Agnes Suu, an AMS and MSC alumna (graduated in 2013), has in the course of her PhD work made an achievement that literally redefines the way the pH of HPLC mobile phase (eluent) is measured. Her work (a continuation of her AMS master’s thesis) introduces a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) mobile phases and has been published in the Analytical Chemistry journal: Agnes Suu et al, Unified pH Values of Liquid Chromatography Mobile Phases. Anal. Chem. 2015, 87, 2623–2630.

Mobile phase pH is very important in LC, but its correct measurement is not straightforward and all commonly used approaches have deficiencies. The new approach is based on the recently introduced unified pH (pH_abs) scale, which enables direct comparison of acidities of solutions made in different solvents, based on chemical potential of the proton in the solutions.

The work by Agnes represents the first experimental realization of the pH_abs concept using differential potentiometric measurement for comparison of the chemical potentials of the proton in different solutions (connected by a salt bridge), together with earlier published reference points for obtaining the pH_abs values (referenced to the gas phase) or pH_abs^H2O values (referenced to the aqueous solution). The liquid junction potentials were estimated in the framework of Izutsu’s three-component method.
pH_abs values for a number of common LC and LC-MS mobile phases have been determined. The pH_abs scale enables for the first time direct comparison of acidities of any LC mobile phases: with different organic additives, different buffer components etc. Agnes has developed a possible experimental protocol of putting this new approach into chromatographic practice and has tested its applicability. She has demonstrated that the ionization behavior of bases (cationic acids) in the mobile phases can be better predicted by using the pH_abs^H2O values and aqueous pKa values than by using the alternative means of expressing mobile phase acidity. Description of the ionization behavior of acids on the basis of pH_abs^H2O values is possible if the change of their pKa values with solvent composition change is taken into account.