Effect of the eluent pH on the thermospray molecular ion intensity of nucleosides

The protonated molecular ion intensities of 15 nucleosides obtained by thermospray ionisation have been measured using 0.1 M ammonium acetate mobile phase at neutral and acidic pH. To explain the dependence of the molecular ion intensity on the mobile phase pH, the hydrophobicity, the pK(a) values and reversed-phase high-performance liquid chromatographic retention data (log k') at neutral and acidic pH values were studied. Significant correlations (above 95% probability level) were found between the change in the protonated molecular ion intensity and the hydrophobicity as well as the pK(a) values of the compounds. The reversed-phase chromatographic retention parameter (log k') obtained at pH 3.5, showed significant correlation together with the pK(a) values to the molecular ion intensity change caused by decreasing the mobile phase pH. None of the investigated nucleosides showed an increased molecular ion intensity change at low pH when more than 5% methanol was present in the mobile phase.