Cation ionic eluents

In principle, Eq. (184) applies to all ion chromatographic methods. It reveals that the detector signal not only depends on the solute ion concentration, but also on the equivalent ionic conductances of eluent cations and eluent and solute anions, as well as the degree of dissociation of eluent and solute ions. The latter two parameters are determined by the pH value of the mobile phase. 

In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K", Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

Separations of metal cations with ionic eluents has been limited mostly to the alkali metals, ammonium, magnesium(II), calcium(Il), strontium(ll) and barium(ll). Separations of other metal cations are usually performed with eluents that complex the sample cations to varying degrees (see Section 7.4). Some organic cations have also been separated with ionic eluents, although this appears to be an under-utilized area of cation chromatography. 

Typical applications of ion exchange in sugar analysis include (a) complexation of borates, which accentuates ionic interactions with the exchanger (27), (b) use of hydroorganic eluents, especially acetonitrile/water, with rigid, fine-particulate anion columns (28), (c) use of basic eluents, since most carbohydrates are weak acids with pKfl of 12-13 (29), (d) complexation with cations, Pb2+, Ca2+ and Ag+ being the most frequently employed, (e) the use of cation exchangers in a heavy metal form, e.g., Aminex HPX-85. 

Changes in mobile-phase components such as pH, ionic strength, and water content have been systematically studied [3,310,316,317]. These studies indicate that retention of basic analytes is mediated primarily by the cation-exchange properties of the silica [2]. Interestingly, it has been suggested from retention data of various pharmaceuticals that the retention mechanisms of silica with aqueous eluents and reversed-phase systems are similar [317,318]. Due to the ion-exchange properties of silica, mobile-phase pH adjustments are useful in changing the retention of ionic compounds. 

In IPC, the potential difference between the stationary phase and the bulk eluent develops because of the different adsorbophilicities of the anion and cation of the IPR. The potential determining ion is the ionic species of the IPR that experiences the strongest tendency toward adsorption onto the stationary phase it may be either the anion or the cation. The potential determining ion is the actual ionpairing reagent. 

The electrified stationary phase carries the same charge status of the IL ion that shows the strongest adsorbophilic attitude. Furthermore, ionic interactions between the analyte ion and the IL anion and cation, respectively, are contradictory and concur to modulate analyte ion retention in a complicated way. It follows that by increasing IL in the eluent, overall retention of the analyte may potentially (1) decrease [4] or (2) increase [5,6], or (3) remain almost constant if the conflicting effects of the IL cation and anion balance each other [7], depending on the specific IL concentration in the mobile phase [8]. Furthermore a reversal of elution sequence with increasing IL concentration is possible [9]. The multiplicity of interactions in the presence of a mixture of these ionic modifiers offers wide versatility related to selectivity adjustment. 

For icNi-exchange HPLC, the retention of a cationic peptidic solute of charge z can be related to the ionic strength of the eluent and the pH such that... 

The choice of eluent system depends on the polymer type. For most non-ionic hydrophilic polymers, water can be used. However much more complex eluent systems are needed, for anionic and cationic polymers where interactions with the column based on ion exclusion, inclusion and exchange, adsorption by hydrogen bonding or hydrophobic interactions and intramolecular electrostatic effects, are possible. This can often make method development in aqueous SEC extremely difficult and time-consuming. 

---