Laser microprobe mass spectrometry studies of polycyclic aromatic hydrocarbons in soils, sediments, and interstellar ice analogs.

摘要

Microprobe two-step laser desorption/laser ionization mass spectrometry (muL2MS) was used to (1) identify and characterize PAHs found as contaminants on geosorbents, and (2) detect PAHs and PAH byproducts in analogs that are created under conditions that mimic the interstellar medium (ISM). The presence of PAHs in geosorbents was examined in one laboratory prepared sample and three aged field samples. The data show lateral variations in the extent of sorption at the sub-particle scale, which indicates that sorption phenomena are heterogeneous at sub-particle dimensions. Complementary spectroscopic techniques were employed to provide information at the microscale on where PAH contaminants in sediment and soil particles are located. muL 2MS was used for PAH measurements, infrared microspectroscopy was used for organic carbon measurement, and scanning electron microscopy with wavelength dispersive X-ray spectroscopy was used for elemental microanalysis. PAH concentrations on coal- and wood-derived particles were found to be several orders of magnitude higher than on silica particles. It was found that most PAHs are concentrated on external surface regions indicating near-surface sorption mechanisms. PAH desorption kinetic studies on these separated fractions revealed a relatively low availability of PAHs from the coal/wood fractions and a high availability from the clay/sift fraction. The creation and analysis of interstellar analogs allows the indirect investigation of processes that occur in the interstellar medium (ISM). To simulate the photoprocessing of organic dust mantles in the ISM, a gas mixture of simple carbon and nitrogen-containing molecules with water was co-deposited on a 10 K surface and exposed to ultraviolet radiation. muL2MS indicated that the photoprocessing created a rich mixture of polycyclic aromatic hydrocarbons and other conjugated organic molecules, which may explain how PAHs are replenished in space. Ice samples containing one part of PAHs in 500 or more parts of water were exposed to ultraviolet radiation and the resulting photochemical products were analyzed using infrared spectroscopy and muL2MS. Peripheral carbon atoms were oxidized to yield alcohols, ketones, and ethers. Deuterium labeling experiments in these ices demonstrate that hydrogen exchange with water is facile under irradiation, which may explain the deuterium enrichments of PAHs in various meteoritic materials.