M. A. Smith

University of Houston, Houston, Texas, United States

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Publications (11)10.97 Total impact

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    ABSTRACT: Titan, the moon of Saturn with a thick atmosphere and an active hydrocarbon-based weather cycle, is considered the best target in the solar system for the study of organic chemistry on a planetary scale. Microfluidic devices that employ liquid phase techniques such as capillary electrophoresis with ultrasensitive laser-induced fluorescence detection offer a unique solution for in situ analysis of complex organics on Titan. We previously reported a protocol for nonaqueous microfluidic analysis of primary aliphatic amines in ethanol, and demonstrated separations of short- and long-chain amines down to -20 °C. We have optimized this protocol further, and used it to analyze Titan aerosol analogues (tholins) generated in two separate laboratories under a variety of different conditions. Ethylamine was a major product in all samples, though significant differences in amine content were observed, in particular for long-chain amines (C12-C27). This work validates microfluidic chemical analysis of complex organics with relevance to Titan, and represents a significant first step in understanding tholin composition via targeted functional group analysis.
    Earth and Planetary Science Letters 01/2014; 403:99–107. · 4.35 Impact Factor
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    ABSTRACT: Abstract The discovery of large (>100 u) molecules in Titan's upper atmosphere has heightened astrobiological interest in this unique satellite. In particular, complex organic aerosols produced in atmospheres containing C, N, O, and H, like that of Titan, could be a source of prebiotic molecules. In this work, aerosols produced in a Titan atmosphere simulation experiment with enhanced CO (N(2)/CH(4)/CO gas mixtures of 96.2%/2.0%/1.8% and 93.2%/5.0%/1.8%) were found to contain 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases. Very high-resolution mass spectrometry of isotopically labeled samples confirmed that C(4)H(5)N(3)O, C(4)H(4)N(2)O(2), C(5)H(6)N(2)O(2), C(5)H(5)N(5), and C(6)H(9)N(3)O(2) are produced by chemistry in the simulation chamber. Gas chromatography-mass spectrometry (GC-MS) analyses of the non-isotopic samples confirmed the presence of cytosine (C(4)H(5)N(3)O), uracil (C(5)H(4)N(2)O(2)), thymine (C(5)H(6)N(2)O(2)), guanine (C(5)H(5)N(5)O), glycine (C(2)H(5)NO(2)), and alanine (C(3)H(7)NO(2)). Adenine (C(5)H(5)N(5)) was detected by GC-MS in isotopically labeled samples. The remaining prebiotic molecules were detected in unlabeled samples only and may have been affected by contamination in the chamber. These results demonstrate that prebiotic molecules can be formed by the high-energy chemistry similar to that which occurs in planetary upper atmospheres and therefore identifies a new source of prebiotic material, potentially increasing the range of planets where life could begin. Key Words: Astrochemistry-Planetary atmospheres-Titan-Astrobiology. Astrobiology 12, 809-817.
    Astrobiology 08/2012; · 2.80 Impact Factor
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    ABSTRACT: Groups of peaks with mass spacing of ∼13.5 are observed in all of the tholin spectra. Analysis of the Orbitrap tholin measurements indicates that these groups EPSC Abstracts Vol. 6, EPSC-DPS2011-1627, 2011 EPSC-DPS Joint Meeting 2011 c Author(s) 2011 can be defined by molecules that have a constant number of heavy atoms (C+N). The average number of C, N, and H atoms in each group increases linearly as a function of increasing mass. These trends can therefore be extrapolated to the higher masses observed in the CAPS measurements as a first estimate of the possible composition of those molecules. The spacing appears to be a consequence of the degree of saturation of the molecules. The INMS spectra and the CAPSIBS (positive ions) spectra exhibit groups of very regularly spaced peaks as well. The spacing in the INMS data is ∼12.5 but seems to increase towards 13.5 for the heavier masses in the CAPS-IBS data. The spacing change may be indicative of the transition from gas phase molecules to aerosols.
