Douglas P. Ridge

Evidence for iron, copper and zinc complexation as multinuclear sulphide clusters in oxic rivers

The availability and toxicity of trace metals in fresh water are known to be regulated by the complexation of free metal ions with dissolved organic matter. The potential role of inorganic sulphides in binding trace metals has been largely ignored because of the reduced persistence of sulphides in these oxic waters. However, nanomolar concentrations of copper and zinc sulphides have been observed in four rivers in Connecticut and Maryland. Here we report dissolved (< 0.2 µm particle diameter) sulphide concentrations ranging up to 600 nM, with more than 90% being complexed by copper, iron and zinc. These complexes account for up to 20% of the total dissolved Fe and Zn and 45% of the total dissolved Cu. Fourier transform mass spectrometry reveals that these complexes are not simple M(HS)+ protonated species but are higher-order unprotonated clusters (M3S3, M4S 6, M2S4), similar to those found in laboratory solutions and bio-inorganic molecules. These extended structures have high stability constants and are resistant to oxidation and dissociation, which may help control the toxicity of these and other less abundant, but more toxic, trace metals, such as silver, cadmium and mercury.

Signal suppression in electrospray ionization Fourier transform mass spectrometry of multi-component samples

The high resolution, mass range and sensitivity of Fourier transform mass spectrometry (FTMS) suggest that it could be a valuable tool for the quantitative analysis of biomolecules. To determine the applicability of electrospray ionization combined with FTMS to the quantitation of biomolecules in multi-component samples, mixtures of varying compositions and concentrations of cytochrome c, angiotensin II, insulin and chicken egg white lysozyme were examined. The instrument used has an electrospray source with a hexapole trap to accumulate ions for injection into an ion cyclotron resonance mass analyzer. Linear responses for single component samples of angiotensin II and insulin were in the range 0.031-3 microM and those of both cytochrome c and lysozyme were between 0.031 and 1 microM. In examining various mixtures of the proteins with angiotensin II, it was found that the presence of the large molecules suppresses the signal of the smaller molecules. This is suggested to be a result of ion-ion interactions producing selective ion loss from either the hexapole trap or the ion cyclotron resonance mass analyzer trap. More massive, more highly charged ions can collisionally transfer large amounts of translational energy to smaller, less highly charged ions, ejecting the smaller ions from the trap. Mass discrimination effects resulting from the trapping voltage were also examined. It was found that relative signal intensities of ions of different masses depend on trapping voltage for externally produced ions. The effect is most significant for spectra including masses that differ by 30% or more. This suggests that for quantitation all samples and standards be run at a constant trapping potential.

Harmonic signal enhancement in ion cyclotron resonance mass spectrometry using multiple electrode detection

Abstract Ion cyclotron resonance signals at higher harmonics of the cyclotron frequency are discussed. If dissipation of the charge in an orbiting charge packet depends only on time, the linewidths of the signals at all harmonics are the same. The spacing between mass lines increases with harmonic order, therefore resolution increases linearly with harmonic order. Selection rules are developed for a class of detection schemes that will detect selected harmonics. The detection electrodes for this class of detectors consist of M (where M is an integer) identical electrodes arranged with M-fold symmetry about the axis of the coherent cyclotron motion of the observed ions. The sum of the signals from all the electrodes contains harmonics of order Mk (k is an integer). The difference between the sum of the signals from every other electrode and the sum of the signals from the remaining electrodes contains harmonics of order M(2k − 1)/2 (in this case M must be even). This suggests that it is possible to detect harmonics of arbitrary order in the absence of harmonic signals of lower order. This could be useful in improving resolution in ion cyclotron resonance mass spectroscopy without increasing data acquisition time or magnetic field strength.

Double metal-to-metal bonds in metal carbonyl clusters formed in the gas-phase negative ion chemistry of iron pentacarbonyl

Fe(CO) 4 − forme Fe 2 (CO) 8 − dans une reaction en phase gazeuse avec Fe(CO) 5 . Constante de vitesse

Fragmentation Energetics of Clusters Relevant to Atmospheric New Particle Formation

The exact mechanisms by which small clusters form and grow in the atmosphere are poorly understood, but this process may significantly impact cloud condensation nuclei number concentrations and global climate. Sulfuric acid is the key chemical component to new particle formation (NPF), but basic species such as ammonia are also important. Few laboratory experiments address the kinetics or thermodynamics of acid and base incorporation into small clusters. This work utilizes a Fourier transform ion cyclotron resonance mass spectrometer equipped with surface-induced dissociation to investigate time- and collision-energy-resolved fragmentation of positively charged ammonium bisulfate clusters. Critical energies for dissociation are obtained from Rice-Ramsperger-Kassel-Marcus/quasi-equilibrium theory modeling of the experimental data and are compared to quantum chemical calculations of the thermodynamics of cluster dissociation. Fragmentation of ammonium bisulfate clusters occurs by two pathways: (1) a two-step pathway whereby the cluster sequentially loses ammonia followed by sulfuric acid and (2) a one-step pathway whereby the cluster loses an ammonium bisulfate molecule. Experimental critical energies for loss of an ammonia molecule and loss of an ammonium bisulfate molecule are higher than the thermodynamic values. If cluster growth is considered the reverse of cluster fragmentation, these results require the presence of an activation barrier to describe the incorporation of ammonia into small acidic clusters and suggest that kinetically (i.e., diffusion) limited growth should not be assumed. An important corollary is that models of atmospheric NPF should be revised to consider activation barriers to individual chemical steps along the growth pathway.

