Waters ACQUITY UPLC Operator's, Overview And Maintenance Manual page 18

Light transmission through a light-guiding flow cell
Rays of light
This information complements the foregoing illustration:
• The core of the light guide is the fluid sample, with refractive index n
• The cladding is a Teflon AF tube, with refractive index n
• The cross-sectional area of the tube is A and the length d. Cell volume =
Ad.
In the preceding figure, two rays of light are shown reflecting from the
core-cladding interface. In a flow cell, the number of "bounces" depends on the
length of the Teflon AF tube, its inside diameter (lumen), and the ray angle,
"α". The light beam (which represents the energy transmitted through the
cell) is comprised of many such rays, up to a maximum whose angle is
theoretically set by the refractive index of the core and cladding. In the
ACQUITY UPLC PDA detector, this angle is mechanically controlled by
components external to the flow cell so that the variation in refractive index
arising from different mobile phases does not materially influence the
efficiency of the transmitted energy.
The following schematic diagram of the flow cell shows the light-guiding
portion of the cell inside the cell assembly.
6
α
Core (sample fluid) Cladding (Teflon AF)
. Index n
2
.
1
< n .
2 1