Anal. Chem. 1998, 70, 2205-2212
Electrically Polarized Ion-Exchange Beds in Ion Chromatography: Ion Reflux Hamish Small*
HSR, 4176 Oxford Drive, Leland Michigan 49654 John Riviello
Dionex Corporation, 450 Lakeside Drive, Sunnyvale, California 94088
All forms of liquid chromatography use eluents that must be prepared and replenished; a widely applied form of ion chromatography (IC) also requires a suppressor and a means for regenerating it. “Ion reflux”, as applied to IC, is a new ion-exchange technique where an electrically polarized ion-exchange bed becomes the source of eluent as well as its means of suppression. Using water as the pumped phase, such polarized beds enable the “perpetual” generation and suppression of eluent with little intervention by the user. In one embodiment of ion reflux, continuous eluent generation, ion separation, and continuous suppression are accomplished within a single bed. In another case, where separation is uncoupled from the other two functions, the ion reflux device may be used with existing separators. This paper describes the principles of ion reflux, the advantages and disadvantages of various embodiments, and gives examples of their use in both isocratic and gradient modes of ion separation. These new means for automating eluent generation and suppression should open pathways to new forms of IC instruments and systems.
When suppressed conductometric detection (SCD) was introduced in 1975,1 it offered chromatography a new and sensitive means for the determination of ionic species that lacked chromophores and were deemed, at that time, to be inaccessible to spectrophotometric detection. Employed in the new technique known as ion chromatography (IC), SCD found immediate application in the chromatography of inorganic ions, where many species are weak light absorbers at all wavelengths, and later to a wide variety of organic ions similarly lacking in useful spectral properties. Subsequently, because of its universal sensitivity to all ionic species, conductometric detection was applied even in cases where the ions were detectable by other means.2-4 Suppressed conductometric detection also introduced a concept that was new to chromatography and drew its share of (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 18011809. (2) Small, H. Ion Chromatography; Plenum Press: New York, 1989. (3) Haddad, P. R.; Jackson, P. E. Ion Chromatography: Principles and Applications; Journal of Chromatography Library 46; Elsevier: Amsterdam, 1990. (4) Weiss, J. Ion Chromatography, 2nd ed.; VCH: Weinheim, 1995. S0003-2700(98)00075-4 CCC: $15.00 Published on Web 05/05/1998
© 1998 American Chemical Society
skeptical acceptance. It used a component in the chromatographic system whose capacity to function was partly consumed in each run, thus requiring periodic interruptions of the chromatography to restore that component to its original capacity. In the first embodiment of IC, this device was called the “stripper” or suppressor bed. This suppressor was simply a column of ionexchange resin that enhanced the sensitivity of conductometric detection by converting eluent to a weakly conducting form while leaving the conductance of the analytes relatively unaffected. In due course, however, this bed became exhausted and had to be converted to its initial ionic form, a simple ion-exchange procedure but an undesirable interruption nonetheless. These interruptions, along with other problems associated with suppressor beds,5 provided the motivation to develop suppressors that would be less obtrusive in the total IC process.6-8 This evolution in suppressor technology culminated in the design of continuous, electrochemically regenerated membrane devices,9-11 requiring so little attention that the suppressor has become, in a sense, invisible to the user. While eluent suppression is now an unobtrusive feature of IC, the “front end” of IC, like most forms of liquid chromatography, demands continuous and careful attention. The user must still prepare and replenish eluents with accuracy and precision. If gradient elution is required, then two or more eluents must be dispensed, usually by a complex, expensive gradient pump. Dasgupta and co-workers have pioneered the application of electrodialytic methods to purify eluents for IC and to electrically control their concentration, thus circumventing the need for a gradient pump.12,13 Their techniques still require that the user prepare eluent, although no great precision is necessary since the electrical control takes care of that. A chromatographic system that completely avoided eluent preparation, in other words, a (5) Reference 2, p 170. (6) Stevens, T. S.; Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488-1492. (7) Stevens, T. S.; Jewett, G. L.; Bredeweg, R. A. Anal. Chem. 1982, 54, 12061208. (8) Stillian, J. R. L. C. Mag. 1985, 3a, 802-812. (9) Strong, D. L.; Dasgupta, P. K. Anal. Chem. 1989, 61, 939-945. (10) Rabin, S. A.; Stillian, J. R.; Barreto, V.; Friedman, K.; Toofan, M. J. Chromatogr. 1993, 640, 97-109. (11) Stillian, J. R.; Barreto, V. M.; Friedman, K. A.; Rabin, S. A.; Toofan, M. U.S. Patent 5, 248, 426, issued September 28, 1993. (12) Strong, D. L.; Dasgupta, P. K. J. Membr. Sci. 1991, 57, 321-336. (13) Strong, D. L.; Dasgupta, P. K.; Friedman, K.; Stillian, J. R. Anal. Chem. 1991, 63, 480-486.
