OPTICAL SENSING OF OXYGEN USING PHOSPHORESCENCE QUENCHING

Optical measurements that exploit the ability of oxygen to quench the photoexcited states of other molecules provide an alternative to the standard electrochemical and paramagnetic methods of measuring molecular oxygen concentration. In the present work we have re-examined the phosphorescence quenching technique as a means of monitoring oxygen concentrations in a gas stream at room temperature. Camphorquinone (CQ) is selected because it has strong absorption and emission bands in the visible region, enabling the use of inexpensive silica fibre-optic light guides. Determination of oxygen in the range 0.1-25% is easily achievable and, for a 10 mu m thick PVC film containing CQ, the time to attain 90% response is of the order of 10 s. Photooxidation and photoreduction account for a significant loss of material under certain conditions of illumination and film composition. The use of triplet lifetime as a measure of oxygen concentration offers several advantages. In particular, by using an intrinsic property of the sensing molecule, variations in lamp intensity and photomultiplier sensitivity are avoided and the effects of photobleaching reactions are minimized.