Laser mass spectrometry studies of complex aromatics in terrestrial samples and laboratory analogs

Microprobe two-step laser-desorption/laser-ionization mass spectrometry (μL2MS) was used to (1) analyze polycyclic aromatic hydrocarbons (PAHs) in terrestrial rocks, (2) determine the photochemistry of PAHs and alkanes, (3) examine photochemical reactions of PAHs and carbon dioxide (CO₂), (4) characterize PAHs in lampblack residuals, and (5) detect polychlorinated biphenyls (PCBs) in standard samples.; In the first study, ancient terrestrial rocks were studied by μL 2MS and gas chromatography/mass spectrometry (GC/MS). μL 2MS provides in situ analysis of very small samples, records the PAHs with no isomer information, and gives quantitative data on alkylated PAHs. GC/MS provides isomer separation and quantitation of PAHs in the bitumen. Combination of these techniques allows analysis of very small samples by μL2MS with GC/MS confirmation of isomers. In the second study, the reaction of PAHs and alkanes was examined in the presence of ultraviolet light. Alkylated PAHs were detected for a wide range of starting materials. Experiments were performed under model prebiotic Earth and interstellar medium (ISM) conditions, and alkylated products were observed. We propose that photoreactions were a plausible pathway for the formation of strong carbon-carbon bonds on the early Earth and in the ISM. In the third study, the photochemistry of phenanthrene and CO₂ was examined in dimethyl sulfoxide (DMSO), water, ocean water, and liposomes. When a solution of phenanthrene in DMSO was irradiated, it readily reacted with CO₂ to yield phenanthrene-carboxylic acid. In water, ocean water, and liposomes, phenanthrene did not photoreact with CO₂. Instead, oxidized phenanthrene products were produced. In the fourth study, five lampblack residuals from four former oil-gas plants in California were characterized. μL2MS analysis indicated that the soils have a large number of PAHs with an alkylation level of less than 5%. The μL2MS spectra of the separated lampblack and sand/clay fractions indicate that PAHs in the sand/clay are associated with the lampblack. In the fifth study, μL2MS was used for detecting PCBs in Aroclor solutions (1232, 1242, and 1262) and spiked samples of activated carbon. All the PCBs present were clearly identified using μL2MS. The detection limit for PCBs on solid surfaces was determined to be between 1 and 10 ppm.