A multireflection cell for enhanced absorbance detection in microchip-based capillary electrophoresis devices

The design, fabrication and testing of a photolithographically fabricated, glass-based multireflection absorbance cell for microfluidic devices, in pa rticular microchip-based capillary electrophoresis (CE) systems is describe d. A multireflection cell was fabricated lithographically using a three-mas k process to pattern aluminum mirrors above and below a flow channel in a c hip, with 30 mu m diameter optical entrance or exit apertures (one in each mirror) positioned 200 mu m apart. Source and detector were positioned on o pposite sides, and the metal mirrors were made 1 cm square, to reduce stray light effects. Calibration curves using bromothymol blue (BTB) with a 633 nm source (He:Ne laser) were linear to at least 0.5 absorbance units, with typical r(2) values of 0.9997, relative standard deviations in the slopes o f +/- 1.3%, and intercepts of zero within experimental error. Effective opt ical pathlengths of 50-272 mu m were achieved, compared to single-pass path lengths of 10-30 mu m, corresponding to sensitivity enhancements (i.e., opt ical path length increase) of 5 to 10-fold over single-pass devices. Baseli ne absorbance noise varied within a factor of two in almost all devices, de pending only weakly on path length. This device can give much higher absorb ance sensitivity, and should be much easier to manufacture than planar, gla ss-based devices previously reported.