SYSTEMATIC-APPROACH TO THE SEPARATION OF MONOCARBOXYLIC AND HYDROXYCARBOXYLIC ACIDS IN ENVIRONMENTAL-SAMPLES BY ION CHROMATOGRAPHY AND CAPILLARY ELECTROPHORESIS

Strategies for method development and optimization of conditions in io n chromatography (IC) and capillary electrophoresis CE are proposed. T he approaches are based on well established theoretical models for ana lyte elution and rely on the inspection of effective capacity factor v ersus specific gradient ramp slope curves for IC and effective mobilit y versus pH curves for CE. Commonly used IC eluent systems, such as so dium hydroxide and sodium berate, and the CE electrolyte 3,5-dinitrobe nzoic acid were investigated. A standard aqueous solution containing f ormate, acetate, alpha-hydroxyacetate (glycolate), propionate, alpha-h ydroxypropionate (lactate), butyrate and beta-hydroxybutyrate anions w as used to test the analytical conditions indicated by the models. Amo ng the IC eluent systems, berate presented a better separation perform ance than hydroxide. However, a complete resolution of all components in the standard mixture was not accomplished experimentally at the cho sen gradient ramp slope of 0.05 mM/ml. The analyte pairs acetate/lacta te and glycolate/butyrate, whose effective capacity factors differ by approximately 0.2 units, co-eluted at 0.17 and 0.24 resolution, respec tively, and the retention time of the last eluting analyte was relativ ely long (20 min). Nevertheless, the IC method provided the best overa ll limit of detection (LOD; 0.016-0.082 mg/l). Under the optimized CE conditions, all seven components in the standard mixture were resolved satisfactorily in less than 7 min. The analyte pair beta-hydroxybutyr ate/butyrate presented the worst resolution, 0.45, and a difference in effective mobility of 1.6%. The CE methodology provided the best colu mn efficiency, roughly a ten-fold improvement in terms of number of pl ates per meter over the IC method, but the limit of detection was comp aratively poorer (0.050-0.36 mg/l). Both proposed IC and CE methodolog ies can be applied to the analysis of mono-and hydroxycarboxylic acids in samples of environmental interest, providing complementary informa tion. The choice of the most appropriate method is a compromise betwee n chemical composition of the sample and concentration level of the an alytes under investigation. For instance, the classical co-elution of acetate/lactate that occurs in IC columns, can be solved by CE. But th e CE method may lack sensitivity for these analytes, which compromises the analysis of certain real samples. The LOD for acetate and lactate as determined by IC is 0.036 and 0.082 mg/l, respectively, while for CE they increase to 0.11 and 0.20 mg/l. Under optimum conditions, the separation and identification of mono-and hydroxycarboxylic acids in a n atmospheric particulate matter sample is illustrated by both techniq ues. (C) 1998 Elsevier Science B.V.