Ion Chromatography: An Overview and Recent Developments

Jul 01, 2010
Volume 28, Issue 7, pg 530–538

Michael Swartz
The term ion chromatography (IC) was first used in 1975 to describe the chromatographic determination of inorganic anions using an anion-exchange separation and conductivity detection (1). This development allowed analysts to simultaneously determine inorganic anions such as fluoride, chloride, nitrate, phosphate, and sulfate in a convenient chromatographic format, replacing the tedious, expensive, and often inaccurate wet chemical assays. IC has been used in industries as diverse as environmental, power, semiconductor, electronics, and pharmaceuticals for a wide range of applications. Common applications include testing for common anions (U.S. EPA Method 314.1) and disinfectant by-products, such as bromate (U.S. EPA 302.0) in tap or bottled water, perchlorate (U.S. EPA Method 332.0, 314.2) and haloacetic acids (U.S. EPA Method 557) in drinking water, iodide and iodate in sea water, amines in wastewater, carbohydrates in fermentation broths, trace-level anions and cations in ultrapure water for the semiconductor and power industries, and more. In the pharmaceutical industry, IC is used throughout the manufacturing and regulation of pharmaceutical products, including the characterization of active ingredients, excipients, degradation products, impurities, and process streams (2). Sample types include raw materials, intermediates (including media and culture broths), bulk active ingredients, diluents, formulated products, production equipment cleaning solutions, and waste streams. IC is especially valuable for the determination of ionic or ionizable (in the mobile phase) analytes that do not have UV chromophores and therefore, by extension, combinations of analytes using multiple detectors in series.

Figure 1: System schematic of a typical IC system.
Due to space limitations, a comprehensive treatment of IC is of course outside the scope of this column. While references are available that can provide additional details on IC, I'll try to cover at least some of the basics here (3,4). An IC instrument is a type of high performance liquid chromatograph, with slight differences in configuration and components. A schematic of a typical modern IC system is illustrated in Figure 1. It has some of the same basic components as a high performance liquid chromatography (HPLC) system: an autosampler, a high-pressure pump, an injector, an analytical column and guard column, a detector, and a data system. It also has some optional components not usually found in an HPLC system, including a suppressor or other form of a postcolumn reactor for derivatizations, and other forms of eluent generation and treatment. Because of the often corrosive nature of the mobile phase, the system components in contact with the mobile phase and the sample are typically made from inert materials, such as polyetheretherketone (PEEK). Traditional IC systems use chemical suppression: after the analytical column, the effluent is passed through a "suppressor" device that chemically reduces the ionic eluent background conductance (and therefore noise), while at the same time increasing the electrical conductance of the analyte ions (and therefore signal). An alternative separation using low capacity ion-exchange resins and low ionic strength eluants sometimes referred to as nonsuppressed IC, first reported in 1979, has also been used successfully for many types of analyses (5). Coupling the ion-exchange separation with any one of a number of detectors, for example, conductivity, UV, mass spectrometry (MS), inductively coupled plasma (ICP), and pulsed amperometric detection (PAD), expands IC applications to increased sensitivity and specificity. Because IC typically uses dilute acids, alkalis, or salt solutions as opposed to organic solvents in the mobile phase, reagent and disposal costs are more economical.

Separation mechanisms for IC are primarily based upon ion exchange, although ion exclusion and ion-pairing approaches also are used. Selectivity is created by differences in charge density, hydration, size, and so forth of the ion-exchange site and polymer backbone, and on the valence, size, and polarizability of the individual ionic species to be measured. In some cases, reversed-phase separations are also performed on the basis of differences in the hydrophobic character of the ionic species.

Stationary and Mobile Phases

Since its inception in the early 1970s, progress in IC stationary phase development including the base material, the ligands or latex attached, the capacity, and smaller particle sizes has helped to meet new analytical challenges and maintain its popularity. An increasing number of column materials have been developed for IC, often targeted at specific analytical problems, aided by a better understanding of the various separation mechanisms. High speed, monolithic, and capillary columns have also been recently introduced (6,7).

