International Journal of Mass Spectrometry最新文献

Single-frequency ion parking, a useful technique in electrospray mass spectrometry (ESI-MS), involves gas-phase charge-reduction ion/ion reactions in an electrodynamic ion trap in conjunction with the application of a supplementary oscillatory voltage to selectively inhibit the reaction rate of an ion of interest. The ion parking process provides a means for limiting the extent of charge reduction in a controlled fashion and allows for ions distributed over a range of charge states to be concentrated into fewer charge states (a single charge state under optimal conditions). As charge reduction inherently leads to an increase in the mass-to-charge (m/z) ratio of the ions, it is important that the means for storing and analyzing ions be able to accommodate ions of high m/z ratios. The so-called ‘digital ion trap’ (DIT), which uses a digital waveform as the trapping RF, has been demonstrated to be well-suited for the analysis of high m/z ions by taking advantage of its ability to manipulate the waveform frequency. In this study, the feasibility of ion parking in a 3D quadrupole ion trap operated as a DIT using a slow-amplitude single-frequency sine-wave for selective inhibition of an ion/ion reaction is demonstrated. A recently described model that describes ion parking has been adjusted for the DIT case and is used to interpret experimental data for proteins ranging in mass from 8600 Da to 467,000 Da.

Molecular systems including clusters often manifest multiple photodissociation pathways upon absorption of photon energy enough to break down chemical bonds. This certainly raises fundamental questions to chemists: which pathway will be most favored and how can we predict it with precision? To address these issues, we had previously introduced a rather crude but highly simplified and straightforward calculation method, Rice-Ramsperger-Kassel-Marcus (RRKM) calculation method complemented by the concept of extreme loose transition state (eLTS). This approach has proven effective in estimating branching ratios in photodissociation of C₆H₄BrCl⁺. Here, we have extended this method to interpret results in IR photodissociation of (C₆H₅NH₂)⁺-H₂O–H₂¹⁸O for further evaluation and refinement of this method. We compared branching ratios derived from RRKM-eLTS with those obtained via phase-space theory (PST) to find that our calculation results through RRKM-eLTS were quite in line with the experimental data while those from PST calculation fluctuated significantly depending on the calculation levels and basis sets. This indicates that RRKM-eLTS model not only aligns well with experimental observations giving insights into the relevant rate constants but also intuitively explains these results. We, hereby, suggest that RRKM-eLTS model is a robust and user-friendly method for computing branching ratios, with possible applications to other molecular systems.

Hydration energies of CaOH⁺(H₂O)_x, x = 1–6, were obtained using threshold collision-induced dissociation (TCID) with xenon (Xe) as conducted with a guided ion beam tandem mass spectrometer (GIBMS). The primary reaction pathway observed for all complexes is the loss of one water ligand, followed by the loss of additional water molecules at higher collision energies for x > 1. The kinetic-energy-dependent cross sections for dissociation of CaOH⁺(H₂O)_x complexes were modeled to obtain 0 K binding energies after accounting for lifetime effects, energy distributions, and pressure effects. Except for x = 5, experimental threshold energies measured through TCID agree well with theoretical hydration energies determined using B3LYP/6-311+G(d,p) structure geometries followed by single point energies calculated at B3LYP, B3P86, M06, and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set. B3LYP-GD3BJ and ωB97XD calculations were also used, with the former yielding results having the best agreement with the threshold energies extracted from the analysis of the TCID cross sections.

