Ion Chromatography edited by
Waters Symposium
Adrian Michael University of Pittsburgh Pittsburgh, PA 15260
Ion Chromatography: An Account of Its Conception and Early Development Hamish Small 4176 Oxford Drive, Leland, MI 49654;
[email protected] In September 1975 Dionex Corporation, then a new company, introduced the first instrument to practice what became known as ion chromatography. That same September three chemists from the Dow Chemical Company published an article describing the new technology and its status at that time (1). This is an account of the conception of ion chromatography and of its development into a technique ready for commercialization. In 1955 I joined the Dow Chemical Company in Midland, Michigan, in what was known as the Physical Research Laboratory (PRL), commonly called “The Physics Lab”. The Physics Lab had a great reputation throughout the company for invention and innovation; polystyrene, polyethylene, saran, polymer foams, and the extraction of magnesium from seawater were a few of the Dow technologies that had been pioneered there. The director of the laboratory at that time was Bill Bauman. One of the company’s great scientist–man-
agers, he had been the leader in establishing Dow as a major producer of ion-exchange resins. Shortly after I arrived at Dow, Bill moved out of the lab to a position of higher responsibility, but we kept up a contact that would bear fruit many years later. My early background in ion-exchange research at Harwell in England had attracted me to Dow and to the ion-exchange group there. The head of the ion-exchange group was Bob Wheaton. Bob, in collaboration with Bill Bauman, had invented ion exclusion (2, 3), a technique for large-scale separation of ionic and nonionic compounds that would later find analytical application. Another member of PRL was Mel Hatch, the inventor of ion retardation, a process for separating electrolytes using water as an eluent (4). When I arrived at Dow, Mel was exploring the application of ion retardation on a large scale to a number of Dow’s separation problems.
The 12th Annual James L. Waters Symposium at Pittcon: Ion Chromatography The James L. Waters Symposium has been held at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy (Pittcon) annually since 1990. The Waters Symposium is unique, in that it explores the origins, development, and commercialization of scientific instrumentation of established and major significance. James Waters, founder of Waters Associates, Inc., arranged with the Society for Analytical Chemists of Pittsburgh (SACP) to offer this annual symposium to ensure that the early history of this cooperative process be preserved, to stress the importance of contributions of workers with diverse backgrounds, objectives, and perspectives, and to recognize pioneers and leaders in the field of instrumentation. Benefits of the symposia include the creation of awareness of the ways in which significant new instruments emerge and how they pave new avenues of scientific investigation. The symposia also promote an interchange among the inventors, development engineers, marketing personnel, and entrepreneurs who play distinct and crucial roles in that emergence. Publication of the proceedings of the Waters Symposia is a high priority of the SACP. The proceedings of the first symposium, on gas chromatography, were published in LC.GC Magazine and those of the next four symposia (on atomic absorption spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectrometry, and mass spectrometry) appeared in Analytical Chemistry. Proceedings of all subsequent symposia have been published in the Journal of Chemical Education: the sixth, on high-
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performance liquid chromatography, appeared in the January 1997 issue (pages 37–48); the seventh, on ion selective electrodes, appeared in the February 1997 issue (pages 159–182); the eighth, on lasers in chemistry, was featured in the May 1998 issue (pages 555–570); the ninth, on immunoassay, appeared in the June 1999 issue (pages 767–792); the tenth, on atomic emission spectroscopy, appeared in the May 2000 issue (pages 573–607); and the eleventh, on X-ray diffraction of powders and thin films was featured in the May 2001 issue (pages 601– 616). The subject of the twelfth annual Waters Symposium was ion chromatography, and the proceedings of that symposium are featured in this issue of the Journal. The first paper in the series is by Hamish Small and describes the very early steps in the invention of the technique and instrumentation that evolved into today’s version of ion chromatography. Barton Evans provides a detailed view of the critical role that the Dionex Corporation played in converting ion chromatography from a research curiosity to a finished, commercially successful product. Finally, Paul Haddad of the University of Tasmania describes a software package that simulates the retention process of ion chromatography, which reflects the advanced state of current understanding of these mechanisms.
