Lawrence L. Griffin

Sediment-water exchange of Mn, Fe, Ni and Zn in Galveston Bay, Texas

In-situ benthic flux studies were conducted at three stations in Upper Galveston Bay twice during March 1996 to directly measure release rates of dissolved Mn, Fe, Ni and Zn from the sediments. Results showed reproducible increases with time in both replicate light and light–dark benthic chambers, resulting in average fluxes of −1200±780, −17±12, −1.6±0.6 and −2.4±0.79 μmol m−2 day−1 for Mn, Fe, Ni and Zn, respectively. Sediment cores collected during 1994–1996 showed that surficial pore water concentrations were elevated compared to overlying water column concentrations, suggesting diffusive release from the sediments. Diffusive flux estimates of Mn and Zn agreed in direction with chamber fluxes measured on the same date, but only accounted for 5–38% of the measured flux. Diffusive fluxes of Fe agreed with measured fluxes at the near Trinity River station but overestimated actual release in the mid and outer Trinity Bay regions, possibly due to inaccurate determination of the Fe pore water gradients or rapid oxidation processes in the overlying water at these stations. In general, measured fluxes of Mn and Ni were higher in the mid Trinity Bay region and suggested a mechanism for the elevated trace metal concentrations previously reported for this region of Galveston Bay. However, the fluxes of Fe were highest in close proximity to the Trinity River, supporting the elevated Fe concentrations measured in this region during this and other studies, and decreased towards middle and outer Trinity Bay. Trace metal turnover times were between 0.1 and 1.2 days for Mn, between 1.3 and 4.6 days for Fe, and between 27 and 100 days for Ni and 12–20 days Zn, and were considerably shorter than the average Trinity Bay water residence time (1.5 years) for this period. Comparing area averaged benthic inputs to Trinity River inputs shows the sediments to be a significant source of trace metals to Galveston Bay. However, while benthic inputs of trace metals were measured, water column concentrations remained low despite rapid turnover times for Mn and Fe, suggesting removal of these metals from the water column after release from the sediments.

Trace metal analysis of natural waters by ICP-MS with on-line preconcentration and ultrasonic nebulization

Concentrations of Mn, Ni, Cu, Zn and Pb in natural waters were measured by inductively coupled plasma mass spectrometry (ICP-MS) with on-line preconcentration using Toyopearl TSK-immobilized 8-hydroxyquinoline resin columns and ultrasonic nebulization. Trace metal concentrations, quantified after analyzing calibration standards, were measured in 3 mL samples in under 15 min with better than 5% precision. Method detection limits were 0.26, 0.86, 1.5, 10 and 0.44 ng L –1 for Mn, Ni, Cu, Zn and Pb, respectively. The accuracy of the method was demonstrated by results from runs of certified reference materials SLRS-3 and CASS-3, which have very different ionic strengths. This on-line system was successfully applied to measure water column trace metal concentrations in Galveston Bay, Texas, and the results compared favorably with those obtained using state-of-the-art off-line preconcentration techniques.

Benthic Exchange of Nutrients in Galveston Bay, Texas

Nutrient regeneration rates were determined at three sites increasing in distance from the Trinity River, the main freshwater input source, to Galveston Bay, Texas, from 1994 through 1996. Diffusive fluxes generally agreed in direction with directly measured benthic fluxes but underestimated the exchange of nutrients across the sediment-water interface. While the fluxes of ammonium and phosphate were directed from the sediment into the overlying waters, the fluxes of silicate and chloride changed in both magnitude and direction in response to changing Trinity River flow conditions. Oxygen fluxes showed benthic production during both summer 1995 and winter 1996, while light-dark deployments showed production-consumption, respectively. Benthic inputs of nutrients were higher at either the middle or outer Trinity Bay regions, most likely due to a higher quality and quantity of the autochthonous organic matter deposited. This feature is consistent with and gives evidence for previously observed non-conservative mixing behaviors reported for nutrients in this region of Galveston Bay. Calculated turnover times, between 7 to 135 d for phosphate, 4 to 56 d for silicate, and 0.3 to 10 d for ammonium were significantly shorter than the average Trinity Bay water residence time of 1.5 yr for the period September 1995 through October 1996. During periods of decreased Trinity River flow and increased residence times, benthic inputs of ammonium and phosphate were 1 to 2 orders of magnitude greater than Trinity River inputs and were the dominant input source of these nutrients to Trinity Bay. The sediments, a sink for silicate when overlying water column concentrations of silicate were elevated, became a source of silicate to the overlying waters of Trinity Bay under reduced flow, high salinity conditions.

