Electrophoresis and fluorescence detection apparatus

An improved electrophoresis and fluorescence detection apparatus has an electromagnetic radiation sensor juxtaposed with a sensing region. The output signal from the electromagnetic radiation sensor is a current signal, and the current signal is converted to a voltage signal. The voltage signal is summed with a programmable offset generated by an inexpensive eight-bit D/A converter. The offset signal is selected to establish a lower starting point for the dynamic range of the A/D conversion, and is selected to null some or all of the background electromagnetic radiation level. The summed signal is amplified and integrated in an integrator. The integrator is switchable under program control. The integrator is switched on for long and short intervals. The short intervals permit sensing over a dynamic range accommodating very high levels of fluorescence; very high peaks may be measured and features of the peaks distinguished. The long intervals permit sensing over a dynamic range that is optimized for the peaks associated with the smaller peaks of individual nucleotides. In this way, the dynamic range of the A/D convertor is set to permit the highest possible resolution over the range of interest during the time in which the sequencing of the nucleotides takes place. The sequencer is fast and economical and yields data with high resolution.