what other species might also extract at the same potential. Figure 3 compares the HDVs of the three ions studied in the present experimental conditions (10 mM LiCl and 10 mM BTPPATPBCl in NB-PVC). HDV analysis shows that 4-OBSA- is not extracted from the flowing aqueous phase into the organic gel for ∆wo φ > ∼0.062 V. Lower potentials permit the extraction of this anion, and the current increases as ∆wo φ goes toward more negative values. At sufficiently negative values of the applied potential, the peak magnitude does not increase further, reaching the HDV voltammetric plateau. It can be seen from Figure 3 that TEA+ and p-TSA- demonstrate HDV behavior similar to that of 4-OBSA-, with the important differences being that TEA+ extraction leads to positive currents and p-TSAextraction to negative extraction currents but at more negative potentials than those obtained for 4-OBSA-. It can be observed that potentials of > -0.188 and < -0.213 V lead to extraction of TEA+ and p-TSA-, respectively, from the aqueous into the organogel phase. These results suggest the use of this system to selectively extract analytes from specific mixtures. Calibration Curve. The concentration dependence of the amperometric signals for ion extraction exhibited good linear correlations. For example, in the case of the potentiostatic extraction of 4-OBSA-, at a constant potential of ∆wo φ ) -0.188 V in order to provide sufficient negative (low) potentials to allow the transfer of 4-OBSA- into the organic gel, the peak current maximum was linearly dependent on the concentration in the range from 1 µM to 0.5 mM (Figure 4). For all analyses, multiple sample injections (n ) 3) were performed. The repeatability of these was typically in the range of 11-21% relative standard deviation (RSD), depending on the concentration level, with lower concentrations having higher RSDs. Flow Rate Studies. Preliminary studies were carried out on the variation of the background amperometric signal for injection of blank analyte solutions, i.e., 10 mM LiCl, this solution being identical to that already flowing through the electrochemical extraction cell. This was evaluated at different solution flow rates. It was found that there were no significant background signals at ∆wo φ values relevant to the extraction of the three model analyte ions chosen for study. However, using more positive (>0.213 V) and negative (< -0.438 V) extraction potentials, some instability was observed, due to the transfers of electrolyte salt species across the interface. Figure 5 shows the reproducibility attained at different flow rates tested for the system proposed in this work. The RSD varied between 14 (at low flow rates) and 2% (at the higher flow rates employed). The amperometric peak maximum (or extraction response) was observed to vary linearly with the cube root of the volume flow rate (V1/3), as expected.14,44 The linearity of the amperometric peak response with the cube root of the flow rate was observed not only from the amperometric peak current (and the peak area) response (Figure 5A inset) but also from the CV peak current (results not shown). Panels A and B of Figure 5 show an example of the repeatable behavior obtained between injections and the increase of the extraction peak response when the flow rate is increased. The amperometric peaks (44) Hill, S. S.; Dryfe, R. A. W.; Roberts, E. P. L.; Fisher, A. C.; Yunus, K. Anal. Chem. 2003, 75, 486-493.

Figure 6. (A) CV of blank LiCl (10 mM) and BTPPATPBCl (10 mM) in NB-PVC. (B) CV of TEA+, and (C) of 4-OBSA- extracted into the organogel phase via the FIA-amperometric system. All CVs were performed under stationary conditions. Scan rate, 5 mV s-1.

were larger and sharper when the flow rate was increased, due to the concentration gradient formed at the interface being steeper at higher flow rates, thus resulting in greater and sharper peak responses. Cyclic Voltammetry. As already discussed above, CV analysis of the blank was always performed as the first experiment prior to any extraction test. Figure 6A shows the characteristic voltammetric response of the blank. An example of the CV performance obtained from this system is shown in Figure 6B and C. Figure 6B shows the CV of TEA+ after its potentiostatic extraction into Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