The effect of ion size on rate of dissociation: RRKM calculations on model large polypeptide ions

The larger an ion is, the less likely it is to decompose on mass spectrornetry time scales at given critical and internal energies. This is an obstacle to obtaining structural information on large molecules by mass spectrometry. We performed RRKM calculations on model ions with masses from 0.27 kDa to 102.4 kDa to explore what such calculations predict regarding this limitation. According to the calculations, it is impractical to add enough energy to fragment very large ions unless the decomposition has a low critical energy. It is suggested that ion-molecule reactions that are either very low in their critical energies or exothermic may be a feasible approach to fragmenting ionized macromolecules.

Size effects in ion-neutral complex-mediated alkane eliminations from ionized aliphatic ethers

The effects of the size of the ionic and neutral partners on ion-neutral complex-mediated alkane eliminations from ionized aliphatic ethers were determined by obtaining metastable decomposition spectra and photoionization ionization efficiency curves. Increasing the size of the ionic partner decreases the competitiveness of alkane elimination with alkyl loss. This is attributed to decreasing attraction between the partners with increasing distance between the neutral partner and the center of charge in the associated ion. Increasing the size of the neutral partner lowers the threshold for alkane elimination relative to that for simple dissociation when the first threshold is above ΔHf(products). This is attributed to increasing attraction between the partners with increasing polarizability of the radical in the complex. Adding a CH2 to the radical in a complex seems to increase the attraction between the partners by about 24 kJ mol−1.

Why are smaller fragments preferentially lost from radical cations at low energies and larger ones at high energies? : An experimental and theoretical study.

Abstract A long-standing mystery in gas phase ion chemistry is: why are smaller fragments preferentially lost at low energies and larger fragments at higher energies in competing α-cleavages? This is addressed here by studies of dissociations of ethanol, 2-propanol, 2-butanol, 2-methylpropane and 2-butanone radical cations. The onsets and energy dependencies of the reactions were obtained by photoionization mass spectrometry. Stationary point geometries, critical energies and vibrational frequencies were generated by B3LYP/6-31G(d) and B3LYP/6-311G(d,p) theory (DFT). Dependencies of reaction rates on internal energy were calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Results obtained establish the generality of a previous finding that loss of H is slower than competing losses of polyatomic fragments. This is attributable to substantial lowering of the frequencies of the vibrations that are converted to rotations and translations in the transition states for the latter reactions, but not in transition states for H losses. It is concluded that lower dissociation thresholds produce preferred losses of smaller alkyl fragments at low energies and that looser transition states favor losses of larger fragments at higher energies. DFT results and abundances of parallel alkane eliminations (two step processes also initiated by simple CC bond cleavage) indicate that cleavage of the CC bond to the larger alkyl fragment is most frequent even below the threshold for complete dissociation (energies at which ion-induced dipole alkyl complexes are formed), i.e., at all energies of bond cleavage, even if not reflected in simple cleavage abundances near threshold. Obtaining realistic rates by RRKM theory required increasing the lowest transition state vibrational frequencies (for modes that become rotations and translations) to well above those obtained by DFT for alkyl losses, further evidence that the rates of those reactions are controlled by minimum entropy transition states occurring earlier than the single imaginary frequency transition states found by DFT. In addition, comparison of DFT critical energies, photoionization critical energies and critical energies that give the best RRKM rates support relaxation of the ground states to lower energy species with long CC bonds following ionization. Preferred losses of larger alkanes at low energies, dominant losses of larger alkyl groups at high energies and formation of long CC bonds to the larger alkyl in the ground state ion all appear to have a common origin.