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system where the front end was as unobtrusive as the current suppressor, could offer significant advantages over present technology. Simplification of operation and greater ease in the automation and in the application of IC to remote or unattended operation are just a few of the benefits that come to mind. This paper describes a new concept for creating such a system. This new IC method retains much of current technology; analytes are still separated by ion exchange and it uses continuously regenerated suppressors with conductometric detection. But instead of an electrolyte solution, water is the pumped phase. It is important to emphasize the distinction between the terms “eluent” and “pumped phase”. The active eluent in the separator is an electrolyte solutionsthe ion-exchange process requires itsbut the fluid pumped to the chromatographic system is, in the simplest embodiment, pure water. In this new technique, the eluent is generated in situ and eluent generation and suppression are linked in a critical way. The key to this new process is an electrically polarized ion exchange bed, operated in what we call the ion reflux mode. We will use the analysis of anions by IC to describe and exemplify the principles of “ion reflux”. PRINCIPLES OF ION REFLUX Ion-exchange resins are good conductors of electricity; the mobile counterions of the resins contribute entirely to their conductance while the highly charged but immobile resin matrix contributes nothing.14,15 How good conductors are ion-exchange resins? A typical strong cation-exchange resin, Dowex 50 × 8, in the sodium form and fully swollen with water, with a capacity of about 2.5 mequiv/g, is as conducting as 0.23 M NaCl. In the hydrogen form, it is as conducting as 0.55 M HCl.14 Not surprisingly, therefore, a packed bed of ion-exchange beads is an electrical conductor, although a rather complex one involving the resin beads, their regions of contact, and the aqueous fluid in the void space.14,15 One embodiment of an electrically polarized ion-exchange bed is, simply, a column of ion-exchange beads retained between metal electrodes to which a dc potential is applied. The electrodes, which make direct contact with the resin, are porous, so they allow a liquid, usually water, to be pumped through the bed in either direction. Let us consider what occurs when a bed of highcapacity cation-exchange resin is polarized and pumped as depicted in Figure 1. A dc potential applied to a packed bed of cation-exchange resin, such as shown in Figure 1, will induce the positive counterions to move toward the cathode and an electrical current will be sustained if there are means for replenishing the counterions leaving the anode and for disposing of those arriving at the cathode. The electrode reactions with water provide the means. The anodic oxidation of water produces hydronium ions and oxygen gas thus,
Figure 1. An electrically polarized cation-exchange bed as a generator of potassium hydroxide. (A) is one way of illustrating an electrically polarized packed bed of cation-exchange resin particles, upper layer in the potassium form, lower layer in the hydronium form. We have found the more abstract representation of (B) to be a better way of conveying the idea of the resin phase as a continuous electrical pathway between the electrodes (the crosshatched regions in (A) and (B)). The split rectangle in the left side of the column (B) represents the resin phase in its two-layer form, while the rectangle in the right side of the column represents the contiguous mobile phase. (B) represents a polarized bed being used as a generator of potassium hydroxide. (C) represents the directions of ion migration and fluid flows in the generator; the arrows on the left of (C) represent electromigration of the ions in the resin phase; the arrows on the right of (C) represent fluid flows. The horizontal broken lines in (C) represent the porous electrodes.
displace other hydronium ions and, in turn, potassium ions toward the cathode.16 The cathode reaction produces hydrogen gas and an equivalent number of hydroxide ions as companions for the potassium ions arriving at the cathode,
H2O + 2e f 2OH- + H2 (gas)
(2)
The hydronium ions are injected into the resin, where they
If we supply water to the bed in the anode-to-cathode direction, the potassium hydroxide solution is carried from the bed (Figure 1B). The directions of ion migration and flow of water and electrode products are shown in Figure 1C. If the water flow rate and current are maintained constant, then the device of Figure 1 can provide a stream of potassium hydroxide of steady concentration, useful, for example, as an eluent for IC. This method of eluent generation will be the subject of subsequent papers. In this paper, we will address the consequences of changing the direction of water flow in the system as depicted in Figure 2. In the Figure 1 device, water flow and cation electromigration are in the same direction, whereas in the Figure 2 device, they are opposed. Thus, while the potassium hydroxide produced at the cathode of the Figure 1 device is flushed from the bed, in the
(14) Small, H. Some Electrochemical Properties of an Ion Exchanger. M.Sc. Thesis, The Queen’s University of Belfast, Belfast, 1953. (15) Sauer, M. C.; Southwick, P. F.; Spiegler, K. S.; Wyllie, M. R. J. Ind. Eng. Chem. 1955, 47, 2187-2193.
