Attenuated total reflection techniques have been used extensively as analytical tools for the analysis of thin films and analytes imbedded in complex scattering matrices. However they have not been commonly utilized as detectors in common analytical techniques such as Flow Injection Analysis because of their relatively low sensitivity. The feasibility of using a thin film planar waveguide as an absorption sensor in the Flow Injection Analysis of Urea was investigated.

Urea was hydrolyzed to ammonia and carbon dioxide with the enzyme Urease. The ammonia produced was quantitated colorimetrically using Berthelot’s reaction. The reaction product, indophenol blue, was detected using the combination planar waveguide 9.2 microliter flow cell sensor.

The planar waveguides used had 2 to 3 orders of magnitude greater sensitivity than typical internal reflection elements. The analytical working range obtained for urea determinations was from 0 to 20 mM urea at a rate of 30 samples per hour.

A description of the investigation and the various factors involved in designing and optimizing a planar waveguide for absorption spectrometry is included.