Determination of organic anions in atmospheric aerosol samples by capillary electrophoresis after reversed pre-electrophoresis

This work presents the determination of low-molecular-mass (LMM) organic acids by CE after extraction with MeOH and preconcentration by evaporation. The interference arising from the simultaneous concentration of fast mobile anions (chloride, nitrate, and sulfate) at levels 200 times higher than those of LMM organic acids was reduced by the application of reversed pre-electrophoresis (RPE) as clean-up technique. This methodology allows for an effective elimination of high levels of fast mobile anions from the capillary before the separation has taken place, although analytes are also partially eliminated according to their electrophoretic velocities. In order to achieve an accurate quantification of organic anions, the use of a mathematical correction based on the linear relation between the analyte's effective volume removed during RPE and its mobility is proposed. Methods based on the use of one and two internal standards are discussed and evaluated. The proposed method is applied to the determination of organic acids in atmospheric particulate matter samples.