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The ion-exchange group was involved in three main areas of research: synthesizing ion-exchange resins, defining their properties, and finding applications for them. The work engaged us in a diversity of chemical disciplines, each with its own set of challenges to be met and problems to be solved. One problem that cropped up regularly was the determination of common inorganic ions such as the alkali-metal and alkaline-earth ions, the ammonium ion, the halide ions, sulfate, nitrate, phosphate, carbonate, et cetera. The determination of halides was accomplished in-house by applying potentiometric titration, but the others required a variety of methods of such widely differing chemistries and instrumentation that only specialists in Dow’s analytical laboratories were equipped to handle them. Thus the analytical problems added significantly to the cost of many projects. Additionally, many of the analytical methods of those days lacked the sensitivity that the projects demanded. Consequently a few of us in the Physics Lab began to imagine the benefits of replacing these many wet-chemistry methods with a single chromatographic technique that would be universally applicable to all ionic species. There were two major problems to solve: separating the various ionic species and then detecting and measuring each. At first, the chromatographic separation of the inorganic analytes did not seem to be a problem; there was an abundance of ion-exchange separation methods that we could draw on. It was the detection of the separated ions after separation that presented the major challenge. At that time, in the mid1950s, fraction collectors were much in vogue in chromatography. The effluent from a chromatographic separation was split into many fractions that were then laboriously analyzed for the species of interest. We decided that any acceptable technique must dispense with fraction collectors and should, instead, provide prompt analysis of the column effluent as it left the column. In other words, it should incorporate continuous monitoring of the effluent by some sort of universal detector. Therein lay the problem. Because most of the ions of interest did not absorb light or offered no convenient means of generating light-absorbing species, we deemed them undetectable by light-absorption methods.1 Monitoring electrical conductivity looked promising; electrical conductance is a universal property of ions in solution and it has a simple dependence on ion concentration, at least in dilute solutions. Furthermore we anticipated that conductivity detectors would be relatively easy to build and would be robust in use. However, conductivity detection following directly on ion-exchange separation had what we perceived to be a serious drawback. Since the eluents necessary for ionexchange separation are themselves electrolytes and therefore highly conducting, we foresaw the difficulty of distinguishing between the small conductance changes due to the analytes and the conductance “noise” of the eluent. So early in the evolution of what became ion chromatography we decided that we would use conductivity detection but we would not use it in conjunction with ion-exchange separation. If we did not use ion exchange as the separation mode what then might we use? Mel Hatch’s ion-retardation resins offered a possible solution; they would allow us to separate the electrolytes using water—the ideal eluent—where the analyte ions would be easily detectable in this low conductivity, essentially noise-free background. Although Mel’s resins 1278
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worked well in large-scale applications where column loading was high, we found that the few unbalanced ionic sites in the resins were sufficient to irreversibly bind the small quantities of electrolyte typical of analytical applications. So for a time the project was stalled because of the inability to devise a suitable stationary phase that would function with a water eluent. However, I recall that Bob Wheaton, in his annual research-planning statement for upper management, always included some remarks on what we had now begun to call “inorganic chromatography”. So what would eventually become ion chromatography (IC) had a quiet start. No task forces or crash programs were set up at Dow, nor was there any clamor at scientific meetings for chromatography to be expanded to inorganic analysis. It was just a dream that a few of us shared as far back as the late fifties and many years would pass before we made the breakthrough that would start IC on its way to success. 1971, A Breakthrough: The Suppressor By 1971 the ion-exchange group in PRL had dispersed; Bob Wheaton had moved to Dow’s research lab at Walnut Creek, CA, and Mel Hatch had gone into academia. I was still in Midland but was devoting most of my research to a new chromatographic technique, hydrodynamic chromatography. Concurrently I was exploring a new idea on the inorganic chromatography problem. I had synthesized a type of ion-retardation resin where the charges were perfectly balanced and was having some success using it with water eluent and conductivity detection, when, quite by chance, I learned that Bill Bauman, who was now in Dow’s Texas division in Freeport, had expressed an interest in inorganic chromatography. Knowing that I was likely to get a thoughtful response from Bill, I summarized my thoughts and ideas in a letter to him. In the exchange of correspondence that followed we made one of the important breakthrough steps toward ion chromatography. There were two key ideas in our new line of attack. First of all we would revert to ion-exchange chromatography as our means of separating the analytes. We would still use conductivity to detect them but we would not place the conductivity detector directly after the separation since we still felt that this seriously impaired the detectability of analytes. Instead, we would place a device between the separator bed and the conductivity detector. We called this device the “stripper”, and later “the suppressor”. The suppressor was a column of ion-exchange resin that converted eluent to a low conductivity form, while leaving the conductance of the analytes relatively unaffected—or in some cases enhancing it (1, 6). Figure 1 illustrates how a suppressor might be used for the analysis of a mixture of anions: chloride and nitrate in the example. An anion-exchange resin pumped with sodium hydroxide solution separates the ions with chloride eluting ahead of nitrate in a background of the highly conducting eluent. At this point the eluent passes directly into a bed of cation-exchange resin in the hydronium form where two ionexchange reactions take place. The resin neutralizes the sodium hydroxide Resin-H+ + NaOH → Resin-Na+ + H2O
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initial embodiment of eluent suppression showed that we had a serious problem. We can illustrate the problem by a question. If we use a separator of 1-mL capacity, what size of suppressor will be necessary to accommodate N samples before regeneration is required? The answer is in the equation Vsupp =
Figure 1. A scheme for the ion-exchange separation of two anions, chloride and nitrate, suppression of the sodium hydroxide eluent and conductometric detection of the analytes as their respective acids.