    10/2011;
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    ABSTRACT: The observations by the Cassini Ion Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) clearly demonstrate the importance of complex organic chemistry in the upper atmosphere of Titan; a complex coupling of neutral and ion chemistry for organic aerosol generation induced by EUV photons and Saturn's magnetospheric charged particles. To understand the dominant energy source for aerosol formation and its formation chemistry, we comparatively investigate the chemical mechanism in N2/CH4 gas mixtures resulting from EUV-VUV synchrotron radiation (50-150 nm) and tunable mono-energetic electron beam irradiation (5 eV - 2000 eV). These excitation energy sources cover the dominant energy source available in Titan's upper atmosphere. Our previous study of the EUV-VUV photolysis of N2/CH4 gas mixtures revealed the unique role of nitrogen photoionization in the catalytic formation of complex hydrocarbons and in the major nitrogen fixation process in Titan's upper atmosphere (Imanaka and Smith, 2007, 2009, 2010). However, relative roles of ion-molecule reactions and radical/neutral reactions in such complex chemistry remain to be determined. We characterized the electron energy distribution by conducting the Langmuir probe measurements. Degradation of the primary photoelectron from N2 photoionization at 20.6 eV photons is clearly observed, and the electron density rapidly decreases down to 109-10 cm-3, which suggests the complex coupling of ion-molecular reactions and dissociative ion-electron recombination reactions for the observed development of complex organic molecules. The electron beam irradiation experiments at energy larger than 200 eV shows distinct gaseous product distribution with nitrogenated gaseous species from those with EUV irradiation products. The generation of secondary electrons and multiple inelastic collisions of fast electrons might increases the nitrogen fixation efficiency. The much less stringent spin selection rules could provide many additional chemical pathways not open to optical absorption processes. We acknowledge support from the NASA grants NNX08AO13G, NNX09AM95G, NNX10AF08G, and the NAI program.
    10/2010;
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    ABSTRACT: The large abundance of NH3 in Titan's upper atmosphere is a consequence of coupled ion and neutral chemistry. The density of NH3 is inferred from the measured abundance of NH4+. NH3 is produced primarily through reaction of NH2 with H2CN, a process neglected in previous models. NH2 is produced by several reactions including electron recombination of CH2NH2+. The density of CH2NH2+ is closely linked to the density of CH2NH through proton exchange reactions and recombination. CH2NH is produced by reaction of N(2D) and NH with ambient hydrocarbons. Thus, production of NH3 is the result of a chain of reactions involving non-nitrile functional groups and the large density of NH3 implies large densities for these associated molecules. This suggests that amine and imine functional groups may be incorporated as well in other, more complex organic molecules.
    Faraday Discussions 01/2010; 147:31-49; discussion 83-102. · 3.82 Impact Factor
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    ABSTRACT: Experimental simulations of the initial steps of the ionic chemistry occurring in the ionosphere of Titan were performed at the synchrotron source Elettra in Trieste. The measurements consisted in irradiating with a monochromatic beam of variable wavelength (from the methane ionization edge at 12.6 eV, up to the molecular nitrogen dissociative ionization edge, beyond 24.3 eV) 3 gas mixtures of increasing complexity. The resulting ionic chemistry was recorded by means of a high-resolution ion trap. Experimental results give conclusive information on the validity of ion-chemistry reaction rates used to perform kinetic simulations, but reveal as well new processes, never described earlier. These will be discussed and compared to model results.
    09/2008;
  • Hiroshi Imanaka, M. A. Smith
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    ABSTRACT: The recent Cassini mission revealed the organic haze formation in the ionosphere of Titan, where the ionization of the dominant N2 molecules is the primary process. Previous study of the vacuum UV photolysis of N2/CH4 gas mixtures indicates that photoionization of N2 by EUV radiation plays a major role in initiating the production of complex organic molecules such as benzene and toluene (Imanaka and Smith, 2007). It is not clear, however, how much nitrogen is incorporated in Titan's organic haze. Many aspects of the nitrogen fixation process by EUV-VUV photochemistry have not been fully understood. We demonstrate the first evidence of nitrogenated organic haze production by EUV-VUV irradiation of a N2/CH4 gas mixture. A N2/CH4 (= 95/5) gas mixture at 0.066 mbar in a 0.7 m length windowless photocell is irradiated by EUV-VUV light using a tunable synchrotron radiation source at the Advanced Light Source. The solid and gaseous species generated at the photon wavelengths of 60 nm and 82.5 nm are investigated in detail. The accumulated solid materials are characterized with Laser Desorption Ionization- Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry. The obtained ultra-high-resolution mass spectra enable the unambiguous CHN formula assignments, and they show the predominance of highly nitrogenated compounds in both solid materials. They consist of the distinct functional groups dominated with highly unsaturated compounds. The solid material produced at 82.5 nm contains more nitrogen than that generated at 60 nm. Although the presence of NH3 is clearly detected in the gas phase products at both photon wavelengths, the gaseous neutral species are dominated by hydrocarbons with less nitrogen incorporation. This implies that dissociated nitrogen favors to be partitioned into the solid phase and that substantial amounts of nitrogen may exist as organic haze in Titan's atmosphere. We acknowledge support from the NASA Exobiology program (grant NNG05GO58G).
    08/2008; 40:421.
