Introduction

Hydrogen-bonding interaction offers unique selectivity based on number of “interaction points” available for hydrogen bonding. One of the useful characteristics to determine retention patterns in hydrogen-bonding mode is the molecular polar surface area (PSA). This calculated parameter is usually used for prediction of drug transport properties, but we successfully applied it to hydrogen-bonding interactions. Polar surface area is defined as a sum of surfaces of polar atoms (usually oxygens, nitrogens and attached hydrogens) in a molecule.  Since those polar  atoms can participate in hydrogen-bonding interaction, estimation of elution order can often be made based on PSA. While PSA is a good indicator of elution time, it must be noted that polar surface area does not account for the accessibility of hydrogen-interaction sites. Not every polar surface participates in intermolecular hydrogen interactions with the stationary phase. Proximity of “interaction points” to each other within one molecule also needs to be considered since molecules can form an intramolecular hydrogen-bonding, which competes with intermolecular interaction between analyte and stationary phase. This reduces retention time in hydrogen-bonding mode.  Such structural factors provides unique selectivity among similarly structural (isomers, homologs, degradation products, precursors) molecules. Since SHARC 1 column is a mixed-mode column, pKa is another useful parameter in method development for these columns. SHARC columns operate in non-aqueous mobile phase, but some effect of charge interaction of stationary phase and ionizable molecules still exists and contributes to the retention profile.

Very distinguished separation and selectivity can be obtained for molecules containing very different numbers of hydrogen-bonding site. A good example is nucleosides and nucleobases. Depending on the conditions various order of elution can be obtained.

Application Notes: Pyridines and aminopyridines are hydrophilic basic compounds. Traditionally these compounds have been separated and analyzed by GC and HPLC. In the case of HPLC, reversed-phase chromatography with ion-pairing reagent is used along with alternative modes like HILIC for separation. Mixed-mode chromatography can also be used to successfully separate isomers of substituted pyridines. However, we developed a new mode of separation for these compounds with hydrogen bonding. Isomers of aminopyridine are separated based on hydrogen bonding interaction between analyte and stationary phase. Mobile phase utilizes combination of acetonitrile and methanol with additives. Retention time and selectivity are  sensitive to variations of mobile phase.  The order of elution changes depending on the amount of acetonitrile, methanol, formic acid and ammonium formate. This method and approach is compatible with  LC/MS and prep chromatography and can be used for separation of other pyridine based compounds and pyridine based isomers.

Application Columns: SHARC 1, 3.2x100 mm, 5 um, 100A, To learn more about SHARC 1 columns click here. To order this column click here. To see more chromatographic separations check our web site.

Application Compounds: pyridine, 2-aminopyridine, 3-aminopyridine, 4-aminopyridine

Detection Technique: UV, LC/MS