Aqueous Normal Phase: Not Just for Hydrophilic Compounds

                In HPLC analyses of hydrophobic molecules, reversed phase is the most commonly used separation method. For hydrophilic analytes on the other hand, Hydrophilic Interaction Liquid Chromatography (HILIC) is used in some cases. However, the drawback to these two retention modes is that they cannot simultaneously retain both hydrophilic and hydrophobic analytes in the same run.

                This is in contrast with Cogent TYPE-C Silica™ columns, in which can retain both types of compounds with the same column. In addition to conventional reversed phase, the columns can retain by Aqueous Normal Phase (ANP) as well. What may be surprising is that even hydrophobic analytes can be retained by an ANP mechanism. Cyclobenzaprine for example has a reported log P of approximately 4.6–4.9 and yet still shows excellent ANP retention, as shown in the chromatogram below. This behavior has been observed for hydrophobic peptides as well and has been published in a 2011 article in the Journal of Chromatography A.

                What this means for the practicing chromatographer is that in some cases it is possible to retain both types of compounds not just with the same column but in the same run. This feature is very useful for any analysis that involves compounds encompassing a wide range of polarity, such as metabolomics. The work of two columns could be done in this case with only one.