Laser Desorption Ionization of Large Condensed Polycyclic Aromatic Hydrocarbons in Petroleum Pitch and Thermally Treated Products

Laser desorption ionization (LDI) mass spectrometry has been widely applied for the analysis of pitch-related materials. LDI is particularly useful when samples have very low solubility. However, LDI conditions, such as laser power output, can have significant impact on the resulting mass spectra. In this work, we examined the LDI of coronene and two petroleum pitch samples, a M-50 isotropic pitch and a thermally treated M-50 pitch that contained a mesophase. LDI at varying laser power is coupled to ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to determine the impact of laser power on the elemental compositions of the pitch samples. Coronene is shown to form large polycyclic aromatic hydrocarbon (PAH) oligomers at high laser powers. Variations in laser power clearly affect mass distributions and compound-type distributions of the pitch samples. The impact of laser power is more significant and visible for the thermally treated pitch sample, where increased laser power generated high levels of fully dealkylated (or denuded) polycyclic aromatic hydrocarbons (dPAHs) and fullerenes. The presence of two types of PAH ions containing even and odd numbers of hydrogen atoms were observed. Even-hydrogen-number PAHs are molecular ions produced by direct laser ionization. The origins of odd-hydrogen-number PAHs are more complicated. They can result from dealkylation of larger PAHs, protonation of the parent molecule, and/or ionization of neutral PAH radicals. The latter can be a significant contributor to the odd-hydrogen-number PAHs. For analytical applications, a balance in laser power is needed to vaporize the non-volatile pitch molecules while also minimizing potential secondary thermal reactions during the LDI process. When laser power is controlled at a similar level, LDI-MS provides useful information to understand pitch compositional change from thermal treatment.