Integrated light collimating system for extended optical-path-length absorbance detection in microchip-based capillary electrophoresis

We have developed an integrated light collimating system with a microlens and a pair of slits for extended optical path length absorbance detection in a capillary electrophoresis (CE) microchip. The collimating system is made of the same material as the chip, poly(dimethylsiloxane) (PDMS), and it is integrated into the chip during the molding of the CE microchannels. In this microchip, the centers of an extended 500-mu m detection cell and two optical fibers are self-aligned, and a planoconvex microlens (r = 50 mu m) for light collimation is placed in front of a light-delivering fiber. To block stray light, two rectangular apertures, realized by a specially designed three-dimensional microchannel, are made on each end of the detection cell. In comparison to conventional extended detection cell having no collimator, the percentage of stray radiation readout fraction in the collimator integrated detection cell is significantly reduced from 31.6 to 3.8%. The effective optical path length is increased from 324 to 460 mu m in the collimator integrated detection cell. The detection sensitivity is increased by 10 times in the newly developed absorbance detection cell as compared to an unextended, 50-mu m-long detection cell. The concentration detection limit (S/N = 3) for fluorescein in the collimator integrated detection cell is 1.2 mu M at the absorbance detection limit of 0.001 AU.