Comparison of capillary zone electrophoresis with standard gravimetric analysis and ion chromatography for the determination of inorganic anions in detergent matrices

Abstract

Low relative molecular mass inorganic anions (chloride, sulfate and nitrate) were determined on a capillary electrophoresis (CE) system (Dionex CES-1) employing a deuterium lamp for indirect detection at 254 nm. All of the anions were separated in conventional fused silica capillaries. The background electrolyte contained an ultraviolet absorbing ion, pyromellitate, and a cationic flow modifier, hexamethonium hydroxide, to reverse the electroosmotic flow in the capillary. Using this proposed method, complete resolution and quantification of three anions (Cl^–, SO₄^2– and NO₃^–) was achieved with calibration graphs for the individual components linear for up to 500 ppm of SO₄^2– and 400 ppm of Cl^– and NO₃^–, with correlation coefficients (r²) ranging from 0.994 to 0.997 and limits of detection for Cl^–, NO₃^– at the sub-ppm level (two times the baseline noise). The upper operating range was limited to 50 ppm owing to loss of baseline resolution in higher concentrations of mixtures of analyte anions. The method was applied to the determination of sulfate in a range of different granular detergents. The instrumental and analytical precisions of the method were tested and calculated as: instrumental precision 1.5% RSD (relative standard deviation), analytical precision 1.7% RSD and an RSD of the migration times as 0.3%. The accuracy of this method was tested by comparison with standard gravimetric analysis and conventional ion chromatography. Statistical analysis of the results indicated that there was no evidence for systematic differences between the three methods. In addition, a number of potential advantages of CE are discussed.