Mobile Phases for IC

Mobile phases (or eluents) for IC are similar to those used for regular ion exchange separations in that the eluent must contain a competing ion (of the same charge sign as the analytes to be separated) which serves to displace the analyte ions from the stationary phase and ultimately to elute the analytes from the column. However, eluents in IC must also satisfy the stringent requirement that they should be compatible with conductivity detection. In the case of nonsup-pressed IC (in which the eluent is not involved in further reaction before reaching the detector) this means that eluent-competing ions of low limiting equivalent ionic conductance (see discussion of conductivity detection below) are required if direct conductivity detection is to be used. Aromatic car-

boxylates (such as benzoate and phthalate), aromatic sulfonates (such as toluenesulfonate) and complex ions (such as the anionic complex formed between gluconate and borate) are ideal for anion separations, whereas aromatic bases are useful for cation separations. All of these species are bulky ions having low ionic mobility (and hence low conductance) so that direct detection of more mobile analyte ions (such as chloride, sulfate, etc.) is possible using conductivity. Alternatively, indirect conductivity detection is possible using eluent-competing ions having very high values of limiting ionic conductance, such as hydronium ions for cation separations and hydroxide ions for anion separations.

Suppressed IC offers the opportunity for further reaction of the eluent before detection. The purpose of this reaction is to reduce the conductance of the eluent; in most cases acid-base reactions are used. The mechanism of eluent suppression will be discussed further below, but for the present it can be assumed that the process works best when applied to eluents comprising competing ions that can be easily neutralized in an acid-base reaction. For example, carbonate and bicarbonate (or mixtures of the two) can be used for anion separations, while dilute solutions of mineral acids can be used for cation separations.

Many cation separations cannot be achieved simply through correct choice of a suitable eluent-com-peting cation. Polyvalent cations show such strong electrostatic attraction to sulfonic acid cation exchangers that they can only be displaced by using concentrated eluents. This, in turn, renders conductivity detection difficult. Practical alternatives are created by the use of a complexing agent as the eluent, or by the addition of a complexing agent to an eluent that already contains a competing cation. This serves the dual purpose of reducing the effective charge on the analyte cation (and hence its affinity for the cation exchange sites) and also introduces a further dimension of selectivity between analytes that does not exist when ion exchange is the only retention mechanism in operation. The above approaches are illustrated schematically in Figure 5, which shows the equilibria existing between a divalent metal analyte ion M2 +, a complexing agent (H2L), and an ethylenediamine (en) eluent, at the surface of a cation exchange resin. In Figure 5A, the eluent contains only the ligand species. Retention of the analyte ion on a cation exchange resin is moderated by the complexation effect of the deprotonated ligand, which can be said to exert a pulling effect on the analyte. The eluent pH determines the degree to which the ligand is de-protonated, which in turn governs the retention of the analyte. Retention is also regulated by the type and concentration of the ligand. Tartrate, oxalate, citrate

Figure 5 Schematic illustration of the equilibria existing between an analyte cation (M2 + ), ethylenediamine (en) and an added ligand (H2L) at the surface of a cation exchanger. In (A) the eluent contains only the ligand, while in (B) the eluent contains both ligand and ethylenediamine.

and a-hydroxyisobutyric acid are examples of typical ligands used as eluent additives.

Figure 5B shows the case where the eluent contains both a ligand and a competing cation (enH2+ ). The retention of the analyte ion, M2 +, is influenced by the competitive effect for the sulfonic acid groups exerted by enH22+, and also by the complexation of M2+ by the deprotonated ligand L2. Once again, complexa-tion reduces the effective concentration of M2+ and the analyte is therefore less successful in competing for the cation exchange sites. This shows that elution of the analyte results from a combination of the pushing, or displacement, effect of the competing cation in the eluent and the complexation, or pulling, effect of the complexing agent. The eluent pH influences both the protonation of ethylenediamine and the deprotonation of the added ligand, which in turn controls the degree of complex formation and hence the retention of the analyte. The type and concentration of the added ligand again play a major role in determining analyte retention. For analyte ions of similar ion exchange selectivities, the retention order closely follows the reverse sequence of the conditional formation constants for the analyte-ligand complexes.

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