Waters ACQUITY UPLC Operator's, Overview And Maintenance Manual page 58

Light transmission through a light guiding flow cell
Light path
α
In the figure above, the light path through the cell is depicted by a pair of rays
(dashed lines) that bounce off the cell wall. The energy carried by each ray is
conserved after each bounce. One hundred percent of the light is reflected,
hence the term "total internal reflection". The Teflon AF tubing is an active
component in the flow cell light path.
In contrast to the light-guiding flow cell, a conventional cell is typically an
all-metal body with lenses at each end.
Light transmission through a conventional flow cell
The light path through a conventional flow cell is designed to avoid contact
with the cell walls, primarily to prevent rays that encounter the walls from
contributing to the measured signal. The energy associated with errant rays is
highly variable depending on the mobile phase composition, quality of wall
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Mobile phase
Cell wall
Teflon AF
Teflon AF
Teflon AF