REVERSIBLE FIBEROPTIC FLUOROSENSING OF LOWER ALCOHOLS

Low molecular weight alcohols, such as methanol and ethanol, are optic ally sensed in nonhydroxylic media using the highly fluorescent dyes 5 -(4-aminophenyl)-2-(2-pyrazinyl)-1,3-oxazole (appzox) or -thiazole(app zth). The novel indicator molecules display an absorption band in the blue (up to 450 mn), high emission quantun yields (up to 0.82), and sm all 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 ph otoreaction that produces an intramolecular charge-transfer excited st ate, which constitutes the basis of the sensor response. Fabrication o f a fiber-optic sensing head for reversible quantification of such ana lytes is possible via deposition of the indicator onto silica gel (for gas-phase monitoring) or covalent binding to cross-linked chloromethy lated styrene-divinylbenzene copolymer (for both gas- and liquid-phase measurements). The performance of the optical monitoring device has b een tested in the analysis of lower alcohols in hydrocarbon solvents a nd 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 re lative standard deviation for repeated measurements being 1.5-3%. Of a ll the two- to five-carbon alcohols tested, as well as water (up to sa turation), only ethanol may be regarded as interferent for methanol qu antification in gasoline. Fiber-optic measurements are also insensitiv e to other fuel additives such as methyl tert-butyl ether, lead tetrae thyl and colored or fluorescent stains. The temperature coefficient of the sensor in the 10-40 degrees C range is 0,0057/degrees C.