Rearrangements of the methyl butanoate ion and its isomers: time and energy dependence of reactions on a complicated potential surface

Abstract Isotopic labeling, translational energy releases and photoionization appearance energies are used to demonstrate that pathways whose energy requirements vary considerably are traversed by metastable methyl butanoate ions on their way to loss of methyl. RRKM calculations imply that kinetic shifts raise the energy required for metastable decomposition following passage through the lowest energy isomers. These shifts substantially increase translational energy releases. is found to be an important decomposition of ionized methyl-2-methyl propanoate.

Why CH3CH3+· formation competes with h· loss from CCCO C3H6O+· isomers

How formation of CH3CH3+· competes with H· loss from C3H6O+· isomers with the CCCO framework has been a puzzle of gas phase ion chemistry because the first reaction has a substantially higher threshold and a supposedly tighter transition state. These together should make CH3CH3+· formation much the slower of the two reactions at all internal energies. However, the rates of the two reactions become comparable at about 20 kJ mol−1 above the threshold for CH3CH3+· formation. It was recently shown that losses of atomic fragments increase in rate much more slowly with increasing internal energy than do the rates of competing dissociations to two polyatomic fragments. This occurs because fewer frequencies are substantially lowered in transition states for the former type of reaction than for the latter. The resulting lower transition state sums of states cause the rates of dissociations producing atoms as fragments to increase much more slowly than competing processes with increasing energy. Here we show that this is why CH3CH3+· formation competes with H· loss from CH3CH2CHO+·. These results further establish that the dependence on energy of the rate of a simple unimolecular dissociation is usually directly related to the number of rotational degrees of freedom in the products, a newly recognized factor in determining the dependence of unimolecular reaction rates on internal energy.

Preference for an ion-neutral complex-mediated pathway over a five-membered-ring H shift in the isomerization of CH3O+HCH2CH2. to CH3CH2CH2OH+. by Ab initio theory

Ab initio theory is used to explore whether the path from CH3OH+CH2CH2· (1) to CH3CH2CH2OH+· (5) goes by way of a conventional 1,4-H shift to form ·CH2OH+CH2CH3 (2), or via the ion-neutral complex-mediated H transfer [CH3OHCH2=CH2]+· (3) → [CH3CH2· CH2OH+] (4). Five levels of theory all place the highest energy point in the complex-mediated reaction 3 → 4 slightly below that for the 1,4-H shift 1 → 2, but both routes appear energetically feasible near the threshold for the dissociation of 1 to CH3CH2 + CH2=OH+. Thus, 1 may take both paths to 5. It is concluded that when both a conventional and a complex-mediated pathway seem plausible in a given system, the latter should be considered to be as likely as the former. Ab initio descriptions of other species involved in the isomerization of 1 to 2 also are presented.

CH4 Loss from (CH3)4N+ Revisited: How Does This High Energy Elimination Compete with •CH3 Loss?:

The rate of CH4 elimination from the tetramethylammonium ion increases faster than simple •CH3 loss with increasing internal energy at both high and low energies. Improved understanding of this highly unusual competition is sought by ab initio theory and RRKM calculations. Geometries and energies of stationary points and pathways as traced by intrinsic reaction coordinate calculations are given. A transition state at an energy much higher than those of both the reactant (414 kJ mol−1 above) and the products (361 kJ mol−1 above) was found for methane elimination from (CH3)4N+. More importantly, this transition state was also 14 kJ mol−1 above one found for methyl loss. However, according to results obtained and presented by RRKM theory, methane elimination through this transition state would be too slow to compete with methyl loss. This transition state may be for a concerted or a complex-mediated process. It is unlikely that CH4 is actually lost by a concerted elimination because only a small fraction of the reverse activation energy becomes translational energy, whereas concerted eliminations usually convert substantial fractions of their reverse activation energies into translational energy. Also, complex-mediated CH4 loss is unlikely because such eliminations are usually very quickly overwhelmed by simple dissociation of the partners just above threshold with increasing energy, opposite to the behavior of the system studied. Thus it is concluded that CH4 elimination from (CH3)4N+ occurs by loss of •CH3 followed by loss of H• at all energies, even though that is a higher energy process than methane elimination. Kinetic shifts and a reverse activation energy for •CH3 loss appear to raise the energy in the ions dissociating in the field free regions high enough to dissociate (CH3)4N+• + to (CH3)2N=CH2+• + •CH3 + H•.