International Journal of Mass Spectrometry最新文献

The effect of deprotonation on the structure and stability of the 21st proteinogenic amino acid selenocysteine, generated as bare deprotonated species by electrospray ionization, has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy over an extended frequency range (700-3600 cm⁻¹), encompassing both the fingerprint and X–H (X = C, N, O) stretching ranges. IRMPD spectra, interpreted by anharmonic DFT calculations, provide evidence of a thermally averaged ion population of two types of low-lying canonical conformers deprotonated at the selenol (Se–H) group and involved in different H-bonding motifs.

The broadened and diffuse band structures observed in the H-stretching range are well interpreted by Born-Oppenheimer molecular dynamics computations that provide a valuable description of the flexible backbone arrangements of Sec and of the proton sharing dynamics in the Se–HO and Se–HN moieties.

Other prototropic isomers, deprotonated at the carboxylic group or with a zwitterionic structure, should not be significantly populated, according to their higher free energy and calculated IR spectra inconsistent with experimental evidence.

Thermo Scientific™ Orbitrap™ analyzers represent a prominent class of high-resolution mass analyzer commonly used in life sciences, and for interrogation of complex samples. Injected ions, trapped within a quadro-logarithmic field, orbit a central electrode and oscillate up and down the axis. A new class of multi-reflection time-of-flight mass analyzer has been developed based on the Orbitrap field structure plus an additional series of periodic lenses wrapped around the central axis to constrain beam dispersion. The axial and angular velocity of the injected ions was balanced so that with each axial oscillation, the ions passed through the next lens in the series, to form a tightly folded 25-m long, 3-dimensional ion path, ending with ions striking a detector surface.

Performance was interrogated via experiment and simulation. 70k resolving power was observed within the relatively compact analyzer, albeit at cost to transmission. A larger design with an integrated extraction trap and greater flight energy is discussed.

Investigation of the metastable dissociations clarifies the characteristics of ionic states. Tandem mass spectrometry using an ion trap mass spectrometer, a time-of-flight mass spectrometer (TOF-MS), and the combination of those spectrometers were used for the analysis of metastable dissociation. However, investigation of the metastable dissociations of multiply charged ions is not straightforward because collision-induced charge transfer reactions to lose charges should be avoided. TOF-MS equipped with a reflectron (refTOF-MS) maintained under high vacuum condition is one of the suitable instruments. However, another difficulty arises due to the working principle of refTOF-MS: The product ions whose m/z is greater than that of multiply charged precursor ions can pass through a reflectron under the experimental conditions for detecting the product ions of a singly charged precursor ion. In this study, we report the ionization of decafluorobiphenyl (DFB) by femtosecond laser pulses and the determination of the product ions emerged from doubly charged precursor ions using a refTOF-MS. Detection of ions behind a reflectron by varying the retarding potential of a reflectron enables us to identify the m/z of product ions in two ways: measurement of the threshold retarding potential reflecting product ion; comparing the relative flight time of the product ions obtained by experiments and ion trajectory simulations. We have reported three and one metastable dissociation channels of DFB⁺ and DFB²⁺, respectively, by a conventional product ion analysis, that is the selection of a precursor ion and its product ions using an ion gate followed by the reflection and separation of them by a reflectron. In this study, we further identified the products that passed through a reflectron: charge transfer product of C²⁺; the product ion of C₄F₂²⁺ which is a secondary product of DFB ion; the product ion of DFB²⁺. The detection of the product ions that passed through a reflectron expanded the measurable m/z range of product ions emerged from doubly charged precursor ions.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), useful for the detection of macromolecules, is generally not suitable for recording the mass spectrum of small molecules, especially within the range of m/z 500, because of the appearance of matrix cluster peaks in the small molecular weight range. To circumvent this problem, a matrix-free variant of MALDI-MS, namely Label-assisted laser desorption/ionization mass spectrometry (LALDI-MS) has been developed in recent years where analytes get covalently attached to a laser absorbing label which induces the desorption/ionization process thus facilitating the recording the mass spectra and avoiding the problem of formation of cluster peaks. In general, the probes used for such detections are all pyrene-based scaffolds which despite being a good laser-active chromophore have both reactivity and solubility problems. Inspired by the right chromophoric nature, we have undertaken a systematic study to design and optimize cinnamaldehyde derivatives and have found that p-methoxy cinnamaldehyde (PMC) as a simpler, cost-effective, less hydrophobic laser-active label, which can detect a series of primary amines, including the neurotransmitters and amino acids. The label PMC can also discriminate between aliphatic and aromatic primary amines.

