Off-line sample preparation by electrophoretic concentration using a micropipette and hydrogel

An off-line electrophoretic sample concentration technique for charged analytes in aqueous samples is presented. As a demonstration, nine anions including inorganic ions, a dye and benzenesulfonate derivatives were enriched from a 10 mL sample solution into 20 μL electrolyte inside a glass micropipette. A hydrogel was placed at one end of the micropipette while the other end was immersed in the sample. The electric field caused the movement and concentration of anions into the high conductivity electrolyte. The technique was applied to purified, drinking and river water and was optimised by changing applied voltage and voltage application time. The LODs after analysis by capillary electrophoresis was 1–19 ng/mL, 4–133 ng/mL and 18–80 ng/mL for purified, drinking and river water, respectively. The linear range was 0.002–0.048 to 0.1–2.4 μg/mL (R² of 0.993–0.999), 0.02–0.24 to 1.0–24 μg/mL (R² of 0.995–0.999) and 0.02–0.24 to 1.0–24 μg/mL (R² of 0.998–1.000), correspondingly. The intraday and interday repeatability (%RSD, n = 6) was ≤ 7.4% and 14.0%, respectively. The concentration factor was from one to two orders of magnitude. The technique was directly compatible with a liquid phase analytical technique, thus eliminated the additional steps (e.g., evaporation, elution and/or reconstitution) which are typically performed in sample preparation (e.g., liquid and solid phase extraction).