Carbonic Acid Eluent Ion Chromatography

Alkali metals, amines and alkanolamines are separated on a poly(butadiene)-maleic acid on silica stationary phase using a carbonic acid (H2CO3*) eluent with and without a mineral acid. The H2CO3* eluent is prepared in situ by high pressure permeative introduction of gaseous CO, through thin membranes supported upon a porous steel disk The permeation flux and thus the eluent concentration is controlled by varying the applied CO2 pressure. This novel fit-supported membrane device tolerates much higher liquid and gas pressures than Teflon AF capillaries, permitting [H2CO3*] exceeding 0.53 M and attaining a pH of 3.3. Silicone was presently preferred over Teflon AF, both as planar membranes, as mechanical properties of the latter change as large amounts of CO2 dissolve in it. After separation, the CO2 can be efficiently removed via another gas permeable membrane device permitting detection of the eluting bicarbonate salt conductometrically in a background of nearly pure water. Most analytes are more sensitively detected after anion conversion to hydroxide using a standard suppressor, permitting 3-17 pmol LODs on 2 mm bore columns. The data, particularly comparisons with an HNO3 eluent, with or without H2CO3*, indicate that proton exchange alone does not account for the retention behavior; some reactive addition of HCO3- is involved.