Surface induced dissociation of chromium hexacarbonyl on fluorinated alkanethiolate surface in ion cyclotron resonance mass spectrometer: studies of energetics of the process using recursive internal energy distribution search method

Abstract Energy transfer in ion-surface collisional activation is characterized for 0–30 eV collisions of chromium hexacarbonyl molecular cations with a monolayer of fluorinated alkanethiolate self-assembled onto a solid gold surface. This surface was mounted on the back trapping plate of the Infinity® cell of a Bruker BioApex 7T ion cyclotron resonance mass spectrometer on the B-field axis orthogonal to the ion beam direction. Internal energy deposition was deduced from fragmentation spectra using a recursive internal energy distribution search method. The efficiency of energy transfer into the ion slowly increases with incident ion energy to a maximum value of 20% at about 23 eV collision energy. Approximate kinetic energy distributions of the fragments were measured by deducing the dependence of ion abundance on trapping potential. From the kinetic energy dependence on mass we infer that rapid decomposition of the molecular cation occurs after it recoils from the surface. Knowledge of both internal and kinetic energy distributions of collisionally activated ions enabled us to deduce the energy deposited into the self-assembled monolayer as a function of collision energy.

Molecular mass determination of saturated hydrocarbons: reactivity of η5-cyclopentadienylcobalt ion (CpCo•+) and linear alkanes up to C-30

The present study demonstrates the feasibility of the η5-cyclopentadienylcobalt ion (CpCo·+) as a suitable cationization reagent for saturated hydrocarbon analysis by mass spectrometry. Ion/molecule reactions of CpCo·+ and three medium chain-length n-alkanes were examined using Fourier-transform ion cyclotron resonance mass spectrometry. Second-order rate constants and reaction efficiencies were determined for the reactions studied. Loss of two hydrogen molecules from the CpCo-alkane ion complex was found to dominate all reactions (≥80%). Furthermore, this dehydrogenation reaction efficiency increases with increasing chain length. These preliminary results suggest that the CpCo·+ ion may be a promising cationization reagent of longer chain saturated hydrocarbons and polyolefins.

Apparent proton affinities of highly charged peptide ions

The apparent proton affinities (PA) of various charge states of three highly basic peptides [(KAP)10, (KAP)8, (KAA)8] were measured by the “bracketing” method. For all three peptides the apparent PA decreases as the charge state increases and the magnitude of the decrease is consistent with an increase in coulombic repulsion in the highly protonated species. Based on a simple electrostatic model, theoretical PAs were predicted for each charge state and the values for (KAP)10 and (KAP)8 were within 10 kcal/mol of the experimental values. The maximum charge state of these peptides was observed in all cases even when the most volatile solvent was sufficiently basic to deprotonate that charge state in the gas phase. In solution (KAP)8 exhibits a random coil secondary structure while (KAA)8 exhibits an α-helix structure. Comparison of measured and calculated apparent PAs suggests that (KAP)8 retains its solution random coil structure in the gas phase and (KAA)8 retains the solution compact α-helix structure in the lower charge states but opens up to a β structure in the gas phase to minimize electrostatic repulsions in higher charge states.

Double resonance with a frequency-scanned ion cyclotron resonance detector applied to the study of metal clusters

Abstract The replacement of a fixed-frequency amplitude-modulated marginal oscillator detector with a frequency-scanned null detector facilitates the use of mul

Activation barriers in the growth of molecular clusters derived from sulfuric acid and ammonia

Unraveling the chemical mechanism of atmospheric new particle formation (NPF) has important implications for the broader understanding of the role of aerosols in global climate. We present computational results of the transition states and activation barriers for growth of atmospherically relevant positively charged molecular clusters containing ammonia and sulfuric acid. Sulfuric acid uptake onto the investigated clusters has a small activation free-energy barrier, consistent with nearly collision-limited uptake. Ammonia uptake requires significant reorganization of ions in the preexisting cluster, which yields an activation barrier on the order of 29-53 kJ/mol for the investigated clusters. For this reason, ammonia uptake onto positively charged clusters may be too slow for cluster growth to proceed by the currently accepted mechanism of stepwise addition of sulfuric acid followed by ammonia. The results presented here may have important implications for modeling atmospheric NPF and nanoparticle growth, which typically does not consider an activation barrier along the growth pathway and usually assumes collision-limited molecular uptake.