and the analyte peak (of sodium chloride or sodium nitrate) is converted to its corresponding acid Resin-H+ + NaCl → Resin-Na+ + HCl Resin-H+ + NaNO3 → Resin-Na+ + HNO3 Thus the conductance of the eluent is suppressed by capturing one of the conducting components (Na+) on an ion-exchange resin and combining the other (OH−) with released hydronium ions to produce the essentially nonconducting water, the ideal environment for sensitive conductometric detection. In short, what we proposed to do was reduce the background noise by essentially eliminating the background signal. As well as being a key in the evolution of IC, the suppressor also brought a new concept to chromatography. Since its capacity to function was partly consumed in each run, the chromatography had to be interrupted periodically to restore the suppressor to its original ionic form. This idea of a consumable component was radically new to chromatography and some doubted that it would be accepted. Since I shared that skepticism, I felt that if our technique was to have any chance of success, then we must make the interruptions for suppressor regeneration as unobtrusive as possible. Our www.JCE.DivCHED.org
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NKC sep C supp
(1)
where Vsupp is the volume of the suppressor, N is the number of samples injected between regenerations of the suppressor, K is a selectivity coefficient that expresses the affinity of the separator bed for the last eluting analyte ion in the set of target ions, Csep is the specific capacity of the separator resin, and Csupp is the specific capacity of the suppressor. In order for suppressor regeneration to be unobtrusive, N, has obviously got to be reasonably large—let us say ten or greater. We knew from our experience with ion exchange that selectivity coefficients, K , of ten or higher were common. Since we were proposing in our initial embodiment of the suppressor concept to employ separation and suppressor resins of approximately equal specific capacity (i.e., Csep = Csupp), it was clear that, to accommodate a reasonable number of samples, the volume of the suppressor would need be vastly greater than the volume of the separator. As a result, the massive dead volume of the suppressor would have at least two adverse effects on the chromatography; it would greatly extend elution times and it would seriously degrade chromatographic efficiency. Therefore, from a chromatographic point of view our initial embodiment of the suppressor concept was unacceptable. But eq 1 also suggested a solution to the problem; make the term CsepCsupp very small to counterbalance the large value of NK. To accomplish this we would use suppressor resins of the maximum possible specific capacity and separators of very much lower specific capacity, perhaps orders of magnitude lower in capacity. In addition, these low-capacity ion exchangers would have to be chemically stable in the very basic and acidic eluents that we proposed to use. Where might we find such materials? In 1971 the only commercially available low-capacity ion exchangers had been developed for the then burgeoning field of HPLC but they were based on silica and I doubted that they would be stable in the eluents that we were planning to use. So I decided that we would have to make our own more rugged alternatives. Where might we start? Two early experiences at Dow were pivotal in the evolution of low-capacity ion-exchange resins. The First Stationary Phases for IC In the mid-1950s I had invented a new separation method called gel–liquid extraction (GLX) that used crosslinked polystyrene particles swollen with water-immiscible solvents. To make these materials perform efficiently as a chromatographic packing I found it necessary to give the polymer particles an initial superficial sulfonation. These surface-sulfonated particles were in fact low-capacity cationexchange resins although their function in GLX had nothing to do with their ion-exchange capabilities; the sulfonation was simply to make them water-wettable.
