| Abstract
| - Fluorescence background interference from the device isinherent in plastic microchips, particularly with blue orUV excitation. Conventionally, microchip background hasbeen reduced with confocal optics or circumvented withspecialized long-wavelength fluorophores. We show thatmicrochip background can be rejected with analyte velocity modulation. In this scheme, the driving voltage ismodulated at low frequency, typically in the range of 7−20Hz. Consequently, migration velocities and analyte signalsare modulated at the same frequency. Microchip fluorescence is unmodulated, so that lock-in detection (synchronous demodulation) easily separates the analyte signalfrom background. The technique does not require a lasersource. In our implementation, a blue (485 nm) LED isthe light source. Simple optics are used to shape thesource and focus it to a spot ∼50 μm in diameter insidea microchip. Photomultiplier detection is employed, anda lock-in amplifier is used to demodulate the signal.Apertures in the system generate a derivative response,which can be converted to conventional bands by integration. Fluorescence rejection provided by our currentsystem lowers detection limits by ∼1 order of magnitudecompared to dc measurements with the same opticaltrain.
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