ACIDITY DETERMINATION IN DROPLETS FORMED BY ELECTROSPRAYING METHANOL-WATER SOLUTIONS

Dissociations of Fe2+(bpy)(3) and Ni2+(bpy)(3) complexes (bpy = 2,2'-bipyridyl) upon electrospraying their methanol-water solutions are governed by their solution chemistry, as established by deuterium isotope exchange and collision-induced dissociation. The intensities of bpyH(+) ions detected by mass spectrometry are linearly proportional to the bpy concentration in solution up to 10(-4) M, but do not follow the equilibrium concentration of bpyH(+) due to the bulk solution acidity. A 10(3)-10(4)-fold increase of acidity from the neutral bulk solution is found:in electrospray droplets corresponding to pH 2.6-3.3. The H3O+ ions formed by electrospray are confined in a similar to 5-27-nm-thhick surface layer not to exceed the Rayleigh stability limit.