Analysis of highly saline samples by capillary zone electrophoresis: enhanced direct UV detection of inorganic anions using on-capillary preconcentration and clean-up techniques

The ability to analyze samples with disparate levels of analyte and matrix ions is among the important benefits defining the practical utility of mode rn capillary electrophoresis. To compensate for the sensitivity Limitations regarding trace-level inorganic anions, a number of on-line approaches tha t should offer an improved SIN ratio in direct UV detection were examined. The novel use of reversed pre-electrophoresis (at the applied voltage oppos ite to the separation voltage) made it possible to efficiently remove the m ost part of high chloride levels from the sample and hence to lower the bac kground signal and to inject increased quantities of fast analyte anions. S pecifically, by taking these advantages the sensitivity response of iodide was improved by a factor of 5 over normal CE mode. Using isotachophoretic s ample stacking, a two-fold increase in detectability was obtained for moder ately mobile anions, nitrate and nitrite, that corresponds to the minimum d etectable concentrations close to their natural occurrences in seawater. Fu rthermore, field-amplified sample injection at increased electrolyte-to-sam ple matrix concentration ratios enabled the maximum SIN enhancement, with d etection Limits at the level of 10(-6) M and lower in the presence of great er than or equal to 5.10(4)-fold molar excess of chloride. (C) 2000 Elsevie r Science B.V. All rights reserved.