Retention of ionizable compounds on high-performance liquid chromatographyVII. Characterization of the retention of ionic solutes in a C-18 column by mass spectrometry with electrospray ionization

The elution of ions from a C-18 column with mobile phases containing methan ol (60%, v/v) and aqueous buffers is studied by mass spectrometry. It is de monstrated that the anions are excluded from the stationary phase by the io nized silanols. However, the ionized silanols interact strongly with cation s, which are retained in the column. These cations are later eluted from th e column by ion exchange with the cations present in the pH buffered mobile phase. The size of the ions, the mobile phase cation concentration and the mobile phase pH are the main parameters that affect elution of the retaine d cations; It is also demonstrated that there are at least two different ty pes of ionizable silanols, with different acidities, that contribute to the retention of cations. An estimate of the pK(a) values of these two groups of silanols in 60% methanol is given. (C) 2001 Elsevier Science B.V. All ri ghts reserved.