Fiber optic sensor for NO_X

Aquocyanocobinamide was used as a selective reagent in the development of a fiber-optic gas sensor for NO_X. This reagent was trapped in front of a bifurcated fiber-optic bundle by using a gas-permeable membrane. NO_X species were generated by the addition of a known amount of nitrite to an acidic sample solution (pH 1.7). These gaseous species diffused through the gas-permeable membrane into the buffered reagent solution (pH > 5) and were partially converted back to nitrite, which replaced the water molecule as the axial ligand in aquocyanocobinamide. Two wavelengths (355 and 550 nm) were used to monitor the ligand exchange. Detection limits as low as 5 × 10^-6 M nitrite were obtained while monitoring the sensor at 550 nm. The selectivity for NO_X species was greatly improved over a potentiometric sensor based on pH detection.