An electrophoretic ion analyzer for on-site autonomous water monitoring

An on-site ion analyzer based on capillary electrophoresis with pressure-driven flow through injection and capacitively coupled contactless conductivity detection has been developed for field monitoring of cations and anions in environmental waters. Automated time-pressure based hydrodynamic injection provides stable pL-nL scale injection (RSD = 1.96%, n  = 30). A mixture of 400 mM Bis-Tris, 400 mM MOPS and 2 mM 18-crown-6 is used as the background electrolyte to provide repeatable separations. A proprietary hydrophilic coated 25 μm id capillary is used to suppress the electroosmotic flow. Separations of anions (Cl - , NO 3 - , NO 2 - , SO 4 2- , F - and PO 4 3- ) and cations (NH 4 + , K + , Na + , Ca 2+ and Mg 2+ ) are achieved by switching the polarity of the high voltage power supply in two individual runs. Signal fluctuations caused by the temperature or viscosity changes in on-site monitoring are corrected by on-line introduction of internal standards. RSDs of the migration time and the corrected peak height over ~35 h and 350 analysis cycles are <4.06%. The LODs of inorganic ions are in the range of 2.1 μM (K + ) to 6.8 μM (PO 4 3- ).  The feasibility for on-site water monitoring with this system has been validated by a standard Ion chromatography method with comparable results obtained.