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A micromachined absorbance and fluorescence detection cell for application to capillary electrophoresis within planar glass substrates (chips) is described. A microfabricated U-cell for absorbance provides a longitudinal path 120−140 μm long parallel to the flow direction and gives at least a 10-fold increase in absorbance compared to an absorbance path transverse to the flow direction. Absorbance detection limits of 0.003 AU gave ∼6 μM detection limits for hydrolyzed fluorescein isothiocyanate dye. The same device can be used for longitudinal fluorescence excitation with a 20-fold improvement in signal-to-background levels due to reduced scattering, utilizing a form of sheath flow. Fluorescence detection limits of ∼20 000 molecules and 3 nM were obtained for fluorescein.
Liang et al. (Mon,) studied this question.