Flow trough cells for spectrophotometry should be designed in such a way that:
The length of light path is maximized
Internal cell volume is minimized
I.D. of flow cell is the same as of the flow channel
Flow cell can be thermostated
Flow cell can be visually inspected
Obviously, increasing the length of light path increases sensitivity of an assay, while decreasing cell volume allows miniaturization, as it decreases volume of sample needed to fill the flow cell. If the internal diameter of channel leading to flow cell is smaller than I.D. of the flow cell (A), the flowing stream at the orifice expands and bubbles will form due to Venturi effect.
It was Garth Klein who designed flow cell that fulfills all the above requirements. Garth's flow cell (B) is simpler and more rugged then currently used flow cells, since many of them comprise unnecessary components such as lenses, quartz windows and gaskets.
The key component of Garth’s cell is a straight piece of PEEK tubing, precisely aligned with two quartz optical fibers, which deliver and collect light at the opposite ends of the tubing. The optical fibers ( O.D. 0.5mm), cemented in 0.8mm I.D. PEEK tubing and polished, are butted against the ends of the tubing (C) (0.8mm I.D.PEEK) that form the light path. The internal volume of flow cell made of 0.8mm I.D. tubing is 5 microliters/cm. Flow cells of 5, 10 and 15cm long have been used in this chapter. Since also the rest of flow channel, including holding coils is made of the same tubing, the flow path, as well as the flow cell, have the same internal diameter. The length of the light path can be changed by changing the length of the tube and of the supporting tubular fitting, that keeps the PEEK tubing straight. Since longer flow cells extend from LOV module, they can be thermostated (D 15cm light path).
Note that Garth’s cell functions as a collimator. Thus its performance does not deteriorate, because in contrast to long light path flow cells, based on multiple internal reflection, contamination of cell walls does not diminish the light throughput.