Several different separation modes are used in IC including ion exchange, ion exclusion, and ion pair–reversed phase. Retention in ion-exchange processes is governed by the interaction between the mobile phase and ion-exchange groups bonded to the column stationary phase base material. Ion-exchange stationary phases traditionally consist of polystyrene, ethylvinylbenzene, or methacrylate resins copolymerized with divinylbenzene and modified with ion-exchange groups. Used for the separation of both inorganic and organic anions and cations, anion exchange usually is accomplished with quaternary ammonium groups attached to the polymer, whereas sulfonate, carboxyl, or phosphonate groups are used as ion-exchange sites for the separation of cations. Typically, anion separations use dilute bases, most commonly hydroxide or carbonate, as mobile phases on polymer-based anion-exchange stationary phases. Polymeric phases are more stable to extremes in pH compared to silica-based materials, and in many cases are still compatible with organic solvents. Silica-based columns that exhibit significantly higher efficiency can be used for cation separations, however, as dilute acids often serve as mobile phases and stability is not as much of a concern. Hydroxide is an eluting ion of choice because it is neutralized to water by the suppressor device. In cation analysis the acid eluent, most commonly methanesulfonic acid, is neutralized to water.

Figure 2: The reagent free ion chromatography (RFIC) process.
Just as with HPLC, mobile phases historically have been manually, and often painstakingly, prepared. Inconsistent mobile phase concentration as well as the presence of contaminating ions — for example the presence of carbonate in a hydroxide eluent — introduces retention time variability. However, with the development of electrolytic eluent generation, commonly referred to as reagent-free ion chromatography (RFIC), mobile phases can be prepared online. In this process, illustrated in Figure 2, an electrolytic eluent generator (EG) is placed inline between the high-pressure pump and the injection valve. Eluent ions are transferred electrolytically across a membrane from a reservoir containing a high concentration of ions into a stream of deionized water in response to an applied current. The resulting eluting ion concentration has greater accuracy, greater precision, and is easier to prepare than eluents made by traditional bench techniques. Because the hydroxide is produced online, there is no possibility of contamination from carbonate. This device also greatly facilitates gradient IC separations because it is more accurate than a pump-proportioning valve, required with manually prepared eluents.

The composition and structure of the ion-exchange sites of the resins often are designed for certain critical separations. Other mechanisms can be added to enhance selectivity of certain target analytes, such as chelation or other complex formation. Fundamentally, retention increases with increasing ion-exchange capacity of the resin and valency of the analyte and decreases with inreasing ionic strength of the mobile phase and valency of the mobile phase. Other thermodynamic properties are also important in the design of the stationary phase and selection of the eluting ion.

The ion-exclusion IC separation mode is governed by ion exclusion, sorption processes, and, depending upon the column type, hydrogen bonding. Most commonly, a high-capacity, totally sulfonated cation-exchange chemistry based upon polystyrene–divinylbenzene is used as the stationary phase. Ion-exclusion chromatography is particularly useful for the separation of weak inorganic and organic acids from completely dissociated acids that are eluted as one peak within the void volume of the column. Ion-exclusion mobile phases generally consist of mineral acids when a UV or amperometric detector is used, although fluorinated organic acid eluents have been used for suppressed ion-exclusion chromatography.

The primary retention mechanism for the ion-pair mode of IC is adsorption. Analytes are separated based upon the ability to pair with a surface-active counter ion rather than based upon the ion exchange selectivity. In ion-pair IC the stationary phase consists of a neutral porous polymeric resin of low polarity and high specific surface area such as divinylbenzene, or in some cases, a traditional HPLC-type C18 stationary phase can be used. The mobile phase controls the selectivity. In addition to an organic modifier, an ion-pair reagent is added to the mobile phase taking into account the chemical nature of the analytes. Selectivity is complementary to ion-exchange selectivity rather than a substitute. Ion-pair chromatography is particularly suited for the separation of surface-active anions and cations, sulfur compounds, amines, and transition metal complexes but generally exhibits poor selectivity for polyvalent species.

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