Operating Principles Of The Light-Guiding Flow Cell - Waters ACQUITY UPLC Operator's, Overview And Maintenance Manual

Absorbance as a function of concentration
Ideal
Actual
Working range
Background absorbance
Concentration

Operating principles of the light-guiding flow cell

Small-bore, high-capacity columns like those used in UPLC produce
small-volume peaks. To avoid bandspreading and maintain concentration, the
volume of a detector's flow cell must be correspondingly small. A good rule of
th
thumb is to hold the volume to 1/10 or less than the peak volume. To achieve
the required volume reduction with conventional absorbance detector flow
cells, the pathlength must be reduced, to avoid a significant decrease in light
throughput. Reduced pathlength results in less analytical sensitivity, as
predicted by Beer's law, yet high light levels are necessary to preserve a high
signal-to-noise ratio.
Using a small-volume, light-guiding flow cell designed with optimum
pathlength and high light throughput, resolves the problem. Such a flow cell is
analogous to an optical fiber, where the core is the fluid sample and the
®
cladding is Teflon AF, a unique, chemically inert, amorphous fluoropolymer
made by DuPont. The refractive index of Teflon AF is lower than that of water
or other HPLC mobile phases. Light rays entering the liquid core, within the
cone half-angle, α, are internally reflected when they meet the Teflon AF
boundary. These rays are transmitted through the flow cell, theoretically
without loss, except for absorption by the sample.
Operating principles of the light-guiding flow cell 5