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Figure 2. The first ion chromatogram of cations; the separation of three alkali metals, lithium, sodium, and potassium, and their measurement by suppressed conductometric detection.
Figure 3. The first description of the concept for making low-capacity, ion-exchange resins for separating anions by ion chromatography (from the author’s laboratory notebook dated 11/9/71).
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Now some ten years later these surface-sulfonated polystyrene beads looked like a good candidate for separating cations in our new form of chromatography. So I loaded a single 9-mm diameter column with a top layer (about 27 cm) of this old surface-sulfonated resin and a bottom layer (about 34 cm) of an anion exchanger, Dowex 1X8 in the hydroxide form, to act as the suppressor. The size of both resins was in the range 37–74 µm. The column was pumped with 0.02 M HCl at 0.4 mLmin and the effluent passed to a conductivity cell and meter of Dow design. An injection of a 100µL sample containing a mixture of lithium, sodium, and potassium chlorides, 10 mM in each, gave the chromatogram shown in Figure 2. The elution times of lithium, sodium and potassium were 63, 82, and 134 min, respectively. Apart from my having used far too much separator resin than was necessary, hence the long elution times, the experiment was a great success. The peaks were nicely separated, the signal-tonoise ratio looked excellent and, for those days, the detectability was impressive. My description of this event is dated November 9, 1971. It was a tremendously exciting moment in the evolution of the new chromatography and the way ahead looked bright indeed. Although we had demonstrated the technique for alkalimetal cations, it seemed that the technique would have a greater impact in anion analysis. Atomic absorption was after all a powerful technique for metal ions but there was nothing to challenge a chromatographic technique that could determine in a single experiment a mixture of anions of widely diverse chemistry. As a first step in developing the anion analog I would need to make a suitable low-capacity anion exchanger since the commercial materials were silica based and unlikely to survive in the basic eluents that I planned to use. Quaternizing the surface of polystyrene particles was a possible route but I could not figure out how to keep the anion-exchanging layer sharp in the way that surface sulfonation produced a sharp cation-exchanging layer. I felt that a nondiffuse active layer was essential for efficient chromatography. Another early experience suggested a solution. Shortly after joining Dow in 1955, I became aware of a problem in the regeneration of large-scale deionizers that use a single large bed filled with an intimate mixture of cationexchange resin (in the H+ form) and anion-exchange resin (in the OH− form). When the bed is exhausted the two resins must be separated before they are regenerated. But “resin clumping”, as the problem was called, prevented the easy disengagement of the two resin types; it was caused by polymer-chain segments on the cation-exchange particles interacting electrostatically with oppositely charged chain segments on the anion-exchange particles. I invented a surface treatment of the particles that prevented clumping in the first place, a process that is still used. This experience with the clumping problem had impressed on me the tenacity of electrostatic bonding between ion-exchange particles of opposite charge and it resurfaced as I considered the problem of making a low-capacity resin. Figure 3 is a reproduction of the pages from my laboratory notebook recording my first thoughts on what we eventually called surface-agglomerated resins. The first anion separator was formed by grinding a large particle size anionexchange resin and then exposing a surface-sulfonated resin www.JCE.DivCHED.org
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Figure 4. The first ion chromatogram of anions; the separation and detection of chloride and nitrate (11/13/71). This experiment also used the then novel device of continuous suppression using a cation-exchange membrane. The membrane was in the form of a single tube of sulfonated polyethylene surgical tubing that received the sodium hydroxide effluent and the analytes as they left the separator column. The tubing was immersed in a bath of a suspension of Dowex 50, in the hydronium form. The bumping contacts of the resin with the membrane wall supplied hydronium ions that passed through the membrane to exchange with sodium ions and at the same time neutralize hydroxide ions in the effluent. The effluent then was passed directly to a conductivity detector.
