SELECTIVE IONOPHORE-BASED OPTICAL SENSORS FOR AMMONIA MEASUREMENT IN AIR

Optical sensors (optodes) based on the incorporation of ammonium ion-selective ionophores and hydrogen ion-selective chromoionophores in plasticized poly(vinyl chloride) (PVC) membranes are applied to the measurement of ammonia in air. Dynamic response characteristics and selectivities for ammonia with respect to other normally occurring gases under varying relative humidity are studied for several membrane formulations. No significant interference occurs from relevant levels of SO2, NO2, or CO2, but a trade-off between selectivity over other amines versus insensitivity to changes in relative humidity is found. An optode formulated with the ionophore valinomycin, which forms a comparatively strong complex with ammonium ion, prefers ammonia over the alkylamines tested but is affected significantly by humidity changes. An optode based on the ionophore ETH 157, which forms a weaker ammonium complex shows no humidity effect but responds approximately equally to low levels of ethylamine, methylamine, and ammonia. In the experimental configuration described, the latter optode has a range of 0.002 to 100 ppm(v) and t95 response times varying from 230 s at 0.05 ppm(v) to 15 s at 100 ppm(v). A proposed optimization of the optical geometry promises to yield sub-ppb(v) detection limits and faster response times in future studies. There is no deterioration in response after 4 months in laboratory air.