OPTICAL-PROPERTIES OF AXIAL-ILLUMINATION FLOW CELLS FOR SIMULTANEOUS ABSORBENCY FLUORESCENCE DETECTION IN MICRO LIQUID-CHROMATOGRAPHY

Simultaneous measurements of absorbance and fluorescence are possible with axial-illuminated flow cells, fashioned with a unique bend geometry. The optical properties of these flow cells have been studied. Effects of variations in lumen refractive index, capillary wall thickness and physical pathlength have been examined. A theoretical understanding of the various light propagation modes and of light intensity distributions in these modes, based upon lumen refractive index, has been attained. Of more practical significance, optical pathlengths from <1 cm to 6 cm are simply attained by positioning the inlet optical fiber along the capillary axis with respect to the bend. The flow cell volumes obtained with different combinations of capillary I.D. and optical pathlength make the flow cell and resulting detector compatible with conventional HPLC and microscale separations. Also, studies have been performed to determine the effects of increased optical pathlength on overall analytical separation efficiency and detectability in the analysis of polynuclear aromatic hydrocarbons using laser-induced fluorescence micro-LC.