Introduction

When separating very polar, water-soluble compounds, eluents containing less than 5% organic modifier are commonly used to achieve sufficient retention. However, operation under such highly aqueous conditions can lead to poor chromatographic reproducibility and decreasing retention times. Conventional C8 and C18 phases undergo 'phase collapse' under these conditions, resulting in a reduction of accessible bonded phase. This phenomenon may either occur very quickly or more gradually.

‘High Aqueous’ Phases

This problem has been addressed by either embedding a polar group in the alkyl phase or by using hydrophilic (polar) endcapping reagents (see Figure 1). Both these approaches, or the use of a C30 phase, have the effect of maintaining the phase surface under fully wetted conditions, even when using 100% aqueous eluent.

•Good retention and resolution for polar compounds

Compared to traditional alkyl phases these 'high aqueous' phases are resistant to retention loss when using highly aqueous eluents, even after several days or weeks. Reproducible retention times and improved peak shapes are achieved for acidic, basic and zwitterionic analytes.

•Alternative selectivity

Conventional C18 phases depend predominantly on hydrophobic interactions between analyte and stationary phase to provide separation. In addition, ‘AQ’ type phases may also show hydrophilic (polar) interactions via H-bonding and dipole-dipole forces. This can influence retention time and improve selectivity for polar analytes.

Eliminate need for ion-pair additives

Many separations of very polar analytes are performed using ion-pair chromatography in order to provide adequate retention. The use of an ‘AQ’ phase generally enables reproducible results to be obtained using conventional aqueous/organic eluents.

•Typical applications.

Typical applications of these ‘AQ’ type phases include carboxylic acids, water soluble vitamins, catecholamines, nucleic acid bases and various polar pharmaceuticals.

‘High Aqueous’ Phases