Scientific Publications 2010
2010. "Characterization of an Ion Mobility-Multiplexed Collision Induced Dissociation- Tandem Time-of-Flight Mass Spectrometry Approach." International Journal of Mass Spectrometry 293(1-3):34-44. Abstract The confidence in peptide (and protein) identifications with ion mobility spectrometry time-of-flight mass spectrometry (IMS-TOFMS) is expected to drastically improve with the addition of information from an efficient ion dissociation step prior to MS detection. High throughput IMS-TOFMS analysis imposes a strong need for multiplexed ion dissociation approaches where multiple precursor ions yield complex sets of fragment ions that are often intermingled with each other in both the drift time and m/z domains. We have developed and evaluated a novel approach for collision-induced dissociation (CID) with an IMS-TOFMS instrument. It has been shown that precursor ions activated inside an rf-device with an axial dc-electric field produce abundant fragment ions which are radially confined with the rf-field and collisionally cooled at an elevated pressure, resulting in high CID efficiencies comparable or higher than those measured in triple-quadrupole instruments We have also developed an algorithm for deconvoluting these complex multiplexed tandem MS spectra by clustering both the precursor and fragment ions into the matching drift time profiles and by effectively utilizing high mass measurement accuracy of the TOFMS. In a single IMS separation with a tryptic digest of bovine serum albumin (BSA), we have reliably identified 20 unique peptides using multiplexed CID approach downstream of the IMS separation. Peptides were identified based upon the correlation between the precursor and fragment drift time profiles and by matching the profile representative masses to those of in silico BSA tryptic peptides and their fragments. The false discovery rate (FDR) of peptide identifications from multiplexed MS/MS spectra was less than 1%.
2010. "Li Ion Diffusion Mechanisms in Bulk Monoclinic Li2CO3 Crystals from Density Functional Studies." Journal of Physical Chemistry C 114(48):20903-20906. doi:10.1021/jp1086569 Abstract Density functional studies of Li+ ion diffusion mechanisms in bulk monoclinic lithium carbonate Li2CO3 crystals are performed to identify the stable Li+ interstitial positions and migration barriers. The migration barrier for Li+ diffusion between the planes defined by Li2CO3 units along the open channels  is found to be very small at 0.28 eV, while a higher migration barrier of 0.60 eV was found for the diffusion across the planes. These results show that diffusion of Li+ in Li2CO3 is favorable along the  channels. The implications for Li+ ion transport in solid electrolyte interphases (SEI) in Li ion batteries are discussed.
2010. "Ab Initio Study of Hydration and Proton Dissociation in Ionomer Membranes ." Journal of Physical Chemistry A 114(25):6904-6912. Abstract We present a comparative study of proton dissociation in various functional acidic units that are promising candidates as building blocks for polymeric electrolyte membranes. Minimum energy structures for four acidic moieties with clusters of 1-6 water molecules were determined using density functional theory at the B3LYP/6-311G** level starting from chemically rational initial configurations. The perfluoro sulfonyl imide acid group (CF3CF2SO2NHSO2CF3) was observed to be the strongest acid, due to the substantial electron withdrawing effect of both fluorocarbon groups. The hydrophilic functional group (CH3OC6H3OCH3C6H4SO3H) of sulfonated polyetherether ketone (SPEEK) membrane was found to be the strongest base with the acidic proton dissociation requiring the addition of six water molecules and the hydrated proton being more tightly bound to the conjugate base. Even though both perfluoro sulfonyl imides and sulfonic acids (hydrophilic functional groups for sulfonyl imide and Nafion ionomers respectively) required only three water molecules to exhibit spontaneous proton dissociation, the largest possible solvent-separated hydronium ion was attained only for the sulfonyl imide moiety. These results provide a scientific basis for understanding the improved conductivity of perfluorinated sulfonyl imide-based membranes relative to that of the widely-used Nafion membrane.
2010. "Dissolution study of Metatorbernite: Thermodynamic Properties and the effect of pH and Phosphate." Environmental Science & Technology 44(19):7521-7526. doi:10.1021/es101619f Abstract Metatorbernite is a host of uranium that has been identified in the shallow vadose zone of the 300 A area at the Hanford site, WA, USA. Consequently, modeling the evolution of U concentrations in vadose zone porewaters driven by meteoric water recharge requires accurate knowledge of metatorbernite solubility. Previous determinations of the solubility constant for metatorbernite were under constrained. In the present contribution, the dissolution of natural metatorbernite crystals was studied at target pH 2.5 and 3.0, using both nitric and phosphoric acid. Steady state was approached from under- and super-saturation. The experiments and calculations yielded a preferred log Ksp = -28.0 ±0.1 that is significantly different than previously determined values. Further, both stoichiometric and non-stoichiometric dissolution was observed as a function of pH and aqueous phosphate concentration.
2010. "Influence of Dynamical Conditions on the Reduction of UVI at the Magnetite-Solution Interface." Environmental Science & Technology 44(1):170-176. Abstract The heterogeneous reduction of UVI to UIV by ferrous iron is a potentially key process influencing the fate and transport of U in the environment. The reactivity of both sorbed and structural FeII has been studied for numerous substrates, including magnetite. The results from UVI-magnetite experiments have been variable, ranging from no reduction to clear evidence for the formation of UIV. In this contribution, we used XAS and high resolution (+cryogenic) XPS to study the interaction of UVI with nano-particulate magnetite. The results indicated that UVI was partially reduced to UV with no evidence of UIV. However, thermodynamic calculations indicated that mixed-valence U phases with average oxidation states below (V) should have been stable, indicating that the system was not in redox equilibrium. A reaction pathway that involves incorporation of U and stabilization of UV and UVI in secondary phases is invoked to explain the observations.