Separation and analysis of high range extractable molecules formed during coal pyrolysis using coupled thin layer chromatography-imaging mass spectrometry (TLC-LDI-IMS)

A new coupled thin layer chromatography-imaging mass spectrometry method was developed to investigate solvent extracts from a coal. The technique was used to assess the separation of complex molecular structures in a tetrahydrofuran (THF) soluble extract taken from a semi-coke (the solid residue obtained after thermal pre-treatment of coal at 430 °C). Thin layer chromatography (TLC) was then used with acetone as a mobile phase and silica as a stationary phase. The component separation was determined by laser desorption/ionisation time-of-flight imaging mass spectrometry (i.e. TLC-LDI-IMS). This provided the molecular weight distributions and spatial concentrations of coal derived structures observed over the TLC plate. In general, acetone lifted and concentrated lighter molecular species in the range of 200-500 Da by up to 55% compared to the THF extract. Structures of 500-1000 Da in molecular weight were lifted, but to lesser extent, distributing these more evenly across the TLC plate. The structures observed in the acetone-concentrate repeated every 12-14 Da as in a homologous series, similar to previous studies. Residual material remaining in the sample (unascended by the mobile phase) showed repeating structures every 24 Da from 600-3000 Da, also similar to previous work.A novel methodology was also evaluated using the TLC plate as a heated substrate for inducing and studying cross-linking reactions. Acetone soluble material between 200-500 Da was loaded onto the TLC plate and heated to 250 °C. This was found to convert the low molecular weight compounds into more complex structures between 1500 and 4000 Da. Direct use of a MALDI-suitable TLC plate as a reaction surface provided a means of characterising the high molecular weight products without the need for a solvent to transfer material onto a second MALDI analysis plate. This avoided limitations in molecular weight with solvent solubility. Overall, the TLC-LDI-IMS method was shown to be a suitable method for tracking the separation and potential cross-linking mechanisms involved in coal derived extracts.