  • Hiroshi Imanaka, D. Archer, M. A. Smith
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    ABSTRACT: Previous study of the vacuum UV photolysis of N2/CH4 gas mixtures indicates that photoionization of N2 by EUV radiation plays a major role in initiating the production of complex organic molecules such as benzene and toluene (Imanaka and Smith, 2007). In this study, we further attempt to understand the role of EUV-VUV light in the nitrogen fixation and haze formation mechanisms in the ionosphere of Titan. Gas mixtures of N2/CH4 in a windowless photocell are irradiated with EUV-VUV light (50 nm to 150 nm) using a synchrotron radiation source at the Advanced Light Source. Various 15N and 13C labeled gas mixtures are irradiated to assess the nitrogen incorporation chemistry and the range of nitrogenous products. Comparison of mass spectra of neutral gas products obtained from 15N2/CH4 and 14N2/CH4 gas mixtures reveals that CxHy chemistry proceeds at much faster rates than nitrogen incorporation chemistry and no significant nitrogeneous gaseous species are observed at UV wavelengths longer than 60 nm. The effective quantum yield of benzene (= produced benzene molecules/total absorbed photons by N2/CH4) at 60 nm is in the order of 10-3, which almost explains the benzene abundance of 1 ppm observed with the Cassini INMS (Wait et al., DPS 2006). This suggests that benzene production with EUV radiation is able to balance against the large photolysis destruction rate at wavelengths longer than 150 nm in Titan's ionosphere. Our laboratory detection of larger molecules, such as xylenes and naphthalene, as well as brownish solid aerosol products implies the production of larger PAHs and haze particles in Titan's ionosphere. We acknowledge support from the NASA Exobiology program (grant NNG05GO58G). Reference: Imanaka, H., Smith, M. A., Role of photoionization in the formation of complex organic molecules in Titan's upper atmosphere, Geophys. Res. Lett.,34, L02204, doi:10.1029/2006GL028317, 2007.
    10/2007;
  • Hiroshi Imanaka, M. A. Smith
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    ABSTRACT: Recent detection of benzene by Cassini's INMS at 1200 km altitude of Titan's atmosphere (Wait et al., 2005) suggests that complex organic chemistry may occur in the upper atmosphere of Titan. Few experimental simulations, however, have been conducted to investigate the complex organic chemistry induced by EUV-VUV irradiation into N2/CH4 gas mixtures. This study attempts to understand the role of EUV-VUV light in the subsequent complex organic chemistry. The occurrences of photoionization and photodissociation of N2 and CH4 depend on the wavelength of irradiated UV light, so that the subsequent organic chemistry from a N2/CH4 gas mixture might strongly depend on the wavelength of irradiated light. In this study, we report the formation of gaseous species from N2/CH4 gas mixtures as a function of EUV-VUV irradiation wavelengths from 50 nm to 150 nm. A N2/CH4 (= 95/5) gas mixture at 0.066 mbar in a 1 m length windowless photocell is irradiated by VUV light using a synchrotron radiation source at the Advanced Light Source. The photon wavelength can be controlled from 50 nm to 150 nm with a 1 nm bandwidth. The gaseous species produced by photochemistry are analyzed using a quadrupole mass spectrometer. The formation of heavy organics up to C8 to C10 by EUV light irradiation is observed, and the gas products depend on the irradiation wavelength. In particular, the efficient formation of benzene and toluene is observed during the irradiation at wavelengths less than 80 nm. This suggests that the photoionization of nitrogen may initiate the formation of benzene via ion-molecule reactions. In the N2 dominant atmosphere of Titan, the extreme ultraviolet radiation may play an important role in activating complex organic chemistry of the upper atmosphere through the photoionization of N2 molecules. We acknowledge the support by the NASA exobiology grant NNG05GO58G.
    09/2006;
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    ABSTRACT: Titan tholins generated by complex processes (including ion–molecule reactions) in a laboratory plasma were investigated by ultrahigh resolution MS and tandem MS/MS measurements. Titan has a special interest in astrobiology because “in situ” measurements by the Cassini–Huygens spacecraft indicate the presence of complex organic molecules of prebiotic interest. The present work focuses on negatively charged ions that have not been systematically studied by ultrahigh resolution MS and MS/MS. The negatively charged ions were generated from a tholin sample by both laser desorption ionization (LDI) and electrospray ionization (ESI). The chemical compositions determined for the negatively charged ions clearly indicate the presence of highly unsaturated (H/C < 1) species with high nitrogen content (presumably related to multiple cyano functionalities). This is characteristically different from the previously analyzed positively charged ions that are more saturated and contain amino and imino functionalities. Based on tandem MS/MS experiments and quantum chemical calculations we propose characteristic structural features for selected ions. They include open chain (C6N3−) and aromatic ring structures (C10N5−). The basic non-aromatic structural unit C2N3− seems to play an important role and several structural “families” can be derived as HCN, HCCH and H2 “adducts” of this ion.
    Int. J. Mass Spectrom. 316-318:157-163.