to a suspension of this finely ground anion exchanger. The separator was a 9-mm × 25-cm column filled with this resin and pumped with 10 mM sodium hydroxide. As well as being the first demonstration of anion analysis by IC, this experiment was also the first demonstration of a continuously regenerated suppressor, as I will explain later. A conductivity cell, meter, and recorder completed the apparatus. The first anion chromatogram is shown in Figure 4. The date was November 13, 1971. Within a week we had demonstrated the feasibility of suppressed conductometric detection for ion chromatography and laid the basis for synthesizing effective stationary phases for the new technique. Grinding of large resin particles was our first source of colloidal anion exchangers but it had a low yield and reproducibility was a problem. Consequently we abandoned that approach in favor of the initial idea of preparing colloidalform ion exchangers by emulsion polymerization techniques. We prepared a useful colloidal resin by cross-linking a polyvinylbenzylchloride latex with diethylene triamine and then quaternizing the remaining groups with trimethylamine. Later, our colleague Jitka Solc drew on Dow’s background in emulsion polymerization technology to devise an effective means for using divinylbenzene as the cross-linker. And as well as learning how to control the degree of cross-linking, Jitka also learned how to control the size of the colloidal polyvinylbenzylchloride latex particles. Quaternization of the DVB cross-linked resin was the final step in producing the colloidal anion exchanger.
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This way of preparing low-capacity anion exchangers introduced several advantages and benefits: • It was a very effective means of controlling the degree of cross-linking of the active layer; the degree of crosslinking has an important effect on the ion exchange behavior of the stationary phase. • It gave uniformly sized particles of controllable size. • The resins had excellent chemical stability in the typical ion-chromatography eluents. • The surface-agglomerated resins had excellent physical stability. Some had doubts about the physical stability of these pellicular composites, but I had developed faith in the tenacity of the electrostatic linkage between the colloidal resin and the substrate; in some serious attempts to separate them I had never been successful. • The active anion-exchanging layer was thin and the boundary between this layer and the substrate was sharp. Both of these circumstances favor rapid mass transfer in the stationary phase and in turn columns of high chromatographic efficiency. • From a manufacturing point of view it was attractive. From, for example, a liter of colloidal resin one could prepare an enormous number of columns with reproducibility of ion-exchange chemistry from column to column absolutely guaranteed.
This route to resin preparation was eventually adopted and successfully elaborated by the Dionex Corporation and is still used as the source of many of its separating columns for both anion and cation analysis. Eluents for Ion Chromatography Developing suitable eluents was another key to IC’s successful development, particularly for anion analysis. Sodium hydroxide was our eluent of first choice since it suppressed to water, the background with lowest conductivity. But hydroxide was a weak displacing ion for many of the analytes of interest and relatively high concentrations were necessary to elute the more intractable species, thus shortening the lifetime of the suppressor. The use of sodium phenate ion instead of sodium hydroxide alleviated the problem immensely; phenate was a high affinity anion and sodium phenate suppressed to phenol, a weak acid whose solutions were feebly conducting (1, 6). Resin-H+ + Na+phenate− → Resin-Na+ + Phenol While this led to great improvements in the speed and efficiency of IC for anions, phenate was never used in commercial IC instruments although some similar eluents such as cyanophenate were. Perplexed by the anomalously high displacing power of a certain “sodium hydroxide” solution, we discovered that its unusual potency was due to carbonate contamination and it was from that accidental discovery that we developed the carbonate–bicarbonate system of eluents (7). In this case the eluent converts to carbonic acid in the
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suppressor thus providing a very low-conducting background. The carbonate–bicarbonate system became the workhorse anion eluent for many years to come. Ion Chromatography Applied to Organic Ions In these early days we showed how the ion-exchange, eluent-suppression technique, while greatly simplifying inorganic analysis, also enabled the analysis of many organic ions that were inaccessible to HPLC because they lacked light absorbing properties. Aliphatic acids, amines, and quaternary ammonium ions were notable examples (1). Today, suppressed conductometric detection, because of its universal ability to detect most ionic species with high sensitivity, is often the chosen method of detection regardless of whether the species are inorganic or organic. Ion Chromatography Enters an Analytical Environment Having the new analytical technique accepted by the analytical community in Dow was a major hurdle. Tim Stevens, at that time a young analytical chemist with responsibility for ion analysis by the old methods, attended a seminar that I gave on the new technique and recognized its potential. So he joined our small group in the Physics Lab and together we improved and devised new eluents for cation analysis by IC and worked on improving the stationary phases. After several months Tim had accomplished enough to convince his management in Dow’s analytical laboratories that what we had developed was a sound and useful technique and he returned to the analytical laboratories where he played a vital part in promoting ion chromatography as a new analytical technique within Dow. When IC began to show promise as a new universal tool for ion analysis, commercialization moved to the top of the list of priorities. There were some eager and competent managers within Dow who wanted to promote IC as a Dow product but I could foresee it faring poorly alongside Dow juggernauts like polystyrene, chlorine, or caustic soda. So I was relieved at the decision to seek an entrepreneurial licensee outside of Dow. George Rock was Dow’s key person in this important stage of IC’s development, and his dogged persistence was rewarded when Dow established a licensing agreement with Durrum Chemical to exploit the Dow technology. This was the beginning of Dionex Corporation. As a side note to George’s efforts to license the new ion chromatography, it is peculiarly relevant to this symposium to note that he had a hidden ally—what I call “the power of the precedent”. Several years earlier, Dow had sought a licensee for another Dow-invented technique, gel-permeation chromatography. The licensee for that technology went on to great success as the Waters Company whose founder, Jim Waters, is the sponsor of this symposium. Legal protection of the new ideas was vital to the new company’s success and Ed Schilling of Dow’s patent department built a bulwark of important patents to protect the fledgling technology. In September 1975 at the fall meeting of the American Chemical Society, the technique we had now begun to call ion chromatography got its first public exposure when Dionex
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Figure 5. The status of ion chromatography ca.1975; a typical anion chromatogram. Eluent: 0.003 M sodium bicarbonate, 0.0024 M sodium carbonate.