Metalated gas-phase complexes, M²⁺(HisAlaAla), M²⁺(AlaHisAla), and M²⁺(AlaAlaHis), where M = Zn and Cd, were examined using infrared multiple photon dissociation (IRMPD) spectroscopy with light from a free-electron laser (FEL). These complexes were chosen because they provide model systems for metal binding to proteins. Complementary simulated annealing calculations were performed to determine energetically low-lying conformers and isomers of these structures. Quantum chemical calculations were used to optimize the structures at the B3LYP level of theory using 6-311+G(d,p) and def2-TZVP basis sets for zinc and cadmium complexes, respectively. IRMPD and calculated linear absorption spectra were compared to evaluate which structures are present. Relative energies of the various species were evaluated using single-point energy calculations for low-lying structures at the B3LYP, B3P86, and MP2(full) levels using 6-311+G(2d,2p) and def2-TZVPP basis sets. For species with histidine at a terminal position (AAH or HAA), the conformations that best reproduce the IRMPD spectra are charge-solvated (CS) conformers, where the metal dication binds to the amine and carbonyl groups of the peptide backbone and to the nitrogen of the histidine side chain, along with contributions from an iminol structure for AAH. The species with the histidine in the center position (AHA) adopt an iminol structure, where the metal dication binds to the backbone iminol nitrogens, the α-amine, π-imine, and the carbonyl of the C-terminus.

Laser desorption mass spectrometry was employed to study rubrene using three different sample preparation methods. Pressed-pellet and films drop-cast from solution were investigated with a laser-desorption time-of-flight spectrometer. Jet-cooled rubrene cations were produced in a supersonic molecular beam by laser desorption from a film-coated metal rod and detected with time-of-flight mass spectrometry. The films for this process were produced by vacuum sublimation of powder samples. The mass spectra from each of these samples contained the parent molecular ion and fragments resulting from phenyl ring elimination - a pattern similar to that produced by electron impact ionization. The amount of fragmentation varied with sample preparation and desorption laser wavelength. The rubrene cation was mass selected and studied with UV laser photodissociation at 355 nm. The resulting fragmentation mass spectrum indicated the loss of one or two phenyl groups, but no more than this. Computational studies of the ion energetics were used to investigate the stable fragment ion structures and understand the energetics of the dissociation process.

In this paper we present basic observations regarding the formation and effect of various polyatomic metallic interferences (combinations of Pb, Al, Si, Mo, W, Fe, Sn, Ti, Ba, and Pt) on uranium isotopic ratios measured via a new Multi Collector – Inductively Coupled Plasma – Mass Spectrometer (MC-ICP-MS) platform: The ThermoFisher Scientific™ Neoma® (without the MS/MS® collision cell option). Our approach is to dope a uranium isotopic standard (IRMM-2020) with various quantities of the metallic elements, all of which have been previously identified as posing isobaric interferences during uranium isotope measurement, and then perform analyses in solution mode and by laser ablation (LA) MC-ICP-MS. As expected, there is considerable variability between the different elementally doped solution and the degree of perturbation exhibited by the uranium isotope standard. However, Pt appears to induce the most drastic shift for ²³⁴U/²³⁸U, ²³⁵U/²³⁸U, and ²³⁶U/²³⁸U. One unexpected result is that the presence of Pb, whether alone or in combination with Al and Si, appears to result in a reduction of the expected uranium signal. This effect was observed (and re-confirmed using a different cup configuration) on the Neoma® MC-ICP-MS during wet plasma analysis, but was not observed during dry plasma conditions (e.g. laser ablation sampling of dried IRMM-2020 containing Pb). Furthermore, the phenomenon was also not observed during dry plasma analysis on a NeptunePlus®. Further experiments will be necessary to fully understand the origin of this signal attenuation, and the results of our study highlight the need for continued investigation of the polyatomic interference issue as new MC-ICP-MS platforms are developed since our results clearly demonstrate that they can drastically skew the observed isotope ratios in sometimes unexpected directions. Lastly, the use of N₂ during LA sampling appears to slightly increase the magnitude of the interferences compared to when only Ar is utilized as the carrier gas. This observation further highlights the importance of plasma conditions on interference formation.