Modeling charge migration resulting from the coherent superposition of cation ground and excited states requires information about the potential energy surfaces of the relevant cation states. Since these states are often of the same electronic symmetry as the ground state of the cation, conventional single reference methods such as coupled cluster cannot be used for the excited states. The EOMCCSD-IP (equation of motion coupled cluster with single and double excitations and ionization) is a convenient and reliable “black-box” method that can be used for the ground and excited states of cations, yielding results of CCSD (coupled cluster with singles and double excitation) quality. Charge migration in haloacetylene cations arises from the superposition of the X and A states of HCCX⁺ (X = F, Cl, Br and I). The geometries, ionization potentials and vibrational frequencies have been calculated by CCSD/cc-pVTZ for neutral HCCX and the X state of HCCX⁺ and by EOM CCSD-IP/cc-pVTZ for the X and A states of HCCX⁺. The results agree very well with each other and with experiment. The very good agreement between CCSD and EOMCCSD-IP for the X states demonstrates that EOMCCSD-IP is a suitable method for calculating the structure and properties of ground and excited states for the HCCX cations.

The ability to observe intact proteins by native mass spectrometry allows measurements of size, oligomeric state, numbers and types of ligands and post translational modifications bound, among many other characteristics. These studies have the potential to, and in some cases are, advancing our understanding of the role of structure in protein biology and biochemistry. However, there are some long-unresolved questions about to what extent solution-like structures persist without solvent in the vacuum of the mass spectrometer. Strong evidence from multiple sources over the years has demonstrated that well-folded proteins maintain native-like states if care is taken during sample preparation, ionization, and transmission through the gas phase. For partially unfolded states, dynamic and disordered proteins, and other important landmarks along the protein folding/unfolding pathway, caution has been urged in the interpretation of the results of native ion mobility/mass spectrometric data. New gas-phase tools allow us to provide insight into these questions with in situ, in vacuo labeling reactions delivered through ion/ion chemistry. This Young Scientist Perspective demonstrates the robustness of these tools in describing native-like structure as well as possible deviations from native-like structure during native ion mobility/mass spectrometry. This Perspective illustrates some of the changes in structure produced by the removal of solvent and details some of the challenges and potential of the field.

Catalytic CO₂ hydrogenation is considered one of the most efficient strategies for CO₂ reduction, provided that we have efficient and selective catalysts. This paper explores a mass-spectrometric approach to rapidly compare the reactivities of 21 metal complexes (iron, cobalt, and nickel complexes with three bidentate N,N′-ligands and four bidentate P,P′-ligands) by energy-resolved MS/MS experiments. The experiments show relative hydricities of these complexes, singling out iron complexes with N,N′-ligands and cobalt complexes with P,P′-ligands as particularly reactive for hydride-donor reactions. Comparing these results with relative affinities of metal hydrides for CO₂ led to the conclusion that iron hydrides should be particularly suited for CO₂ hydrogenation reactions. The results, however, cannot account for the effects of the polar reaction environment in the condensed phase.