(16) Since resins are conducting current, the best choice of resin is usually the resin with the best conductivity, providing no other factor overrides that choice. Potassium form resin is approximately twice as good a conductor as the sodium form.
H2O f 2H+ + 1/2O2+ 2e
2206 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
(1)
Figure 2. Ion electromigration and fluid flows in an ion reflux device.
Figure 2 device, it is immediately returned to the bed wherein it flows through the interstitial void space of the potassium form resin and arrives at the hydronium/potassium boundary. There, the familiar ion-exchange neutralization reaction
KOH + H+resin f K+resin + H2O
Figure 3. An IRD-1 incorporated in a total IC system. The colloidal anion-exchange resin layer on the surface of the potassium form resin is represented by the small circles attached to the upper portion of the split rectangle in IRD-1.
(3)
returns the potassium to the resin phase, where it resumes its electromigration toward the cathode. This movement of a species in one phase in one direction, coupled to its movement in a contiguous phase in the opposite direction, was so reminiscent of reflux in distillation, that the term ion reflux was chosen to describe it. But whatever the name, the key feature of the system represented by Figure 2 is the intimate and harmonious manner in which eluent generation and suppression are linked. Since the current produces hydronium ions at the anode and potassium hydroxide at the cathode in exactly equivalent amounts, the flux of hydronium ions in one direction must balance the flux of potassium hydroxide in the opposite direction, and as a result, the hydronium/potassium boundary remains spatially fixed. Changing the current changes the flux of hydronium ions toward the cathode. However, the concentration of potassium hydroxide flowing in the opposite direction responds to the current change to an exactly equivalent extent and the two opposing fluxes therefore remain in balance. So the boundary remains stationary.17 Thus, the electrochemical splitting of water provides the engine for driving this circulating pool of potassium which, in principle, is a perpetual means for generating base and for suppressing it. How can such a device that incorporates a means of eluent generation and suppression also incorporate the third element of IC, the ion-exchange separator phase? Since the cation-exchange resin has a highly charged negative surface, it provides an (17) Strictly speaking there is a slight displacement of the boundary to a new steady-state position. The concentration of potassium hydroxide in solution is proportional to the current, and since this potassium comes from the “potassium pool”, higher currents cause movement of the boundary toward the cathode and vice versa. Calculations show that for typical ranges of current, the displacements are a few millimeters in an ion reflux device.
excellent electrostatic anchor for oppositely charged, colloidal, anion-exchange resinsthe well-known surface agglomeration technique.18 This layer of tenaciously bound anion exchanger is the stationary phase for the electrochemically generated mobile phase, and thus, all three functions of suppressed IC are incorporated in a single column. We call this the monolithic version of ion reflux and refer to it as an ion reflux device of the first kind, or briefly as IRD-1. Figure 3 illustrates the incorporation of an IRD-1 into a total IC system. It comprises a source of pure water, a precision pump, a sample injection device, IRD-1 polarized by a suitable dc power supply (not shown), a conductivity cell and meter (not shown), and a flow restrictor. When water is pumped to the system with field applied to IRD-1, analyte anions carried into IRD-1 are selectively eluted from the colloidal anion exchanger by the base produced at the cathode. At the hydronium/potassium boundary, base is continuously removed and the separated analytes are converted to their respective acids. From there, the analyte acids are passed to the conductivity cell where they are sensitively monitored in a background of pure water. Thus, apart from the contained analytes, it is a case of water in, water out. When the IRD-1 is operated at constant current, the constant concentration of produced base provides an isocratic elution. Alternatively, if the separation requires it, gradients or step changes in eluent concentration can be arranged simply by programming the current in the IRD. We said earlier that ion reflux devices can, in principle, provide perpetual generation and suppression. It is an appropriate point to add a caveat which relates to the fate of the cations that accompany the analyte anions. Let us consider, for example, that the anions are injected as their sodium salts. While the anions eventually elute from the IRD, the cations do not; they are (18) Reference 2, p 46.