showed the first commercial instrument and we presented the article (1) that described the basis of IC and showed many examples of its application. Figure 5 exemplifies the status of ion chromatography at this time. We had succeeded in developing a method that would provide, in a single measurement, rapid, sensitive analysis for ions of widely diverse chemistry. Dow Research: After 1975 In 1975 the center of gravity of research and development on IC moved to Dionex but in the years immediately following, the scientists at Dow continued to make contributions that would have an impact on the science and marketing of IC.
Suppressor Developments The first commercial instruments for IC used packed bed suppressors and although the suppressor was the device that made conductometric detection a sensitive detector for IC, its packed bed embodiments had some drawbacks. Notably there was the drifting of elution times for some analyte peaks (1, 6) and some analytes such as nitrite would react with the hydronium-form resins and give irreproducible peak areas. At first the interruptions to regenerate were not a serious problem since we had so arranged the size of the suppressor that typically it would last throughout an eight-hour day and regenerate at night. The picture on the regeneration changed in 1979 when it was shown that ion chromatography could be accomplished using ion-exchange and conductometric detection without a suppressor (8). This new development turned the spotlight on the suppressor and the drawback of the interruptions for regeneration. Additionally, the new version of IC was often promoted as avoiding the
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“complexity” that the suppressor added to the technique. The interruptions argument was a legitimate one, however, the complexity argument much less so. The so-called complexity of adding a suppressor is the price that one must pay to extract the maximum sensitivity from conductometric detection. This new development did however stoke the fires under our efforts to devise a better suppressor. A number of years prior to 1971 I had been examining a possible new way of extracting magnesium from seawater. It involved contacting seawater with a countercurrent flow of strong brine that was separated from the seawater by a cation-exchange membrane. I had fabricated the membrane by sulfonating polyethylene surgical tubing and carried out the magnesium extractions by passing brine through the lumen of this sulfonated tube while it was immersed in a bath of simulated seawater. I managed to get a significant concentration of magnesium to build up in the brine phase but not enough to make it commercially viable; dilution of the brine by osmotically driven water from the seawater side of the membrane was one of the problems. So I set the project aside but saved some of the sulfonated tubes. When I attempted the first anion separation by ion chromatography it occurred that one of the old tubular membranes might be used to continuously neutralize (i.e., suppress) the sodium hydroxide from the separator so I attached one of these old devices to the outlet of the separator and immersed it in a stirred bath containing beads of cation-exchange resin in the hydronium form (Dowex 50W). Thus the sodium ion crossed the membrane and exchanged with hydronium ions in the resin particles as they made bumping contacts with the exterior wall of the tubular membrane. The hydronium ions in turn moved in the opposite direction and united with the hydroxide ions to form water. The result of that experiment is illustrated in Figure 4. The membrane device worked well as a suppressor but the membranes were fragile and prone to bursting. Since there were priorities more pressing than perfecting the membrane device, I switched to using packed-bed suppressors since they did the job well and were immune to bursting problems. When suppressorless IC arrived on the scene Tim resurrected the membrane concept and succeeded in fabricating a number of useful devices that were rugged compared to the original one (9). Patent protection was obtained for the new devices, and they went on to be the first in a family of continuous, chemically regenerated eluent suppressors. Later, Dionex would add chemically regenerated flat membrane devices followed by electrochemically regenerated devices that used only water and electrical current to keep them functioning. Thus the suppressor has evolved from being a conspicuous part of IC, and something of a bother, to being practically invisible to the user.
