Laser desorption laser ionization mass spectra of 23 model compounds and 2 petroleum asphaltene samples are presented. These experiments involved desorption by irradiation with the 10.6 μm output of a CO2 laser followed by single-photon ionization with the 157 nm output of a fluorine excimer laser. The average molecular weight of the asphaltene samples agrees closely with that found previously using multiphoton ionization with the 266 nm output of a Nd:YAG laser. The fragmentation behavior as a function of ionization laser pulse energy is studied to evaluate which families of model compounds fragment differently from asphaltenes and, hence, can be excluded from being dominant in asphaltenes. All model compounds having one aromatic core with and without various pendant alkyl groups show little to no fragmentation, mimicking the behavior observed for the two asphaltene samples, whereas all model compounds having more than one aromatic core show energy-dependent fragmentation. These observations support the contention that the dominant structural character of asphaltenes is island-like.

Defined by their solubility in toluene and insolubility in n-heptane, asphaltenes are a highly aromatic, polydisperse mixture consisting of the heaviest and most polar fraction of crude oil. Although asphaltenes are critically important to the exploitation of conventional oil and are poised to rise in significance along with the exploitation of heavy oil, even as fundamental a quantity as their molecular weight distribution is unknown to within an order of magnitude. Laser desorption/ionization (LDI) mass spectra vary greatly with experimental parameters so are difficult to interpret: some groups favor high laser pulse energy measurements (yielding heavy molecular weights), arguing that high pulse energy is required to detect the heaviest components of this mixture; other groups favor low pulse energy measurements (yielding light molecular weights), arguing that low pulse energy is required to avoid aggregation in the plasma plume. Here we report asphaltene mass spectra recorded with two-step laser mass spectrometry (L2MS), in which desorption and ionization are decoupled and no plasma is produced. L2MS mass spectra of asphaltenes are insensitive to laser pulse energy and other parameters, demonstrating that the asphaltene molecular weight distribution can be measured without limitation from insufficient laser pulse energy or plasma-phase aggregation. These data resolve the controversy from LDI, showing that the asphaltene molecular weight distribution peaks near 600 Da and previous measurements reporting much heavier species suffered from aggregation effects.

The source and nature of carbon on Mars have been a subject of intense speculation. We report the results of confocal Raman imaging spectroscopy on 11 martian meteorites, spanning about 4.2 billion years of martian history. Ten of the meteorites contain abiotic macromolecular carbon (MMC) phases detected in association with small oxide grains included within high-temperature minerals. Polycyclic aromatic hydrocarbons were detected along with MMC phases in Dar al Gani 476. The association of organic carbon within magmatic minerals indicates that martian magmas favored precipitation of reduced carbon species during crystallization. The ubiquitous distribution of abiotic organic carbon in martian igneous rocks is important for understanding the martian carbon cycle and has implications for future missions to detect possible past martian life.

Two-step laser mass spectrometry (L 2 MS) is explored as a technique to measure the molecular-mass distribution of asphaltenes. Unlike widely used laser desorption/ionization (LDI) mass spectrometry, in which a single laser pulse is used to desorb and ionize asphaltenes, L 2 MS involves two laser pulses and spatially and temporally separates the desorption and ionization events. This process allows L 2 MS to measure the asphaltene molecular-mass distribution free from artifacts resulting from aggregation and insufficient laser power, as occur in LDI. Studies of relevant model compounds show that L 2 MS detects these molecules without aggregation (unlike LDI), with only a minimum of fragmentation/multiple charging, and with relatively uniform sensitivity across the relevant mass range; however, the efficiency of the resonant ionization process is sensitive to molecular structure. These data suggest that L 2 MS does not suffer from significant mass discrimination in the relevant mass range and that L 2 MS provides a slight underestimate of the asphaltene molecular-mass distribution. Petroleum asphaltenes from different geographical origins are found to have similar mass spectra, all showing a peak at every nominal mass under an envelope beginning at 200 units, peaking at 500-600 units, and extending to 1000-1500 units. Coal asphaltenes are found to be considerably lighter and less complex, showing pronounced clusters of peaks separated by 14 units under an envelope beginning at 200 units, peaking at 300 units, and extending to 500 units. These results bring the molecular-mass distribution of asphaltenes as measured by laser desorption mass spectrometry in accordance with many other mass spectrometry and diffusion measurements.