Plastic microchip electrophoresis with analyte velocity modulation. Application to fluorescence background rejection

Fluorescence background interference from the device is inherent in plastic microchips, particularly with blue or UV excitation. Conventionally, micro chip background has been reduced with confocal optics or circumvented with specialized long-wavelength fluorophores. We show that microchip background can be rejected with analyte velocity modulation. In this scheme, the driv ing voltage is modulated at low frequency, typically in the range of 7-20 H z. Consequently, migration velocities and analyte signals are modulated at the same frequency. Microchip fluorescence is unmodulated, so that lock-in detection (synchronous demodulation) easily separates the analyte signal fr om background, The technique does not require a laser source, In our implem entation, a blue (485 nm) LED is the light source. Simple optics are used t o shape the source and focus it to a spot similar to 50 mu m in diameter in side a microchip. Photomultiplier detection is employed, and a lock-in ampl ifier is used to demodulate the signal. Apertures in the system generate a derivative response, which can be converted to conventional bands by integr ation, Fluorescence rejection provided by our current system lowers detecti on limits by similar to 1 order of magnitude compared to de measurements wi th the same optical train.