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Figure 7. (A) Potentiostatic extraction of p-TSA- (1 mM) (three injections), ∆wo φ ) -0.300 V (top) and CV analysis of p-TSA- extracted (bottom). (B) Potentiostatic extraction of 4-OBSA- (1 mM) (three injections), ∆wo φ ) -0.100 V (top) and CV analysis of 4-OBSA- extracted (bottom). Note: gray line in CV analysis is the CV of blank (no target ions added). Black lines correspond to the response of the ions extracted in the organogel. Scan rate, 5 mV s-1.

the organogel using positive extraction potentials. This exhibits a forward peak-reverse peak separation close to 60 mV, as expected for reversible transfer of a singly charged ion.45 Figure 6C shows the voltammetric response of 4-OBSA- previously extracted into the organogel phase. In this case, the 4-OBSA- was extracted from a series of solutions of different concentrations ranging from 1 µM to 0.5 mM (i.e., the CV after a potentiostatic extraction of all concentrations such as that shown in Figure 4), showing a forward peak-reverse peak separation of 60 mV, again as expected for reversible transfer of a singly charged ion.45 However, the positive peak in the CV analysis of the 4-OBSAextracted is due to the back-extraction of 4-OBSA- into the aqueous phase, while the negative peak is caused by the extraction of such anionic species into the organogel. Conversely, the positive peak transfer in the CV of TEA+ is caused by the extraction of TEA+ into the organogel phase, and the negative peak is its backextraction into the aqueous phase. However, the CV of p-TSA- (not shown) was more complicated as the transfer of this anion overlapped with the lower potential limit of the potential window established by the electrolytes employed for the aqueous and organic phases. The CV of this anion was difficult to resolve under these experimental conditions. These data demonstrate that, after extraction of an ion into the organogel phase, CV can be used as an in situ test for the (45) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001.

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presence of the analyte in the gel. That is, the extraction process can be monitored during both the extraction process (by the transfer current) and postextraction (by a subsequent voltammetric experiment). Selective Extraction from a Mixture of Two Similar Compounds. In this section, the electrochemical extraction of anions of similar chemical structure is presented. The ability to electrochemically manipulate the selective extraction of an ion from a mixture is studied with a view to its use as a component of the analytical process. Here the emphasis is on selective extraction within the hydrodynamic cell presented in the previous sections. The anions selected for these studies were two aromatic sulfonate ions, 4-OBSA- and p-TSA-. The aqueous phase was Li2SO4 (10 mM), and the organic phase was BTPPATPB (10 mM) in DCE-PVC. The p-TSA- (1 mM) and 4-OBSA- (1 mM) solutions were prepared in Li2SO4 (10 mM). In these studies, aqueous Li2SO4|BTPPATPB-DCE-PVC was used as electrolytesolvent system instead of aqueous LiCl|BTPPATPBCl-NB-PVC, as it was observed that DCE provided a larger potential window than NB, thus allowing the detection of a greater number of target ionic species. Furthermore, the selection of the aqueous and organic electrolyte salts was observed to be of critical importance: CV analysis of p-TSA- shows that its transfer overlaps with the left side of the potential window (electrolyte transfer) when LiCl and BTPPATPBCl in NB-PVC are used as aqueous- and organic-phase electrolytes (not shown). However, p-TSA- transfer

Figure 8. (A) Potentiostatic extraction of 4-OBSA- (1 mM) in a mixture of p-TSA- (1 mM) and 4-OBSA- (1 mM) (three injections, ∆wo φ ) -0.100 V, black line), and of p-TSA- (1 mM) and 4-OBSA- (1 mM) in a mixture of p-TSA- (1 mM) and 4-OBSA- (1 mM) (three injections, ∆wo φ ) -0.375 V, gray line). (B) CV analysis of 4-OBSA- selectively extracted from the mixture using ∆wo φ ) -0.100 V. (C) CV analysis of both p-TSA- and 4-OBSA- extracted at ∆wo φ ) -0.375 V. Scan rate, 5 mV s-1.

