Stationary phase effects in hydrophilic interaction liquid chromatographic separation of oligonucleotides

The use of liquid chromatography coupled with mass spectrometry (LC-MS) for the characterization of oligonucleotides and nucleic acids is a powerful analytical method. Recently, hydrophilic interaction chromatography (HILIC) has been proposed as a reasonable alternative to ion-pair reversed phase separations of oligonucleotides prior to MS. A wide variety of HILIC stationary phase surface chemistries are currently available. Although their selectivity can be considerably different, few studies have compared these chemistries for LC-MS analysis of oligonucleotides. We evaluated ten different HILIC column chemistries to understand their capabilities for separating a variety of oligonucleotides. In general, we found that most columns were ineffective at separating larger (n > 15-mer) oligonucleotides under the mobile phase and gradient conditions evaluated here. However, several stationary phases were found to be effective for separating smaller oligonucleotides such as endonuclease digestion products. Given that early eluting oligonucleotides were found to be compatible with standard electrospray ionization conditions, several different HILIC stationary phase options are available for LC-MS studies of smaller oligonucleotides including those generated in RNA modification mapping experiments.