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 zwitterioni c stationary phase [formed by coating an octadecyl silica column with a pho sphocholine zwitterionic surfactant (PC-ZS)] and water as the mobile phase has been investigated. The PC-ZS possesses a positively charged group (+) a nd a negatively charged group (-), with the (+) being outermost and the (-) being innermost. In other words, the positions of the functional groups ar e opposite to that for sulfobetaine-type zwitterionic surfactants (SB-ZS) u sed previously for the separation of inorganic anions, With a PC-ZS station ary phase, cations having the same charges (e,g., Mg2+, Ca2+, Ba2+) were se parated successfully. This study shows that the separation selectivity of z witterionic stationary phases towards inorganic anions or cations is determ ined 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 stationa ry phase interacts with analyte ions in a unique manner. A retention mechan ism based on coulombic attraction and repulsion effects and ion-pair format ion is proposed.