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Introduction

1.1 Historical Perspective

"Chromatography" is the general term for a variety of physicochemical separation techniques, all of which have in common the distribution of a component between a mobile phase and a stationary phase. The various chromatographic techniques are subdivided according to the physical state of these two phases.

The discovery of chromatography is attributed to Tswett [1,2], who in 1903 was the first to separate leaf pigments on a polar solid phase and to interpret this process. In the following years, chromatographic applications were limited to the distribution between a solid stationary and a liquid mobile phase (liquid solid chromatography, LSC). In 1938, Izmailov and Schraiber [3] laid the foundation for thin-layer chromatography (TLC). Stahl [4,5] refined this method in 1958 and developed it into the technique known today. In their noteworthy paper of 1941, Martin and Synge [6] proposed the concept of theoretical plates, which was adapted from the theory of distillation processes, as a formal measurement of the efficiency of the chromatographic process. This approach not only revolutionized the understanding of liquid chromatography but also set the stage for the development of both gas chromatography (GC) and paper chromatography.

In 1952, James and Martin [7] published their first paper on gas chromatography, initiating the rapid development of this analytical technique.

High-performance liquid chromatography (HPLC) was derived from the classical column chromatography and, besides gas chromatography, is one of the most important tools of analytical chemistry today. The technique of HPLC flourished after it became possible to produce columns with packing materials made of very small beads (~10?µm) and to operate them under high pressure. The development of HPLC and the theoretical understanding of the separation processes rest on the basic works of Horvath et al. [8], Knox [9], Scott [10], Snyder [11], Guiochon [12], Möckel [13], and others.

Ion chromatography (IC) was introduced in 1975 by Small et al. [14] as a new analytical method. Within a short period of time, ion chromatography evolved from a new detection scheme for a few selected inorganic anions and cations to a versatile analytical technique for ionic species in general. For a sensitive detection of ions via their electrical conductance, the separator column effluent was passed through a "suppressor" column. This suppressor column chemically reduces the eluent background conductance, while at the same time increasing the electrical conductance of the analyte ions.

In 1979, Fritz et al. [15] described an alternative separation and detection scheme for inorganic anions, in which the separator column is directly coupled to the conductivity cell. As a prerequisite for this chromatographic setup, low-capacity ion-exchange resins must be employed so that low-ionic strength eluents can be used. In addition, the eluent ions should exhibit low equivalent conductances, thus enabling detection of the sample components with reasonable sensitivity.

At the end of the 1970s, ion chromatographic techniques began to be used to analyze organic ions. The requirement for a quantitative analysis of organic acids brought about an ion chromatographic method based on the ion-exclusion process that was first described by Wheaton and Bauman [16] in 1953.

The 1980s witnessed the development of high-efficiency separator columns with particle diameters between 5 and 8?µm, which resulted in a significant reduction of analysis time. In addition, separation methods based on the ion-pair process were introduced as an alternative to ion-exchange chromatography because they allow the separation and determination of surface-active anions and cations.

Since the beginning of the 1990s, column development has aimed to provide stationary phases with special selectivities. In inorganic anion analysis, stationary phases were developed that allow the separation of fluoride from the system void and the analysis of the most important mineral acids as well as oxyhalides such as chlorite, chlorate, and bromate in the same chromatographic run [17]. Moreover, high-capacity anion exchangers have been developed that enable the analysis of, for example, trace anionic impurities in concentrated acids and salinary samples. Problem solutions of this kind are especially important for the semiconductor industry, seawater analysis, and clinical chemistry. In inorganic cation analysis, simultaneous analysis of alkali and alkaline-earth metals is of vital importance, and can be realized only within an acceptable time frame of less than 15?min by using weak acid cation exchangers [18]. Of increasing importance is the analysis of aliphatic amines, which can be carried out on modern cation exchangers without adding organic solvents to the acid eluent.

Since the publication of the third edition in 2004, considerable effort has been focused on the development of monolithic separation materials for use in ion chromatography. Monolithic media offer the potential benefit of faster analysis or improved resolution with comparable analysis speed, thus following the trend toward shorter analysis times observed in conventional liquid chromatography. While method speedup in conventional liquid chromatography (UHPLC) is achieved by utilizing smaller particle sizes and smaller column formats, this pathway can be followed only to a certain extent in ion chromatography due to the limited back pressure tolerance of metal-free components in the fluidic system of IC instruments. Most research in the area of monolithic sparation media has been devoted to silica-based materials [19], which are not very suitable for ion chromatography, especially for anion separations due to pH limitations. Polymer monoliths, on the other hand, were so far used only for the separation of biomolecules such as peptides, proteins, and nucleotides [20]. Only very recently has progress been made in developing polymer monoliths for the separation of small-molecular weight ions, first in the form of aggregated particle monoliths to avoid PEEK column wall adhesion [21] and finally in the form of nanobead-agglomerated monoliths covalently bonded to the inner column wall [22].

The scope of ion chromatography was considerably enlarged by newly designed electrochemical and spectrophotometric detectors. A milestone of this development was the introduction of a pulsed amperometric detector in 1983, allowing a very sensitive detection of carbohydrates, amino acids, and divalent sulfur compounds [23,24]. A recent development in the field of electrochemical detection is 3D amperometry. The relationship of 3D amperometry to conventional amperometry is in some ways similar to the relationship of diode array detection to single wavelength UV absorbance detection. Three-dimensional amperometry enables the continuous acquisition of current throughout the entire waveform period rather than only during a predefined period within the waveform when current is integrated. The complete data set enables, among other things, postchromatographic current integration. Because different chemical compounds oxidize differently at a given applied oxidation potential, subtle differences in the amount of current generated through a waveform can provide additional information about the identity and purity of the substances being analyzed.

Applications utilizing postcolumn derivatization in combination with photometric detection opened the field of polyphosphate, polyphosphonate, and transition metal analysis for ion chromatography, thus providing a powerful extension to conventional titrimetric and spectrometric methods.

A growing number of applications are based on hyphenation, thus coupling ion-exchange chromatography with ICP-OES, ICP-MS, or ESI-MS. The advantage of coupling the various application forms of ICP with ion chromatography includes the ability to separate and detect metals with different oxidation states. The analytical interest in chemical speciation is based on the fact that the oxidation state of an element determines toxicity, environmental behavior, and biological effects. Hyphenation with ESI-MS provides the analyst with mass-selective information. Depending on the type of MS (single quadrupoles, triple quadrupoles, ion traps, etc.) coupled to IC, molecular weight and/or structural information can be obtained. The recently published EPA Method 557 [25] for determining haloacetic acids in water at trace levels by IC-ESI-MS/MS, for instance, clearly demonstrates the need for MS hyphenation to achieve the required sensitivity and specificity for challenging applications.

These developments made ion chromatography an integral part of both modern inorganic and organic analyses.

Even though ion chromatography is the dominating analytical method for inorganic and organic ions, ion analyses are also carried out with capillary electrophoresis (CE) [26], which offers certain advantages when analyzing samples with extremely complex matrices. In terms of detection, spectrometric methods such as UV/Vis and fluorescence detection as well as contactless conductivity detection [27] are commercially available today. Because inorganic anions and cations as well as aliphatic carboxylic acids cannot be detected very sensitively, applications of CE for small ion analysis are rather limited compared to IC, with its universal suppressed conductivity detection being employed in most...