Zwitterionic bonded phases

One of the most popular types of HILIC columns today is based on a grafted polymeric layer with sulfoalkylbetaine zwitterionic moieties on wide-pore silica, containing both strongly acidic sulfonate groups and strongly basic quaternary ammonium groups separated by a short alkyl spacer. 

In addition, a 3.5 pm material with a pore size of 100 A and a surfece area of 135m /g is also available. Nudeodiu- HILIC is offered in three different particle sizes (1.8, 3, and 5 pm) with a pore size of 110 A and a surfece area of 340m /g. 

---

Mixed mode or zwitterionic siloxane-bonded phases have been prepared by cobonding hydrophobic ligands, such as octyl or octadecyl, with cationic, anionic or polar ligands, in a single stationary phase [99]. An alternative approach is to first bond a ligand with a reactive terminal group, such as a 3-aminopropyl group, that... 

In the case of electrically neutral stationary phases the zwitterionic phases, on the surface of which a betaine is bonded, have the advantage over the diol phases that with the same mobile phase the retention of the stationary phase is much higher. This is because due to the ionic groups the more polar mobile phase near the surface can form a thicker layer and, therefore, the retention of the polar analytes is increased. The selectivity of zwitterionic stationary phases is similar to that of diol phases. 

W. Jiang and K. Irgum, Covalently bonded zwitterionic stationary phase for simultaneous separation of inorganic cations and anions. Anal. Chem., 71,... 

Zwitterionic mbeed-mode columns are also offered by Merck Millipore (Darmstadt, Germany) under the trade name SeQuant ZIC-HILIC. Separations of polar and ionizable compounds on this sulfobetain-type bonded phase that is available in a 3 and 5 pm format, are achieved by hydrophilic partitioning combined with weak ionic interactions. 

Fig. 2.6 Comparison of the calculated structures for glycine in the gas-phase and in water (COSMO solvation model). Note that the central bond angle in the zwitterionic form 1 is distorted by the hydrogen bond length of 1.96A computed for this structure in the gas phase. When solvation effects are included in the calculation using COSMO, the electrostatic interaction is reduced in magnitude due to charge screening by water, and the bond angle distortion is no longer present.

Hydrophilic interaction chromatography (HB IC) or aqueous normal-phase chromatography (ANP) refers to the use of polar stationary phases (e.g., bare silica, silica, or polymeric phases with bonded zwitterionic ligands, diol phases) in combination with a mobile phase rich in organic solvent but containing a significant amount of water (typically at least 3%). Bell [97] summarized the advantages of this technique as follows ... 

Close proximity of the oppositely charged functional groups can be achieved in ion exchangers with covalently bonded zwitterionic molecules in which two or three methylene chains separate the carboxylate or sulfonate and quaternary ammonium groups [ 13-151. This arrangement results in the establishment of a combination of repulsion and attraction electrostatic forces and such stationary phases have been used for the separation of seven or eight anions and cations [13,141. 

Pyrroles having electron-withdrawing substituents in the /3-position are usually less soluble in nonpolar solvents than are the corresponding a-substituted compounds. As the importance of the zwitterionic canonical form to the resonance hybrid is less for the /3-isomers than for the a-substituted compounds, the lower solubility of the -substituted pyrroles most probably results from a difference in the intermolecular association of the two compounds. Pyrroles that have carbonyl substituents in the a-position readily form dimeric complexes via NH-0=C hydrogen bonds (25) in the liquid phase and in concentrated solution. The dimeric species may also exist in the solid phase and only in very dilute solutions (< ca. 10-4 M) do the com-... 

---