Comparison of ion chromatographic methods for the determination of organic and inorganic acids in precipitation samples

Abstract

A comparison of three ion chromatographic methods for the determination of organic and inorganic acids in precipitation samples has been made, in order to identify which method or combination of methods is most suitable for the determination of these acids. If only one method is to be used, the gradient method is the most suitable as it allows the determination of nearly all the acids of interest. However, to cover adequately all the common acids, the best solution is to use a combination of gradient and ion-exclusion methods, as the gradient method provides the most reliable results for formic, hydrofluoric and methanesulfonic acids, and the ion-exclusion method provides the best results for acetic and lactic acids in addition to giving corroborating analyses of some of the other organic acids of interest. The inorganic acids are adequately determined by the gradient method. This combination is preferred over the isocratic–ion exclusion combination as the latter would not allow satisfactory determination of the complete range of acids of interest. The detection limits for the gradient (0.01–0.08 ppm) and isocratic (0.01–0.25 ppm) methods are comparable for most acids but are lower than those obtained by the ion-exclusion method (0.1–0.4 ppm). Overall, the precision of the results generated by the isocratic method is better than that for the gradient method, which is in turn better than the precision of the ion-exclusion method. The recoveries obtained by the three methods are within 100 ± 10% except for some organic acids determined by the ion-exclusion method.