Design of a flow cell for BIS is not a trivial issue since the following criteria must be fulfilled:
beads must be retained in the light path
the beads must be packed in a well-defined geometry
the light beam must interrogate the majority of beads
amount of beads upstream from the beam must be minimized
at the end of measuring cycle all beads must be swept
clear from the flow cell cavity
For UV-VIS spectroscopy these conditions are met in the LOV configuration, by the flow cell design that uses 600μm quartz fibers, clad by steel or PEEK tubing. The gap (2 mm wide) between the fibers defines the optical path that is perpendicular to the flow channel 1.6 mm wide, leading from port #2 through the cell, the outlet of which is blocked by a retaining plug. The plug has an O.D. 30 μm smaller than the I.D. of the channel, while the I.D. of the channel within the plug is 100 μm. These dimensions allow the carrier stream to pass through and alongside the plug, while the beads are retained. The quantity of beads (typically 2μL of packed bed) is defined by the volume of injected bead suspension. Flow programming is the essential component of this design: beads are packed at a flow rate of 50μL/sec, perfused by analyte at 1μL/sec and discarded at a flow rate of 200μL/sec.