Unusual IPRs

Uncommon IPRs were tested recently. Polymerized acyl monoglydnate surfactant was found to be as effective as sodium dodecylsulfate for the resolution of organic amines [126]. For the analysis of pyridine-based vitamins in infant formnlas, dioc-tylsulfosuccinate produced a unique retention pattern [133], Among bizarre IPRs, tris(hydroxymethyl)aminomethane was used for the determination of cyclamate in foods. It was selected over different ion-pair reagents such as triethylamine and dibu-tylamine, based on sensitivity and time economies [134]. Hexamethonium bromide, a divalent IPR, was used successfully to separate sulfonates and carboxylates [135]. 

Interestingly a crystal violet dye was used as a IPR for indirect photometric detection. Common inorganic anions were separated and detected at the absorption maximum of the dye with classical reversed phase columns. The affinity of the analytes decreased in the order of 8203 804 I N03 Br N02 CL, which is the same as that observed with classical anion exchange chromatography [136]. 

5-Dinitrobenzoic acid proved capable of improving reversed phase retentions of chemical warfare agent derivatives [137] and [S-(R,R)]-(-)-bis(-a-methylbenzyl) amine hydrochloride was an effective IPR for zwitterionic analytes [138]. 

An atypical simultaneous presence of both anionic and cationic modifiers was reported to achieve better efficiency and shorter analysis times, probably because of the competition between the solute and the similarly charged IPR [139-142]. Actually this strategy mimics the collaborative behavior of the cation and the anion when ILs are used as IPRs. 

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The model was recently tested to determine whether it was able to model analyte retention in the presence of novel and unusual IPRs (see Chapter 7) such as chaotro-pic salts and ionic liquids. Chaotropes that break the water structure around them and lipophilic ions (classical IPRs and also ionic liquids) that produce cages around their alkyl chains, thereby disturbing the ordinary water structure, are both inclined to hydrophobic ion-pairing since both are scarcely hydrated. This explains the success of the theory, that is predictive in its own right, when neoteric IPRs are used [64]. Recently a stoichiometric model (vide supra) was put forward to describe retention of analytes in the presence of chaotropic IPRs in eluents [18] but its description of the system is not adequate [64]. 

Figure 5.41 Unusual coordinatively unsaturated species stabilized by carbene ligands. Dative bonds are shown with arrows. Dipp = 2,6-iPr CJ-iy...

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