Characterization of Large Synthetic Polycyclic Aromatic Hydrocarbons by MALDI- and LD-TOF Mass Spectrometry

Abstract

Polycyclic aromatic hydrocarbons (PAHs) with high molecular weights represent a class of compounds with are difficult to analyze with conventional analytical methods because of their insolubility. In this paper, we describe their characterization by laser desorption mass spectrometry (LDMD). The huge PAHs show a fragmentation pathway which is unknown so far. It can be explained by the loss of cyclic C 6 -units and a subsequent rehydrogenation of the resulting radical sites of the measured fragments. With these findings we are able to detect structure defects in a PAH induced by rearrangement reactions during synthesis. Since the analysis of large PAHs by LDMS is generally hindred by the formation of fragments we also developed an improved method of sample preparation analogous to matrix assisted laser desorption/ionization (MALDI) mass spectrometry. By this method, the isotopic distributions of the PAH molecular ions can now be measured without interfering signals of fragments to enable a reliable identification. Fragmentation mechanisms proposed for the loss of a cyclic C 6 unit. The mass differences after fragmentation depend on the peripheral structure of the PAHs. The overall mass difference between the molecular ion and the fragment ion is assumed to result from the cleavage of one or more C-C bonds and the subsequent attachment of hydrogen atoms to the reactive radical sites left behind on the bigger fragment. A mass difference of Δm = 72 results from cleavage of three C-C bonds.