can be easily identified within the potential window when Li2SO4 and BTPPATPB in DCE-PVC are used. Figures 7 and 8 show the shift of the voltammetric response for the 4-OBSA- transfer toward the positive limit of the potential window, using Li2SO4 and BTPPATPB instead of LiCl and BTPPATPBCl, which facilitates the analyses of more species transferring at potentials more negative than 4-OBSA-, such as p-TSA-. First, p-TSA- (1 mM) was injected three times (100-µL sample volume injected and 1.5 mL min-1 flow rate) using an extraction

potential of -0.300 V, which was sufficient to promote the extraction of this anion into the organogel phase (Figure 7A upper trace). A CV was then performed to verify the extraction of p-TSAinto the organogel phase. The positive peak (forward sweep) in this CV is due to the back-extraction of the anion into the aqueous phase, and the negative peak in the reverse sweep was due to the re-extraction into the organogel (Figure 7A lower trace). Afterward, the gel was regenerated by the application of positive potentials and high flow rates for the total back-extraction of the Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

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anion into the flowing aqueous phase. The CV recorded after gel regeneration was similar to the initial voltammogram of the blank, which means that the organogel was successfully cleaned or regenerated. Second, 4-OBSA- (1 mM) was injected three times (Figure 7B upper trace), as in the p-TSA- experiments, but using an extraction potential ∆wo φ ) -0.100 V. The subsequent CV showed that the 4-OBSA- extraction was achieved (Figure 7B lower trace). Third, after gel regeneration, a sample containing both p-TSA(1 mM) and 4-OBSA- (1 mM) was injected (three times and under the same conditions as already mentioned) at an extraction potential of ∆wo φ ) -0.100 V (Figure 8A black line). This potential is sufficient for the extraction of 4-OBSA- but not p-TSA-. The CV after this extraction exhibited peaks due to 4-OBSAtransfer only but not for p-TSA-. This postextraction CV analysis confirms that the extraction potential of -0.100 V does not allow the transfer of p-TSA- and thus promotes the selective extraction of 4-OBSA- from the mixture of the two aromatic sulfonates (Figure 8B). Fourth, the use of more negative extraction potentials (∆wo φ ) -0.375 V, Figure 8A gray line) causes the extraction of both 4-OBSA- and p-TSA- when the mixture is injected into the cell. The amperometric response (gray line) is observed to be much wider, due to extractions of both anions at this potential. The CV after this extraction at ∆wo φ ) -0.375 V shows the peaks due to both p-TSA- and 4-OBSA- (Figure 8C), confirming that both compounds may be extracted into the organogel phase by application of a suitable potential. These results demonstrate that, by the application of appropriate potentials, selective extraction of one ion from a mixture of ions of similar chemical structure is possible. By choice of appropriate applied potential, one or both ions in the present mixture can be extracted. CV can be used as a postextraction confirmation of selectivity.

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CONCLUSIONS The studies reported here demonstrate that electrochemistry at the gel-stabilized ITIES can be used as an effective route for the selective extraction and separation of ions. The hydrodynamic four-electrode system proposed is observed to be suitable for the potentiostatic extraction of different ions such as 4-OBSA-, TEA+, and p-TSA-, showing a good extraction performance. By a careful choice of the extraction potentials, it is possible to manipulate the selective transfer of ions across the ITIES into a gel phase. This can confer a simple instrumental control over liquid-liquid extraction methodologies commonly used in the cleanup and preparation of complex samples for instrumental analysis. The benefit of the electrochemical manipulation of the extraction process is that in situ monitoring of the process is possible by virtue of the ion-transfer current. Postextraction confirmation of the process is possible by application of a voltammetric experiment. Hence, such an electrochemical method can be used for the selective extractive separation of ions in mixtures, such as the selective extraction of different aromatic sulfonates of similar chemical structure demonstrated in this work. The ability, by judicious choice of applied interfacial potential difference, to extract selectively one ion from a mixture or to coextract a range of ions of interest from a sample matrix may be realized with this methodology. This paper provides a simple yet easily controllable and manipulable route to the extraction, cleanup, and separation of ionic analytes. Further studies of the processes involved, including optimization and practical application problems are underway. ACKNOWLEDGMENT This work was supported by Science Foundation Ireland (02/ IN.1/B84) and the UK Engineering and Physical Sciences Research Council (GR/M67292). Received for review June 10, 2005. Accepted August 29, 2005. AC051029U