Carbonate interferences by ion chromatographic determination of anions in mineral waters

The high content of hydrogen carbonate anions and dissolved CO2 in some mineral waters interfere with the determination of anions in such samples by ion chromatography. It was found that the irreproducibility of retention times of all anions present in the samples was caused by on-column neutralisation of CO32- to HCO3- by CO2. Microbubbles of CO2 released downstream by the suppressor inhibited exchange of Na+ with H3O+ resulting in lower conductivity and consequently in the decrease of chloride peak. To avoid peak deformation and overlapping caused by carbonate species, they should be removed by acidification followed by CO2 elimination in a gas separation unit. To prevent sample contamination by the anion of the added acid, the electrodialytic clean-up procedure was applied. The liberated CO2 was removed in a microporous Gore-Tex semipermeable tubing connected in series with the dialytic cell. The developed dialytic clean-up procedure was applied to the analysis of some typical mineral waters.