Mixture of Different IPRs in Mobile Phase

During a typical IPC separation, a single IPR is present in the mobile phase, usually at constant concentration. The possibilities offered by the simultaneous presence of more than one hydrophobic ion in the mobile phase were recently investigated. An atypical concurrent use of both anionic and cationic IPRs yielded more efficiency and a shorter analysis time, probably because of the competition between the solute and the similarly charged IPR [1,2]. The same strategy adopted in the IPC separation of oxytetracycUne and its marker residue in edible animal tissues was optimized using an excess of IPR oppositely charged to the analytes [3]. 

Ionic liquids (ILs), introduced to suppress deleterious effects of silanophilic interactions, soon proved to be extraordinary IPRs (see Chapter 7.4 for details). An ILs is an equimolar mixture of cationic and anionic hydrophobic ions, each one able to adsorb onto the stationary phase. The synergistic contributions of both cationic and anionic components generates the unique properties of ILs. 

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. 

A mixture of zwitterionic and cationic surfactants yielded superb separation of common anions on a C18 column, while a mixture of a zwitterionic and anionic modifiers proved valuable in the separation of carboxylic adds under reversed phase conditions. It reduced the chromatographic run time and improved peak shape [10]. 

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