Optical sensors (optodes) based on the Incorporation of ammonium lon-selectlve lonophores and hydrogen Ion-select Ive chromolonophores In plasticized poly(vlnyl chloride) (PVC) membranes are applied to the measurement of ammonia In air. Dynamic response characteristics and selectlvlties 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 S02, N02, or C02, but a trade-off between selectivity over other amines versus Insensitivity to changes In relative humidity Is found. An optode formulated with the lonophore vallnomycln, 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 lonophore ETH 157, which forms a weaker ammonium complex shows no humidity effect but responds approximately equally to low levels of ethylamine, methylamlne, and ammonia. In the experimental configuration described, the latter optode has a range of 0.002 to 100 ppmv and tK response times varying from 230 s at 0.05 ppm, to 15 s at 100 ppm,. A proposed optimization of the optical geometry promises to yield sub-ppb, detection limits and faster response times In future studies. There Is no deterioration In response after 4 months In laboratory air.
A detector system for fibre-optic sensors relying on a light-emitting diode and a photodiode is described. A sensing membrane is irradiated through one branch of a bifurcated optical cable with a blue light-emitting diode of high intensity. The second arm of the cable guides fluorescent light from the membrane back t o the detector. Wavelength separation was found t o be unnecessary and lock-in amplification alone provided excellent discrimination against fluctuations in ambient light intensity. The entire system is therefore significantly simpler than most fluorescence-based sensors as no monochromators or optical filters are employed and both the light source and detector consist only of solid-state electronic components. The lightemitting diode can be square-wave modulated electronically rather than with a mechanical chopper as necessary for other types of light sources. It was possible t o carry out the lock-in detection with a circuitry based on a demodulator contained in one integrated circuit package. The entire detector system is therefore very compact, simple and inexpensive. The performance was tested with t w o types of sensing membranes. An application is demonstrated with the determination of oxygen employing the quenching of the fluorescence of tris(2,2'-bipyridyI)ruthenium(ii) chloride contained in a silicone membrane. A further application is shown with a nitrate-selective membrane based o n poly(vinyl chloride), which contains a tridodecylmethyiammonium salt as ionophore and a lipophilic derivative of fluorescein. Nitrate concentrations over four orders of magnitude could be determined with this system.
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