Reversible fiber-optic 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-oxasole (appzox) or -thiazole (appzth). The novel indicator molecules display an absorption band in the blue (up to 450 nm), high emission quantum 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].sub.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 chloromethyiated 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.sub.100%]) to 0.2-6% (v/v) methanol in the samples are 2-4 rain at 22 [degrees] C, the relative standard deviation for repeated measurements being 1.5-3%. Of all the two- to free-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.