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captured in the cation-exchange resin and added to the potassium pool. Since the total ion-exchange capacity of the device is fixed, it follows that any addition of ions to the metal ion pool must take place at the expense of depleting the hydronium suppressor section by an equivalent amount. While a single sample will normally lead to only a minuscule reduction of the suppressor capacity, it is obvious that a sufficiently large number of samples will eventually consume the hydronium part of the device entirely, thus disabling one of its vital functions. There is a simple solution to this problem; inject the analytes as their acids, thus avoiding the addition of any extra metal ion to the potassium pool. A convenient way to accomplish this is by placing a small bed of cation-exchange resin in the hydronium form, between the sample injection valve and the IRD. Of course the bed will eventually become exhausted and will have to be regenerated or replaced, but that is a relatively simple procedure compared to the corrective regeneration of an IRD that has accumulated cations from the sample. While the monolithic device appears to be the ideal embodiment of ion reflux for ICsand it may be for some applicationss some circumstances argue for less integration of the functions. For example, exposing separator beds to poisoning species can lead to their failure, in which case they must be replaced. If such were the fate of the separator in a monolithic IRD, the complete device would have to be replaced since there is no known way to rejuvenate a spoiled separator. Discarding a device where two of the functions were still unimpaired would be acceptable only if the replacement was cost-effective, that is, if the device had paid for itself in analyses performed before failure. We anticipate this possibility in special cases, for example, in water quality monitoring in the electric power industry where devices will normally be exposed to very “clean” samples. But for the many applications where fouling of the separator is more likely, we saw a need for a version of ion reflux that isolates separation from the other two functions. Furthermore, from a manufacturing standpoint, such a device would be less disruptive than completely revamping the whole IC system and therefore is more likely to be an acceptable step. On the basis of these arguments, we developed a device that allows the use of a separate, conventional separator column, which can be easily replaced when necessary. We call this an ion reflux device of the second kind and refer to it as IRD-2. ION REFLUX DEVICE OF THE SECOND KIND For anion analysis, an IRD-2 (Figure 4) comprises a small bed of high-capacity cation-exchange resin confined between a porous metal electrode (the anode) and a porous plastic support of the type common to chromatography. The cathode is placed in a cathode chamber that is separated from the main resin bed by a cation-exchange membrane or plug of cation-exchange resin. This membrane (or plug) permits electromigration of cations but prevents the flow of fluid between the cathode and resin compartments. Donnan exclusion essentially prevents anion migration through the membrane. Figure 4 illustrates the IRD-2 integrated in a total IC system. Polarization of the IRD-2 in the manner shown causes hydronium ions in the resin to displace potassium ions toward the membrane. The potassium ions pass through the membrane into the cathode compartment, where they receive an equivalent amount of hydroxide ions. Water pumped through the cathode compartment carries the potassium hydroxide to a 2208 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
Figure 4. An IRD-2 in an IC system. The solid black rectangle represents the cation-exchange membrane separating the resin bed from the cathode compartment. The system components are not drawn at the same scale.
sample injection valve, through an anion separator, and then to the IRD-2. There the potassium hydroxide eventually meets the hydronium/potassium boundary where the potassium returns to the resin. The potassium thus circulates in the same way that it does in an IRD-1, providing, in principle, a perpetual means of eluent production and suppression. Unlike an IRD-1, however, where the potassium hydroxide returns immediately to the bed, in an IRD-2, the potassium hydroxide produced in the cathode compartment passes through a conduit before returning to the resin bed. This allows us to “break into” this conduit and insert any separator that we choose. Like an IRD-1, manipulation of the current in an IRD-2 produces changes in the concentration of eluent and permits either isocratic or gradient elution. The potassium/hydronium boundary remains spatially fixed in an IRD-2 just as in an IRD-1 (see ref 17 however). In an IRD-2, as in an IRD-1, the same caveat applies to the claim that eluent suppression is perpetual, but dealing with the problem of cations from the sample is slightly different. Placing a small cation-exchange resin bed in the hydronium form between the sample injection valve and the separator is not a solution because it will be quickly exhausted. So, in the case of IRD-2 type devices, analytes must be converted to their respective acids before being loaded to the sample injection valve. Ion Reflux Devices for Cation Analysis. Designing ion reflux devices for cation analysis involves appropriate modifications of the resins and membranes and changing the polarities of the
