SOLVENT-PROGRAMMED MICROCHIP OPEN-CHANNEL ELECTROCHROMATOGRAPHY

Open-channel electrochromatography in combination with solvent program ming is demonstrated using a microchip device. Channel walls were coat ed with octadecylsilanes at ambient temperatures, yielding stationary phases for chromatographic separations of neutral dyes. The electroosm otic now after coating was sufficient to ensure transport of all speci es and on-chip mixing of isocratic and gradient elution conditions wit h acetonitrile-buffer mixtures. Chips having different channel depths between 10.2 and 2.9 mu m were evaluated for performance, and van Deem ter plots were established. Channel depths of about 5 mu m were found to be a good compromise between efficiency and ease of operation. Isoc ratic and gradient elution conditions were easily established and mani pulated by computer-controlled application of voltages to the terminal s of the microchip. Linear gradients with different slopes, start time s, duration times, and start percentages of organic modifier proved to be powerful tools to tune selectivity and analysis time for the separ ation of a test mixture. Even very steep gradients still produced exce llent efficiencies. Together with fast reconditioning times, complete runs could be finished in under 60 s.