Figure 1 shows emission spectra for a typical No. 2 fuel oil and a typical No. 6 fuel oil. The differences are clear, permitting easy classification. The differences between two No. 6 oils illustrated in Figure 2, however, are slight. Furthermore, in Figure 3, note that the Prudhoe Bay crude oil spectrum is very similar to the data shown in Figure 2. Such close correspondence in emission spectra is common, and can lead to classification errors.
When emission data are not sufficient for conclusive sample identification, synchronous scanning can provide significantly more useful spectral structure. Synchronous scanning entails simultaneous scanning of the excitation and emission monochromators with a constant offset between them. The recorded intensity is proportional to the product of the observed excitation and emission intensities. Accordingly, a significant difference could then be discerned between the maximum peak wavelengths of the No. 6 fuel oils in Figure 4 and the Prudhoe Bay crude oil in Figure 5. Whereas the maximum peak positions were identical for the samples emission spectra, synchronous scanning increases the difference in position by around 50 nm.
Figures 6, 7, and 8 provide practical examples comparing emission spectra of unknown oil samples and library references. Figure 6 shows that an analytical sample extracted from soil yields recognizable and useful spectra. Figure 7 reveals a close correspondence between a known Prudhoe Bay crude oil and an unknown sample received in isopropanol. Figure 8 is interesting because it compares a weathered real-world sample with a known JP-4 jet fuel. Fluorescence anal