electrodes. Thus, in an IRD-2 for cation analysis, a high-capacity anion-exchange resin occupies the resin compartment and an anion-exchange membrane or plug separates it from the anode chamber. The superior layer of anion exchanger is in a counterion form typical of acids used in IC, e.g., chloride or methanesulfonate. The lower layer in direct contact with the cathode is in the hydroxide form. Cathodically generated hydroxide displaces chloride (or methanesulfonate) through the membrane and into the anode chamber where the anodically generated hydronium ion forms hydrochloric (or methanesulfonic) acid, which goes on to perform its eluting function in the separator column. The acid from the separator passes to the IRD-2, eventually encountering the upward flux of hydroxide which returns the acid anion to the resin for recirculation. The analytes pass to the conductivity cell as hydroxides or free bases in a background of pure water. Similar qualifications and caveats apply to ion reflux devices for cation analysis, vis a` vis boundary stability and perpetual suppression, as apply to their anion counterparts. Managing the Electrolysis Gases. The electrode reactions produce the electrolysis gases hydrogen and oxygen, and depending on the device used, one or both gases are carried through the separator and through the conductivity cell. At a current of 100 mA, the total volume of gas produced is ∼1 mL/min. While a current this large probably represents the highest rate to run an IRD, the lower currents typically used will produce sizable volumes of gas. Whether this gas will affect the separation is questionable, but we can expect it to interfere with the proper functioning of the detector unless we can find some way to manage it. We have used what we call the “Boyle’s law” solution; if the gas is of a bothersome volume at ambient pressure, then compress it to an acceptably small volume. Addition of a flow restrictor at the outlet of the IC system raises the pressure in the system and compresses the gases to an insignificant volume. Several meters of plastic capillary of suitable bore diameter, placed at the end of the system, raise the operating pressure in the conductivity cell from 200 to 500 psi at normal flow rates. Other methods for coping with electrolysis gases19 should be adaptable to ion reflux devices. EXPERIMENTAL SECTION Fabrication of an IRD-1. The positions of the various boundaries in an IRD-1 type device are important and, while we have assembled devices in a single piece of plastic tubing, it is easier to control boundary positions if the device is assembled in sections. Step 1. The Separator Section. A 4 mm i.d. × 150 mm length PEEK column was equipped with a plastic bed support and filled with the potassium form of a strong cation-exchange resin, nominally 8% cross-linking, 20 µm in diameter. The resin was prepared by sulfonating cross-linked polystyrene to the fullest extent possible; its ion-exchange capacity was roughly 5.0 mequiv/g of dry resin. The resin bed was then treated by slowly pumping through it a dilute suspension of a colloidal anion-exchange resin, Dionex AS-11 anion-exchange latex resin (Dionex Corp., Sunnyvale, CA). A thorough water rinse removed any unattached latex. This composite, surface-agglomerated resin is the separator section of IRD-1. (19) Strong, D. L.; Chang, U. J.; Dasgupta, P. K. J. Chromatogr. 1991, 546, 159-173.
Step 2. The Suppressor Section. A 4 mm i.d. × 50 mm length PEEK column was coupled directly to what would eventually be the outlet of the separator section. This section was then filled with the same high-capacity cation-exchange resin as used in the separatorsalso in the potassium form. The outlet of this section was equipped with a porous platinum electrode/bed support. The Pt electrodes were custom-fabricated porous disks (Mott Metallurgical, Farmington, CT.) They varied from 4.1 to 4.2 mm in diameter, were ∼1.45 mm thick, and had a 2-µm nominal porosity. Step 3. “The Resin Spring”. Through many trials and failures, we discovered that it is vital to maintain good physical contact between the resin and the electrodes. While fluid flow guarantees good contact at the outlet electrode, it tends to separate resin from the inlet electrode. There are many possible solutions to this problem; we used what we call a “resin spring”. A 4 mm i.d. × 35 mm length PEEK column was equipped with a Pt electrode and filled with Dowex 50W × 4 that had been equilibrated with 20% potassium nitrate to osmotically shrink the resin. This resinfilled column was then attached to the inlet of the separator section and the complete assembly pumped with deionized water in the direction of suppressor-to-resin spring. This washing reswelled the Dowex 50W × 4 resin, thus forcing it against what would become the inlet electrode. Step 4. As a final step, some of the resin in the suppressor section was converted to the hydronium form. This was generally done by pumping 0.01 M nitric acid at 1 mL/min in the direction suppressor-to-resin spring. The amount of acid was precisely controlled; typically 40 mL of acid was pumped, which means that 0.4 mequiv of suppressor capacity was implanted at the exit of the device. Some uncoated potassium form resin between separator and hydronium form resin is recommended to allow for boundary creep. The hydronium ion may also be implanted electrochemically. Fabrication of an IRD-2. IRD-2 type devices do not contain a separator and are thus more compact than IRD-1 devices. The first IRD-2 was fabricated from a single block of plastic (6.5 × 4 × 2.5 cm). The main resin compartment was a cylindrical cavity, 4 mm in diameter and 6.5 cm long, drilled in the block and terminated by a porous Pt disk electrode/bed support. It was filled with the same high-capacity cation-exchange resin (20 µm, 8% cross-linked) used in the IRD-1 for the separator substrate. The resin was initially loaded in the potassium form and then a precisely measured amount of hydronium ion (usually 0.5 mequiv) was injected at the outlet end. The remaining upper portion of the resin in the potassium form was ∼1.3 mequiv. The resin compartment was separated from the cathode compartment by a cation exchange membrane disk, (Membrane International, Glen Rock, NJ) designated AMI-7000 cationexchange membrane. The cathode was a length of fine Pt wire crumpled and formed into a rough disk shape. This more open structure allowed proper irrigation of the cathode/membrane interface. In IRD-2 devices for cation analyses, the resin was an 8% crosslinked polystyrene/divinylbenzene-based anion-exchange resin; the membrane was a disk of Ultrex Membrane AMI-7001 (Membrane International). CAUTION: We know that electrically polarized ion-exchange beds can become hot, particularly when current is high and water Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
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Figure 5. The first IRD-2.
