Conditions for rapid determination of inorganic cations in water by ion-exchange chromatography and evaporative light scattering detection

The simultaneous LC determination of alkali and alkaline-earth cations is p ossible by ion exchange chromatography run in an isocratic elution mode, us ing a polymeric exchanger coated on a spherical silica gel, followed by eva porative light scattering detection. By using an aqueous trifluoroacetic ac id (4 mM) isocratic eluent, a successful single run analysis of Na+, K+ Mg2 + and Ca2+, was achieved in 6 min on a LiChrosil (R) IC CA 2 column. This m ethod provides good linearity for the four calibration curves, and enables a 10 ng absolute detection limit for sodium, magnesium, and calcium and a 2 5 ng absolute detection limit for potassium, at a signal-to-noise ratio of 3. The repeatability determined from six injections was satisfactory for al l inorganic cations studied. This methodology has been successfully applied to the direct analysis of mineral waters containing low levels of magnesiu m in the presence of high levels of calcium as well as low levels of potass ium in the presence of high levels of sodium.