REVERSIBLE FIBEROPTIC FLUOROSENSING OF LOWER ALCOHOLS

Low molecular weight alcohols, such as methanol and ethanol, are optically sensed in nonhydroxylic media using the highly fluorescent dyes 5-(4-aminophenyl)-2-(2-pyrazinyl)-1,3-oxazole (appzox) or -thiazole(appzth). The novel indicator molecules display an absorption band in the blue (up to 450 mn), high emission quantun yields (up to 0.82), and small excited-state lifetimes (1-3 ns) that prevent cross-sensitivity to oxygen. Their large Stokes shift and minute emission (Phi(em) < 0.01) in the presence of alcohols are discussed in terms of an adiabatic photoreaction that produces an intramolecular charge-transfer excited state, which constitutes the basis of the sensor response. Fabrication of a fiber-optic sensing head for reversible quantification of such analytes is possible via deposition of the indicator onto silica gel (for gas-phase monitoring) or covalent binding to cross-linked chloromethylated styrene-divinylbenzene copolymer (for both gas- and liquid-phase measurements). The performance of the optical monitoring device has been tested in the analysis of lower alcohols in hydrocarbon solvents and commercial gasoline samples. Typical response times (t(100%)) to 0.2-6% (v/v) methanol in the samples are 2-4 min at 22 degrees C, the relative standard deviation for repeated measurements being 1.5-3%. Of all the two- to five-carbon alcohols tested, as well as water (up to saturation), only ethanol may be regarded as interferent for methanol quantification in gasoline. Fiber-optic measurements are also insensitive to other fuel additives such as methyl tert-butyl ether, lead tetraethyl and colored or fluorescent stains. The temperature coefficient of the sensor in the 10-40 degrees C range is 0,0057/degrees C.