flow rate is low. We are also aware that anion-exchange resins in the nitrate form have been known to decompose explosively when heated. Therefore, we urge those who experiment with electrically polarized beds to use extreme caution when working with anion-exchange resins with oxidizing counterions such as nitrate or perchlorate. Figure 5 is a photograph of the first IRD-2. Power Supplies. Electrophoresis power supply EPS 600 (Pharmacia Biotech, Uppsala, Sweden) was used in isocratic elutions. A model 220 programmable current source (Keithley Instruments, Cleveland, OH) was used mainly for gradient and step elutions but also in some isocratic experiments. The currents supplied to the various devices were in the range of 0.5-30 mA; the applied voltages ranged from 6 V to a maximum of ∼150 V. The chromatographic pump used was a Dionex gradient pump model GPM-2. The conductivity detector was a Dionex conductivity detector model CDM-1. Pure water was prepared by treating distilled water in a Barnstead EASYpure RF water purifier. This produced water with a specific resistivity of ∼18 MΩ-cm. RESULTS AND DISCUSSION Both types of ion reflux devices were studied from two points of view; is the ion reflux concept feasible and, if so, how stable is chromatographic performance? While we have not made a systematic study of ion elution behavior over long periods of usage, we can report the behavior of both types of devices when they were operated for several days continuously (∼10 h of continuous running each day). On one test of the IRD-1, where the current was 12 mA (129 V) and the water flow rate was 1 mL/min, the elution times of fluoride, chloride, nitrate, and sulfate were initially, 1.02, 1.3, 2.02, and 4.07 min, respectively. After 2 days of continuous running, a test made under the same conditions gave elution times of 1.02, 1.3, 2.02, and 4.18 min for the same set of anions. After a further 8 days of intermittent use, the elution times were 1.02, 1.28, 1.95, and 3.92 min, respectively. Performance that was typical of the IRD-2 is illustrated by the following example. The IRD-2 was run continuously for 8 h in conjunction with a Dionex AG-11 column as separator. During this period, the current was maintained at 6 mA while the water flow rate was steady at 1 mL/min. At the beginning of the 8-h 2210 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
Figure 6. Capacity factors (k′) vs current for an IRD-1. Flow rate was 1 mL/min. Capacity factors are flow rate dependent in ion reflux systems, since, at a fixed current, a higher flow rate produces a lower concentration of eluent, and vice versa.
period, the elution times of fluoride, chloride, nitrate, and sulfate were 0.63, 0.82, 1.38, and 4.00 min, respectively. At the end of the period of continuous running, the elution times were 0.70, 0.90, 1.38, and 4.00 min for the same set of anions. This stability in capacity factors shows that maintainence of a steady current through an IRD of either type produces a steady and reproducible concentration of eluent. We have also demonstrated that the concentration of eluent produced is proportional to the current in the device; furthermore, concentrations agree closely with values calculated by applying Faraday’s laws of electrolysis. This link between current and concentration is reflected in plots such as Figure 6. Plots similar to Figure 6 are commonplace in conventional IC;20 in that case, the abscissa is in units of concentration. In view of the proportionality of current and concentration, plots such as Figure 6 are to be expected for ion reflux devices. With respect to chromatographic efficiency, since IRD-2 systems employ commercial separators with reliably stable efficiency, we expected IRD-2 systems to reflect a similar stability; and they did. In the example where the IRD-2 system was run continuously for 8 h, there was no change in the efficiency. In the IRD-1 device, efficiency declined as the device was run continuously; in the worst case, at the end of a day of continuous use, the efficiency had dropped to half its initial value. The original efficiency could be recovered entirely if flow was stopped for a time, but then declined again when flow was resumed. We believe that this loss in efficiency is caused by the water flow pushing the resin away from the inlet electrode in a nonuniform way, so that the resistance to current is variable from place to place radially in the resin/electrode interface. This, in turn, will lead to a nonuniform radial distribution of eluent concentration, which will distort the peak shape of eluted species and be manifested as impaired efficiency. So although the resin spring was a breakthrough that enabled the first workable IRD-1, it is not a complete solution to the problem that it addresses. Some means of adjusting the position of the entrance electrode may provide a better solution to the problem of unstable efficiency in IRD’s of the first kind. (20) Reference 2, p 67.
