Abstract
Laser-desorption mass spectrometry (LD-MS) method development was undertaken to improve estimates of mass ranges for complex hydrocarbon mixtures. A creosote oil, an anthracene oil, and a mixture of known polynuclear aromatic hydrocarbon (PAH) compounds were examined. The data on the mixture of the four PAHs made it possible to define LD-MS conditions necessary to generate artifacts such as cluster ions by the combination of high laser power and high-mass accelerator voltage. The formation of cluster ions was possible without overloading the detector system. These multimer ions overlapped with higher-mass ion signals from the sample. However, careful balancing of sample concentration, laser power, total ion current, and delayed ion extraction appears to show high-mass materials without generating high-mass multimer (artifact) ions. It is possible to suppress the formation of cluster ions by keeping low target concentrations and, consequently, low gas phase concentrations formed by the laser pulse. The principal method used in this work was the fractionation of samples by planar chromatography followed by successive LD-MS analysis of the separated fractions directly from the chromatographic plates. This method separated the more abundant small molecules from the less abundant large molecules to permit the generation of their mass spectra independently, as well as reducing the concentration of sample by spreading over the PC-plate. The technique demonstrably suppressed multimer formation and greatly improved the reproducibility of the spectra. Results showed the presence of molecule ions in the ranges m/z 1000−2000 for the anthracene oil sample and m/z 600−1500 for the creosote oil sample, tailing off to m/z ∼5,000. The creosote oil contained significantly less of this high-mass material than the anthracene oil sample, and in both cases, high-mass material was only present in low quantities. Ion mass range estimates were in close agreement with molecular mass ranges from size exclusion chromatography, and findings were consistent with changes observed in the UV-fluorescence spectra. The method outlined in the paper appears directly applicable to the characterization of heavier coal and petroleum derived fractions.
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