Zwitterion Stationary Phases

Zwitterion is a German word for an ion with both a positive and a negative charge. A suitable column for ion chromatography is easily prepared by coating a re versed-phase HPLC column with a zwitterionic surfactant. The most common type is a sulfobetaine stationary phase of the formula type  

The R- group is usually an alkyl chain with 12-16 carbon atoms. A steroidal surfactant, such as CHAPS or CHAPSO2, is another type of stationary phase. A phosphochohne surfactant with the negative charge nearest to the R head group is used primarily for the separation of cations  

A zwitterion phase has some unique properties. Sample anions are attracted to the N sites and cations are attracted to the negative sites. If the positive and negative sites are separated by a sufficient distance, these sites have little effect on one another. But in the examples shown, anions are attracted to the positive sites and are simultaneously repulsed by the nearby negative sites. Cations undergo a similar attractive-repulsive effect. Thus, the chromatographic properties of zwitterionic phases differ in several ways from the anion- and cation-exchange columns used in conventional chromatography  

Anions and cations are taken up simultaneously by zwitterion phases. The strength by which they are retained is affected by the properties of both the anion and the cation. 

This is different from conventional 1C where the cation has 

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Monolithic column with a zwitterionic stationary phase... 

As electrostatic interactions exist between the HILIC zwitterionic stationary phase and charged solutes, particular attention has to be paid to the rigorous control of experimental conditions such as pH, buffer type and ionic strength. 

Another approach for separating ions, also using water as the mobile phase but employing a zwitterionic stationary phase, has been developed by Hu et al. [5], When a small amount of aqueous solution containing an analyte (cations and anions) is passed through a zwitterionic stationary phase, neither the analyte cations nor the analyte anions can get close to the opposite charge fixed on the stationary phase, because another charge... 

When a small volume of an aqueous sample containing anions and cations is passed through a column with a zwitterionic stationary phase, neither the cations nor anions can get very close to the opposite charge on the stationary phase. The sample anions and cations are forced into a new state of simultaneous electrostatic attraction and repulsion interaction. Thus the sample ions are somewhat attracted to the zwitterionic stationary phase, but the attraction is weak enough that water alone can serve as the eluent. To preserve electroneutrality, an equal charge of anions and cations must be eluted. The authors have termed this separation method as EKC (electrostatic chromatography). 

With HILIC, various polar stationary phases with differing selectivity are used. Basically, it must be distinguished between three different selective types weak anion exchanger (silica modified with aminopropyl groups) and amide columns, weak cation exchanger (usually unmodified (bare) silica) and neutral supports (diol or zwitterionic stationary phases (ZIC-HILIC)). With ionizable compounds, in addition to the distribution equilibrium between the mobile phase ( pseudo-stationary phase ) near to the polar surface and the less polar mobile phase, ionic interactions can also occur, resulting in differing separation characteristics on the different stationary phases. 

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. 

Wikberg, E., Sparrman, T., Viklund, C., Jonsson, T., and Irgum, K. 2011. A nuclear magnetic resonance study of the state of water in neat sihca and zwitterionic stationary phases and its influence on the chromatographic retention characteristics in hydrophilic interaction high-performance liquid chromatography. J. Chromatogr. A 1218 6630-6638. 

Figure 23-4 Simultaneous separation of anions and cations on a ZIC-HILIC zwitterionic stationary phase with charged aerosol detection. The 150 X 4.6-mm column with S-jim-diameter stationary phase was eluted with a gradient of 20-70% B over 26 min at a flow rate of 0.5 mL/min at 30°C. Solvent A 15 vol% 100 mM ammonium acetate (pH 4.68) in water, 5% methanol, 20% 2-propanol, 60% acetonitrile.

Chapter 10 on zwitterion stationary phases is another new addition to this book. Separations are generally performed on an HPLC column coated with a zwitterion surfactant that contains both positive and negative sites. In some cases cations and anions can be separated in a single run using pure water as the eluent ... 

Yu and Hartwick [4] were apparently the first to use a zwitterion stationary phase in HPLC. The column was packed with silica particles to which the following chain was attached ... 

In 1993, Hu, Takeuchi and Haraguchi [5] described a technique in which anions and cations were separated simultaneously on a zwitterion stationary phase with pure water as the eluent. An octadecyl silica column acquired a semi-permanent coating when a solution of a zwitterionic surfactant was passed through the column. It was demonstrated that analyte cations and anions were eluted together. The ability to separate ions by ion chromatography with a mobile phase containing no chemicals whatsoever is an ideal situation for detection of the ionic analytes. 

As has been suggested [2], it might be appropriate to use the name zwitterion ion chromatography (ZIC) to refer to ion chromatographic separations in which a zwitterionic stationary phase is employed. 

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

Chapter 10 on Zwitterion Stationary Phases describes a fascinating variation of IC. Pure water can often be used as the eluent to separate sample ions when a zwitterion stationary phase is employed. Ions elute as cation-anion pairs. 

Qiu, H.D., Jiang, Q., Wei, Z., Wang, X.S., liu, X, and Jiang, S.X (2007) Preparation and evaluation of a silica-based l-alkyl-3-(propyl-3-sulfonate) imidazolium zwitterionic stationary phase for high-performance liquid chromatography. /. Chromatogr. A, 1163,63-69. 

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