Figure 9. Separation of low-affinity anions. Separator: Dionex AS11, 4 × 250 mm. Sample (in order of elution): fluoride, acetate, formate; 10 µL, 0.001 M in each. Flow rate of water, 1 mL/min. The current was 0.5 mA; the applied potential was 6 V. Figure 7. A separation on an IRD-1 and the effect of flow rate of water. Ions (in order of elution): fluoride, chloride, nitrate, sulfate, and phosphate. Sample: 10 µL, 0.0001 M in each analyte. Elution volumes were, respectively, 1.02, 1.30, 2.02, 4.18, and 19 mL at 1 mL/min and 0.98, 1.11, 1.48, 1.76, and 4.6 mL at 0.5 mL/min. The current was 12 mA; the applied potential was 129 V.
Figure 10. Separation of cations on an IRD-1. Ions (in order of elution) lithium, sodium, potassium. Sample: 10 mL, 0.00033 M in each ion. The flow rate of water was 1 mL/min; the current was 0.9 mA; the applied potential was 57 V.
Figure 8. Separation using an IRD-2 with a conventional separator. Separator: Dionex AG-11; 4 × 50 mm. Sample (in order of elution): fluoride, chloride, nitrate, sulfate, and phosphate; 10 µL, 0.001 M in each analyte. Flow rate: 1 mL/min. Current programmed: first minute, 2 mA; second minute, 3 mA; third minute, 10 mA; third to seventh minute, 20 mA.
It is possible that, for different reasons, there is variation in the quality of membrane/electrode contact in IRDs of the second kind. In this case, however, the effects of nonuniformities are obliterated by mixing in the electrode chamber and in the conduit leading from it, so the behavior is irrelevant to overall efficiency. Separations. Many separations were performed using both devices; some of the more noteworthy chromatograms are shown in Figures 7-10. The chromatogram of Figure 7 is noteworthy in that it shows (a) a separation typical of an IRD-1 and (b) an unusual effect of flow rate on elution volumes; all of the ions have lower elution
volumes at the lower flow rate, the effect being especially pronounced in the case of phosphate. There is a simple explanation for this. Since the current is the same in both experiments, it follows that the concentration of eluent generated at 0.5 mL/ min must be twice that at 1 mL/min. So all ions must elute at smaller volumes at the lower flow rate. The influence of electroselectivity on the displacement of polyvalent ions by monovalent ions20 accounts for the exceptional elutability of phosphate, and to a lesser extent sulfate, by the more concentrated potassium hydroxide produced at the lower flow rate. Since the separator phase of an IRD-1 lies within the electrical field applied to the device, how much of a part does electrophoresis play in the displacement of sample ions from such a device and how much of the displacement is chromatographically induced? We believe for the following reasons that electrophoresis plays an insignificant role. In the first place, the order of elution is that of a chromatographic elution. Second, the voltage gradients in the IRD-1typically less than 6 V/cmsare not high enough to account for the rate of ion displacement that we observe. For example, in Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
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the experiment of Figure 7, at 1 mL/min flow rate, fluoride traverses the 23.5-cm-long IRD-1 in ∼1 min, that is, at a linear velocity through the device of ∼23 cm/min. Under a voltage gradient of 5.5 V/cm (129/23.5 ) 5.5) fluoride ion, with a mobility of ∼0.0005 cm2 V-1 s-1 in free solution, will travel ∼0.2 cm/min electrophoretically. So the calculated electrophoretic contribution is roughly 1% of the observed rate of translation of the fluoride ion through the device. Therefore, we conclude that the major driving force on the fluoride ion is the flow of the mobile phase. How about the slower eluting ions, especially sulfate? Sulfate is displaced more slowly than fluoride because sulfate spends most of its time in the stationary phase (fluoride spends a small fraction of its time in the stationary phase). We do not know precisely the magnitude of ion mobilities in the stationary phase, but we can assume them to be roughly 1 order of magnitude less than they are in free solution.14 On this basis, the electrophoretic mobility of sulfate through the IRD-1 is calculated to be ∼0.03 cm/min. In the experiment of Figure 7, the observed velocity of sulfate is (23.5/19 cm/min), that is ∼1.2 cm/min. So, as with fluoride, the calculated electrophoretic contribution for sulfate is much less than the observed displacement velocity. Finally, when the unpolarized IRD-1 was used as the separator in a conventional IC experiment, employing potassium hydroxide eluent of the same concentration as that generated in the ion reflux mode, the elution times were close to those