Emission FT-IR Analyses of Jet Fuel Deposits

Deposits were collected on small flat and removable strips of metal from an aircraft fuel slowly flowing over them for varying lengths of time (up to 14 h). Both the strips and the fuel were heated to about 250°C. The fuel contained oxygen. After every test the deposit weight was determined with a microbalance, and the infrared emission spectrum of the deposit maintained at 100°C was recorded with a specially adapted Fourier spectrophotometer. Deposit thicknesses varied from 22 nm to 500 nm. Plots of deposit thickness squared vs. time were linear, with an abrupt change of slope after five hours of run time. Corresponding to these different slopes were also different infrared spectra, the earlier ones showing strong -C-O- bands around 1100 cm⁻¹, the latter primarily -C=0 bands near 1700 cm⁻¹. Fuels containing up to 1% of thiphene or pyrrole showed only the latter bands. These results are consistent with a diffusion mechanism dependent on the timely arrival at the liquid-deposit interface of a sufficient concentration of a reaction intermediate.