Stationary Phases for IC

The ion exchange stationary phases used for IC are usually formed by chemical bonding of appropriate functional groups to a suitable substrate such as a polymer or silica. The functional groups used most commonly are sulfonates (for cation exchangers) and quaternary ammonium groups (for anion exchangers). In this respect, IC stationary phases are similar to the conventional ion exchange materials used widely throughout analytical chemistry. However, there are two important factors that differentiate the ion exchangers used in IC. The first is their ion exchange capacity. IC requires ion exchangers with low ionexchange capacity, typically in the range 10100 lequivg-1. This requirement can be attributed chiefly to the fact that IC was developed originally for use with conductivity detection, which introduces a preference for eluents of low background conductance in order to enhance the detectability of eluted analyte ions. The diversity of detection methods now available makes it possible to use columns of much higher ion exchange capacity, but because conductivity is still the most commonly employed detection mode, the majority of separations continue to be performed on low capacity materials. The second characteristic of ion exchangers for IC is their greatly enhanced chromatographic efficiency when compared to traditional ion exchangers.

In practice, both of the above-mentioned differentiating characteristics can be achieved by using ion exchangers in which the functional groups are confined to a thin shell around the surface of the stationary phase particle. This both reduces the number of functional groups (and hence the ion exchange capacity) and also limits the diffusion path of analyte ions, thereby improving mass-transfer characteristics and hence chromatographic efficiency. Two main approaches to synthesizing such ion exchangers can be identified. The first involves the use of only a very short reaction time during which the substrate material, i.e. either silica or a polymer such as poly(styrene-divinylbenzene) or poly(methyl methacrylate) is derivatized in order to introduce the ion exchange functional group. For example, a macroporous poly(styrene-divinylbenzene) bead immersed in concentrated sulfuric acid for less than 30 s will give a material in which sulfonic acid functional groups are confined to a very shallow depth (of the order of 20 nm) around the outside of the particle. This produces a 'surface-functionalized cation exchanger', represented schematically in Figure 2, in which the confinement of functional groups to the outer layer has been achieved by chemical means. Surface-functional-ized anion exchangers can be produced in a similar

Figure 2 Schematic representation of the cross-section of a surface-sulfonated cation exchange resin. The negative charges represent sulfonic acid groups that are located on the surface of the resin bead. Note that the interior of the bead is not sulfonated.

manner. Historically, surface-functionalized ion exchangers have found most use in nonsuppressed IC.

The second approach to synthesis of ion exchangers for IC involves a physical process for confining the functional groups to the outer layer. These ion exchangers, known as 'agglomerated materials', consist of a central core particle, to which is attached a monolayer of small-diameter particles which carry the functional groups of the ion exchanger. Provided the outer layer of functionalized particles is very thin, the agglomerated particle exhibits excellent chromatographic performance due to the very short diffusion paths available to analyte ions during the ion exchange process. Schematic illustrations of agglomerated anion and cation exchangers are given in Figure 3. The central core (or support) particle is

Figure 3 Schematic represenatation of agglomerated (A) anion and (B) cation exchangers.

Electrostatic attraction so; I +

1 T3

S03 Aminated latex

SO I

Figure 4 Formation of an agglomerated anion exchange resin using electrostatic binding. Note that the core and the latex particles are not drawn to scale.

generally poly(styrene-divinylbenzene) of moderate cross-linking, with a particle size in the range 7-30 |im, which has been functionalized to carry a charge opposite to that of the outer particles. The outer microparticles consist of finely ground resin or monodisperse latex (with diameters in the approximate range 20-100 nm) that has been functionalized to contain the desired ion exchange functional group. It is this functional group which determines the ion exchange properties of the composite particle, so that aminated (positively charged) latexes produce agglomerated anion exchangers (as illustrated in Figure 3A), while sulfonated (negatively charged) latexes produce agglomerated cation exchangers (Figure 3B). Electrostatic attraction between the oppositely charged core particles and outer micropar-ticles holds the agglomerate together, even over long periods. Figure 4 shows details of this electrostatic attraction for an agglomerated anion exchanger. Agglomerated ion exchangers are used most frequently in suppressed IC.

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