Comparative Study of Graphite-Supported LDI- and ESI-FT-ICR-MS of a Pyrolysis Liquid from a German Brown Coal

Pyrolysis liquids from coal are complex mixtures of organic compounds with low to high molecular mass and low to high polarity. Compared to low-molecular-weight compounds, little information is available regarding high-molecular-weight compounds in pyrolysis liquids, although their characterization is important for the elucidation of degradation pathways. In this study, laser desorption ionization (LDI) using graphite powder as the support material has been used in conjunction with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) for the analysis of a pyrolysis liquid from brown coal. The acquired data is compared to previous results of the same sample using electrospray ionization (ESI). Using LDI, compounds with masses between 300 and 900 Da were detected by LDI. An evaluation of the spectra was based on the frequency of molecular formulas with a different number of heteroatoms. Hydrocarbon compounds and heteroatomic compounds containing oxygen and sulfur were found. A comparison to ESI results shows that the numbers of observed molecular formulas are virtually the same, but a higher quantity of molecular formulas with a low number of oxygen can be detected by LDI. The observation of molecular formulas without oxygen is a unique feature of the LDI spectra. A more detailed investigation was possible by the utilization of double bond equivalent plots versus carbon number, which revealed a prevalence of LDI for the ionization of compounds with higher DBE.