An apparatus to effect multiple simultaneous separations to measure absorbance, comprising a photodetector array comprising a plurality of photosensitive elements connected to provide a serial output. The elements are typically pixels of a photodiode array (PDA). The elements are illuminated by a light source positioned to illuminate at least a portion of the photodetector array. The light source may be a an AC or DC mercury lamp or other useable light source for chromatography. An array of separation channels is disposed between the light source and the photodetector array, each of the separation channels having a lumen, a sample introduction end and a detection region disposed opposite the sample introduction end. Typically and as disclosed herein the array is a multiple parallel capillary electrophoresis system. The last element is a mask element having at least one aperture for each associated separation channel, each aperture corresponding to its associated separation channel thereby selectively permitting light from the light source to pass through the lumen of its associated separation channel, at least a portion of the light passing through the lumen of the associated separation channel falling on a respective photosensitive element of the photodetector array to effect measurement of absorption of light by a sample introduced into the sample introduction end of the associated separation channel. Also disclosed is an instrument for analyzing the spectral content of chemical samples contained in a large number of capillary tubes which uses a single AC powered light source (which therefore gives a time-varying light output) and a single self-scanned PDA detector. This greatly reduces the number of circuit elements required, since only one amplifier, one A/D converter, one light source and one detector need be provided. The resulting device can be constructed less expensively and take up less space as compared to the conventional system. Further, a control signal having a particular frequency is supplied to both the power supply of the AC light source and the PDA detector. This stabilizes the detector signal's baseline.