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical technique used to monitor volatile organic compounds in real-time. For quantitative analysis, compounds of interest are typically calibrated using gas standards, but PTR-MS is quantitative to uncalibrated compounds if the mass-dependent transmission is well defined. However, long-term measurements are challenging due to the drift in transmission over time. Performing frequent calibrations helps, but the methods are time-consuming and tedious, often leading to instruments being under-calibrated. Here we show the use of long-lived and globally monitored compounds in the atmosphere as a tool to constrain the transmission between calibrations. The major ion of trichlorofluromethane (CFC-11) and carbon tetrachloride (CCl₄) is found at the mass-to-charge ratio (m/z) 116.9, which we propose using to retrieve the transmission of a PTR-MS. We determined the pseudo-reaction rate constants of CFC-11 and CCl₄ to be 0.82 × 10⁻⁹ ± 0.05 × 10⁻⁹ cm³ s⁻¹ molecule⁻¹, and 1.65 × 10⁻⁹ ± 0.08 × 10⁻⁹ cm³ s⁻¹ molecule⁻¹, respectively. The method introduced here can improve data quality and accuracy, especially for long-term atmospheric measurements.

The magnets play a crucial role in sector mass spectrometers, directly influencing the resolution, sensitivity, and stability of these instruments. This study is based on the design requisites for high-resolution scanning magnet, alongside the optical layout schematics for mass spectrometer instrumentation. The analysis is conducted on the design parameters encompassing magnetic structure, field quality, and response dynamics. Utilizing Opera-3D simulation software, optimization designs are executed for the magnetic structure and performance, resulting in a magnet design exhibiting commendable attributes such as response time, magnetic field distortion characteristics, field range, linearity, and homogeneity. Following the completion of magnet fabrication and assembly, comprehensive evaluations of magnetic field performance and instrument testing are conducted. Results demonstrate good magnetic field linearity and minimal eddy current distortion, with a magnetic field homogeneity reaching up to ±0.05 % within the desired field region. The magnet exhibits versatility across a scan range spanning 7 to 238 u, with a scanning speed of 140 ms. Sensitivity and resolution tests, conducted using In-115, validated the magnet's compliance with the stringent design requirements of high-resolution mass spectrometry scanning magnets.

Measuring accurate energetics in mass spectrometry thermochemical and calorimetry experiments depends on detailed knowledge of the energetics of the ion populations. For example, in cases where blackbody infrared radiative dissociation (BIRD) kinetics are not in the rapid energy exchange (REX) limit, threshold dissociation energies obtained directly from experiment will be too low. When ions are not in the REX limit, the ion internal energy distributions can be modeled using a master equation (ME). The ME allows evaluation of ion internal energies over time with a set of rate equations that describe the transfer of energy from one energy state to another. Here, ME modeling that accounts for the radiative absorption and emission, and dissociation rate constants is performed to determine the energetics of two model systems, M²⁺(H₂O)_n with n = 24, 55, 96, 178 and H⁺(AlaGly)_n with n = 4, 8, 16, 32, activated by a blackbody field at temperatures between 120 and 200 K. The hydrated cluster and oligopeptide sizes are chosen such that respective ions have comparable number of internal degrees-of-freedom. The effects of blackbody temperature and inherent properties, such as frequencies and infrared (IR) intensities, molecule size, and dissociation parameters (threshold dissociation energy, E₀, and high-pressure pre-exponential factor, A^∞) on the resulting ion effective temperatures, steady-state energy distributions, and BIRD kinetics are explored. ME results show that at low blackbody temperatures (<∼140 K), the steady-state internal energy distributions of the ion populations resemble those of Boltzmann distributions at the blackbody temperature. At higher blackbody temperatures (>∼140 K), rapid dissociation causes the steady-state internal energy distributions to equilibrate to lower energies where absorption and emission are competitive with dissociation. This results in ion effective temperatures that deviate from and are “colder” than the blackbody temperatures. The temperature where this transition occurs depends on the competition among absorption, emission, and dissociation, and is controlled by the dissociation parameters, vibrational frequencies, and IR intensities, as illustrated for M²⁺(H₂O)_n and H⁺(AlaGly)_n. This work shows that, under certain conditions, the ion effective temperatures can deviate significantly from those of the blackbody field temperatures. ME modeling can be used to determine the energy content of ion complexes in mass spectrometry experiments to improve the accuracy of thermochemical and calorimetry measurements of weakly-bound clusters and for more confident assignments of conformations and structures in action spectroscopy.