Determination of sodium and ammonium ions in disproportionate concentration ratios by ion chromatography

In ion chromatography, samples of very different ammonium-to-sodium concent ration ratios are difficult to quantify since these two cations have simila r selectivities for stationary phases containing commonly used sulfonate or carboxylate cation-exchange functional groups. The IonPac CS15 cation-exch ange column, with carboxylate and phosphonate functional groups as well as a crown ether group, was developed to address this limitation. Selectivity for the common inorganic cations on this column is different from that of c onventional cation-exchange columns in that the separation between sodium a nd ammonium ions has been greatly increased, allowing for determinations of low levels of one in the presence of high levels of the other with an isoc ratic eluent. For larger than 4000:1 sodium-to-ammonium concentration ratio s, an eluent step change or gradient elution is needed. For moderate ratios , combinations of this column with a carboxylate column, containing no crow n ether group, can be used at room temperature with an isocratic eluent con taining no organic solvent. (C) 1999 Elsevier Science B.V. All rights reser ved.