Electrostatic ion chromatography of cations using an N-dodecylphosphocholine zwitterionic stationary phase and water as the mobile phase

The separation of inorganic cations using a phosphocholine-type zwitterionic stationary phase [formed by coating an octadecyl silica column with a phosphocholine zwitterionic surfactant (PC-ZS)] and water as the mobile phase has been investigated. The PC-ZS possesses a positively charged group (+) and a negatively charged group (-), with the (+) being outermost and the (-) being innermost. In other words, the positions of the functional groups are opposite to that for sulfobetaine-type zwitterionic surfactants (SB-ZS) used previously for the separation of inorganic anions, With a PC-ZS stationary phase, cations having the same charges (e,g., Mg2+, Ca2+, Ba2+) were separated successfully. This study shows that the separation selectivity of zwitterionic stationary phases towards inorganic anions or cations is determined chiefly by the positions of the two charged groups on the zwitterion. The elution order for the cations (for example Ba2+ < Mg2+ < Ca2+) differed from that of cation-exchange, demonstrating that the zwitterionic stationary phase interacts with analyte ions in a unique manner. A retention mechanism based on coulombic attraction and repulsion effects and ion-pair formation is proposed.