Approximately 100lL of sample was d

Approximately 100lL of sample was deposited onto the surface of the film using a micropipette and subsequently spread uni- formly using the tip of the micropipette. The viscosity of oil sam- ples was reduced by mixing samples with hexane, which aided deposition with the oil film forming after the solvent evaporated. Films prepared in this manner were maintained in a horizontal position, and the spectra of the oil film were obtained using a PE film background spectrum. The spectra of oils were recorded using these PE film. An effective pathlength was determined from each spectrum and then used to normalize the spectrum to a fixed path- length. This effective pathlength could be used to compare quantitatively results obtained from different films or different sample loadings. The effective pathlengths of oil film spectra were normalized to a fixed pathlength of 0.15 mm by a pathlength cali- bration equation relating the effective path length to the absor- bance at 4334 cm