Ion-molecule reactions in the gas phase are significantly influenced by hydration. Here we investigate the impact of hydration on the reactivity of two atmospherically relevant anions, O^•− and OH⁻, with oxygen and carbon dioxide. A mixture of hydrated anions O^•−(H₂O)_n and OH⁻(H₂O)_n, n < 60, is prepared in a laser vaporization source and reacted in a temperature-controlled ICR cell with O₂ and CO₂. While OH⁻(H₂O)_n does not react with O₂, formation of hydrated ozonide O₃^•−(H₂O)_m is observed in the reaction of O^•−(H₂O)_n with O₂ for all studied cluster sizes. The reaction slows down with increasing cluster size, which compromises nanocalorimetry. Quantum chemical calculations show that ozonide formation is exothermic with ΔE₀ = −52 kJ mol⁻¹ for n ≈ 7–11, while O₂ is very weakly bound to OH⁻(H₂O)_n. Observation of such a non-covalent (O₂)OH⁻(H₂O)_m complex in a mass spectrometer might be possible at significantly lower temperatures than accessible in our experiment. For CO₂, we observe reactions only in a narrow size regime, up to n ≈ 8 for O^•−(H₂O)_n and n ≈ 6 for OH⁻(H₂O)_n, to form CO₃^•−(H₂O)_m and HCO₃⁻(H₂O)_m, respectively. Calculations render both reactions substantially exothermic also for larger clusters, ruling out thermochemistry as an explanation for the size-dependent reactivity.

The ²³⁵U/²³⁸U isotope ratios of uranium standard reference materials and an environmental uranium sample recovered from seawater were measured by thermal ionization mass spectrometry using the single-collector peak-jumping and static multi-collection modes. All the ²³⁵U/²³⁸U isotope ratios measured in the peak-jumping mode were systematically lower than the values obtained in the static measurements. The lowering of the measured isotope ratios became more remarkable with the decrease in the ²³⁵U isotopic abundance, resulting in the maximum 2.2‰-lower value comparable to isotope mass fractionation. The lowering of isotope ratios is not due to the apparent loss of ²³⁵U ion currents caused by the time constant of the Faraday amplifier used but attributable to the actual decrease of ion currents by ion-beam changes in the mass switching step of peak-jumping mode. Furthermore, the proper collection of ion beams to the Faraday cup was achieved by allowing sufficient delay time for eliminating the effect of ion-beam changes, which was significantly longer than the time needed for the response of the amplifiers.

Protein-protected metal nanoclusters (MNCs) represent a new class of highly photoluminescent nanomaterials that have wide applications. Suitable reaction conditions combining protein and metal precursors can produce a vast range of different NC sizes (i.e. different number of metal atoms). The average number of metal atoms per protein can be determined by mass spectrometry (MS). MS coupled with matrix-assisted laser desorption ionization (MALDI) presents a number of advantages such as detection with high sensitivity of nanoclusters with high molecular weights. Although many protein-protected MNCs have been characterized by MALDI-MS, a large dispersion in the number of metal atoms has been reported mainly due to sample preparation. In this work, we optimized the protocols for negative and positive ion detection mode as a general MALDI-MS sample preparation method for protein-protected MNCs (bovine serum albumin and lysozyme and with gold and silver). Negative and positive ion mode detection was compared, showing that negative ion mode detection in MALDI-MS can also be used with acidic matrices. Obvious matrix effects on ion signals and peak positions by MALDI-MS were observed. The average metal numbers of MNCs embedded in proteins are different depending on the MALDI matrix. The matrix effects give a warning for more serious consideration on MALDI-MS measurement and spectra analysis of MNCs.

In present study of electron driven inelastic processes, we report calculated probabilities of inelastic events (including total inelastic and ionizations) occurrence by quantifying the cross sections, $Q_{inel}$ and $Q_{ion}$ for beryllium (Be) and beryllium hydrides viz. BeH, BeH₂, Be₂H₂ and Be₂H₄ from the molecular ionization threshold to 5 keV. The Spherical Complex Optical Potential (SCOP) to compute $Q_{inel}$ has been employed and $Q_{ion}$ is extracted using Complex Scattering Potential –ionization contribution (CSP-ic) approximation. Owing to the toxic character of beryllium compounds we do not find any experimental work for these molecules. In paucity of the experimental data, present work is important and will fill the void of the data needed for particular in plasma physics, aero science, astrophysics and chemical analysis. This study is the maiden report of $Q_{inel}$ for all of these important targets.