Indirect Photometric Detection In 1979 by using light-absorbing ions in the eluent of ion-exchange chromatography we demonstrated that transparent analytes could be detected and quantified from the vacancies they produced in the UV absorbing background. This new detection technique, indirect photometric detection, upset the prevailing dogma that photometers could only detect species containing chromophores (5). Indirect photo-
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metric detection has had limited use in IC but the principles are widely applied in capillary electrophoresis. This contribution to chromatography and capillary electrophoresis was recognized by inclusion of our article in Milestones in Analytical Chemistry (10).
Ion Exclusion Ion exclusion, invented by Wheaton and Bauman, was originally intended as a large-scale industrial processing tool but in 1978 Richards and Turkelson showed how it could be applied on an analytical scale (11). This pioneering development pointed the way to making ion exclusion an important member of the repertoire of IC methods. It is especially useful in the separation and analysis of weak organic acids. Electrochemical Techniques In my pre-Dow days I had studied ion-exchange resins as electrical conductors and had often returned to this earliest experience in the hope of finding a use for this property. In 1974 my notes from that time record an idea “Possibilities for continuously regenerating an ion chromatography stripper bed by electrical means” and show a proposed device comprising an ion-exchange bed clamped between membranes and electrodes. There were more pressing priorities at that time and I never reduced any of the concepts to practice but many years later a revival of these old ideas led to the new process that we call ion reflux (12). With ion reflux it is possible to carry out ion-exchange separation and suppression simply by pumping water and applying current to an ion-exchange bed. Thus ion chromatography has in a sense come full circle. In the very early days at Dow we had considered water as the ideal eluent but had never been able to come up with a suitable stationary phase and had abandoned that approach. Now with the aid of some new electrochemistry we can realize that earliest dream of practicing ion chromatography using only water as the pumped phase. In Retrospect By 1975 there was much still to be done before ion chromatography would become a viable commercial product. Yet we had accomplished a good deal; we had solved some difficult problems and demonstrated that ion chromatography could be a robust and reliable technique. Although problem solving was a key to our success I do not believe it was the
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only one. Dow technology and the climate for research at Dow in those early years were notably positive influences on our work. For example, Dow’s strengths in the field of ion exchange and in emulsion polymerization were deep wells that we could draw on and the openness of Dow research management to new ideas was vital. I for one was fortunate in working for scientist–managers who did not expect a detailed map of all the fertile valleys and mineral-rich hills before they allowed you to go exploring. Most of them had passed that way themselves and knew the risks and the oftentimes lean rewards of exploratory research. Finally, there was what I call “the dream”. We knew that if we could accomplish what is represented in Figure 5 that applications would surely follow. We imagined a few of them but it was impossible to foresee the contribution of Dow’s licensee, Dionex, or the myriad applications that future users would uncover. We had just the dream to urge us on. This has been a brief story of how we brought that dream up to the threshold of success. Note 1. Later we demonstrated that commonly held idea to be false (5).
Literature Cited 1. Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 75, 1801. 2. Wheaton, R. M.; Bauman, W. C. Ann. N.Y. Acad. Sci. 1953, 57, 159. 3. Wheaton, R. M.; Bauman, W. C. Ind. Eng. Chem. 1953, 45, 228. 4. Hatch, M. J.; Dillon, J. A. Ind. Eng. Chem. 1963, 2, 253. 5. Small, H.; Miller, T. E., Jr. Anal. Chem. 1982, 54, 462. 6. Small, H. Ion Chromatography; Plenum Publishing: New York, 1989. 7. Small, H.; Solc, J. The Theory and Practice of Ion Exchange; Society of Chemical Industry: London, 1976. 8. Gjerde, D. T.; Fritz, J. S. J. Chromatogr. 1979, 186, 509. 9. Stevens, T. S.; Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488. 10. Milestones in Analytical Chemistry; American Chemical Society: Washington DC, 1994. 11. Turkelson, V. T.; Richards, M. Anal. Chem. 1978, 50, 1420. 12. Small, H.; Riviello, J. Anal. Chem. 1998, 70, 2205.
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