observed when the IRD-1 was run in the ion reflux mode. Figure 8 demonstrates programmed elution using IRD-2 with a commercially available separator. In this example, which was designed simply to illustrate the separation of ions of widely differing affinities in a single run, the current was increased in steps as the run progressed. No attempt was made to optimize this separation, which accounts for the comparatively broad peak for phosphate; a further increase in current beyond 10 mA would have led to an earlier elution of phosphate and a concomitantly sharper peak. The small peaks at 2.63 and 3.52 min are probably due to impurities. We have shown for example that if we allow the purified water to become even slightly contaminated by carbon dioxide, then carbonate accumulates on the separator during the low-current part of the cycle and is dislodged as a peak when the current is increased to 10 mA. The separation of some low-affinity anions (Figure 9) is noteworthy in that it was accomplished at a very low current of 0.5 mA. At this current, at 1 mL/min, the concentration of base generated is 0.00031 M (calculated). Preparing potassium hydroxide of such a low concentration in the conventional way is no challenge, but keeping it carbonate free is. A relatively small amount of contamination by carbon dioxide will have a major effect on the eluting power of base of such a low concentration.21 The in situ generation of base in the IRD precludes carbon dioxide contamination and guarantees reproducible eluting behavior from
the dilute bases necessary for resolving weakly held species. In the separation of anions using both types of ion reflux devices, background conductivities were consistently less than 0.5 µs. An IRD-1 was prepared to demonstrate that the ion reflux principle also could be applied to cation analysis; a separation is shown in Figure 10. Separation Columns for Ion Reflux. Historically, most anion separation columns used carbonate-based eluents. Even at the low-millimolar concentration range, carbonate-based eluents can efficiently elute polyvalent ions such as sulfate and phosphate. This is not the case for hydroxide eluents; therefore, to realize the full potential of ion reflux, hydroxide-selective separation columns will be required. Fortunately, such columns are being developed22 because ion chromatographers have recognized the advantages of hydroxide-based eluents: low background conductivity after suppression, minimal water dip, and improved resolving power for weakly held species. Dionex AS-11 is such a column.
(21) Small, H. In The History and Preservation of Chemical Instrumentation; Stock, J. T., Orna, M. V., Eds.; D. Reidel Publishing Co: Dordrecht, 1986; pp 97107. (22) Jackson, P. E.; Pohl, C. A. TrAC, Trends Anal. Chem. 1997, 16, 393-401.
Received for review January 27, 1998. Accepted April 13, 1998.
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CONCLUSIONS This work has demonstrated the feasibility of the ion reflux concept. As applied to IC, it retains many of the strengths of present technology while adding some new advantages. It still uses ion exchange as the separation mechanism, so it inherits the superb technology of present day separators. It uses water as the pumped phase, which should greatly simplify the front end of IC. One can even envision a system where the effluent from the conductivity cell is passed back to the water reservoir via a small bed of mixed ion-exchange resin to remove the analytes, and the only water consumed is that electrolyzed to gases. In principle, both ion reflux systems are capable of perpetual in situ generation of eluent. The concentration of the eluent can be easily controlled by controlling the current applied and/or the flow rate of water supplied. It follows that ion reflux systems should have easily controlled gradient capabilities and this has been demonstrated. With appropriate sample pretreatment, ion reflux systems can, in principle, deliver perpetual eluent suppression, thus permitting the use of well-established conductometric techniques. While IRD-2 devices have the advantage that they may be used with existing separators, the special appeal of the highly integrated monolithic devices should motivate their application to suitable problems. For these reasons, we see many opportunities for applying the principles of ion reflux to the design of new IC instruments and systems. ACKNOWLEDGMENT H.S. thanks the Dionex Corp. for their support and especially thanks Nebojsa Avdalovic and his colleagues for the hospitality and help extended to him while working at the Dionex research laboratories in Sunnyvale. The methods and apparatus described in this publication are the subject of pending patents.
AC980075+
