There are many advantages of using on-line liquid chromatography/mass spectrometry (LC/MS) to characterize complex mixtures. By incorporating low-voltage electron-impact ionization/highresolution MS with moving belt LC/MS, differentiation can be made between naphthenoaromatics and alkylaromatics and between aromatic hydrocarbons and "difficult-to-resolve" thiophenes. Alternative on-line LC/MS techniques for heavy hydrocarbon characterization are also discussed.

The mass spectra of two series of aliphatic polynitro compounds are reported and discussed. The fragmentation patterns of aliphatic nitro and polynitro compounds are similar in that no appreciable molecular ion current is observed; however, there are several otherfeaturesinthefragmentation of aliphatic polynitro compounds which differ from that of nitroalkane spectra. Both series of compounds studied-C(NO,),(CH,),-,, where x = 4 to 0 and Cz(N02)z(CH3)6-E, where x = 6,4,2-show