Resins retention factors

The approach is to measure retention times of metal cations on columns containing low-capacity resins with eluents containing perchloric acid or various perchlorate salt.s. The perchlorate anion is used to eliminate any possible complexing of a metal ion by the eluent anion. Retention factors (k) are calculated from the retention data. 

The retention factors were calculated from the retention times and are given in Tables 5.4 and 5.5. The data show that, as expected, eluents containing sodium(l) are more efficient than those containing hydrogen(l). The data in these tables are arranged in order of increasing capacity factors. In this way it is possible to compare the relative affinities of the various divalent and trivalent metal ions for resin sites. 

Selectivity of Sulfonated Cation-Exchange Resin for Metal Cations Table 5.5. Retention factors (k) of various cations when sodium perchlorate eluents are used. 

Table 5.9, Retention factors (k) for cations with resins of different bulk densities. The eluent is 0.75 M perchloric acid.

Now the retention factor will be influenced by the type and concentration of L in the chelating resin, the pH of the eluent, and the type and concentration of E" in the eluent. 

Strength of the eluent required for equal retention factors. If a mobile phase with a low conductivity is desired, as in the case of ion chromatography, a resin with a low capacity is preferred. On the other hand, for high retention and high sample capacity, ion exchangers with a high capacity should be used. 

In analytical applications of ion exchange chromatography two aspects of retention are of main interest (1) How does the retention factor vary with the concentration of eluent salt (2) What factors determine the selectivity between different analyte ions From a theoretical point of view, it is clear that with increasing concentration of electrolyte in the external solution, the magnitude of the electrostatic potential in the resin phase decreases. The reason is that the added electrolyte ions partly shield the charges bound to the resin phase. For analyte counterions this implies that their sorption to the resin phase decreases giving a lower retention factor. 

For equilibrium [11], the most used quantitative relation of as a function of electrolyte salt concentration is based on the assumption of a Donnan potential in the resin phase, i.e., eqn [19] is assumed to be valid. It is also assumed that the concentration of co-ions to the surface charges is negligible in the resin phase so that cr.r is equal to the concentration of surface charges in the resin phase, Cr. This value is therefore constant and independent of the concentration of cb,e in the eluent phase. Combining eqn [19] with eqn [24] gives the following expression for the retention factor as a function of Cb,e ... 

The retention factor is defined as the ratio of the sample ion in the resin phase to that in the liquid phase. 

The data listed in Tables 5.3-5.6 are simply observations concerning the effect of eluent concentration and resin exchange capacity on the retention factors of metal cations. A more fundamental approach is to examine the effect of physical and chemical variations in both the mobile and stationary phases on chromatographic behavior of ions. The factors affecting selectivity of ion chromatography have been reviewed in a recent publication [11]. 

As discussed in the early part of this chapter, macro-cyclic complexes are generally more stable in nonaqueous solvents than in aqueous solution. Therefore, it is expected that adding 18-crown-6 to a nonaqueous mobile phase would increase the retention time of cations to be separated. Fritz s group added 18-crown-6 to a nonaqueous IC mobile phase to study the retention of alkali metal cations and ammonium ion on a sulfonic acid cation-exchange resin. The retention factors of all the ions increased with increasing concentration of 18-crown-6 in acetonitrile eluent containing 1 mM methanesulfonic acid. Most notably. 

To accomplish any separation of two cations (or two anions), one of these ions must be taken up by the resin in distinct preference to the other. This preference is expressed by the separation factor (or relative retention), using K+ and Na+ as the example ... 

Among the other factors to be considered in the choice of separative conditions is temperature. Peak resolution and retention time (RT) depend greatly on this parameter an increase in temperature diminishes the viscosity of the mobile phase and of the sample, and a reduction of RT is observed in addition to an increase in the efficiency of the column. However, this effect is not always produced, nor for all supports some sulphonic ion-exchange resins show for analytes such as ethanol, the opposite effect. It is clear that thermostatization of the system must be very thorough so that repeatable results may be obtained. Very commonly used temperatures are those between 25°C and 70°C, values that must be adjusted each time depending on the complex of analytes to be separated. 

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