Energy-resolved Mass Spectrometry Research Articles

Effects of the net charge on abundance and stability of supramolecular surfactant aggregates in gas phase

Self-assembling of amphiphilic molecules under electrospray ionization (ESI) conditions is characterized by quite unexpected phenomenology. The noticeable differences with respect to the condensed phase are attributable to the absence of the surfactant-solvent interactions, the presence of net charge in the aggregates, and the strong deviation from equilibrium conditions. Aiming to investigate the effects of the net charge on abundance and stability of supramolecular surfactant aggregates, positively and negatively charged aggregates of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and sodium methane sulfonate (MetS), butane sulfonate (ButS) and octane sulfonate (OctS) have been studied by ESI mass spectrometry, energy resolved mass spectrometry and density functional theory calculations. The negatively charged aggregates are found to be less stable than their positive counterparts. The results are consistent with a self-assembling pattern dominated by electrostatic interactions involving the counterions and head groups of the investigated amphiphilic compounds while the alkyl chains point outwards, protecting the aggregates from unlimited growth processes.

Plasma Parameters in a Pre-Ionized HiPIMS Discharge Operating at Low Pressure

A high power impulse magnetron sputtering (HiPIMS) discharge assisted by a dc pre-ionizer operating at low pressure has been investigated using the Langmuir probe technique as well as energy-resolved mass spectrometry. It was found that the background plasma with densities of about 1 × 109 cm-3 provided by the dc pre-ionizer can reduce the ignition delay time of the HiPIMS plasma from more than 50 μs to less than 5 μs at a working pressure of about 0.1 Pa. Furthermore, the technique of super-imposing HiPIMS with the dc discharge can be readily employed with a low self-sputtering yield target (Ti) at a working pressure of 0.08 Pa, with a pulse width and a repetition frequency of 100 μs and 100 Hz, respectively. This leads to a power density of 450 Won 2 at the target. At these operating conditions, the probe measurements showed a high-density plasma 6 cm in front of the target. The maximum electron density was found to be 8 × 1011 cm-3, with an effective electron temperature of about 3-4 eV. A study of the ion mass distribution revealed that the intensity of Ti2+ ions is larger than the peak of Ti+ in low-pressure operation. Moreover, the number of metal ions in the high-energy tail of the ion energy distribution function increased with a decreasing pressure from 0.5 to 0.08 Pa.

Triuret as a potential hypokalemic agent: Structure characterization of triuret and triuret–alkali metal adducts by mass spectrometric techniques

Triuret (also known as carbonyldiurea, dicarbamylurea, or 2,4-diimidotricarbonic diamide) is a byproduct of purine degradation in living organisms. An abundant triuret precursor is uric acid, whose level is altered in multiple metabolic pathologies. Triuret can be generated via urate oxidation by peroxynitrite, the latter being produced by the reaction of nitric oxide radical with superoxide radical anion. From this standpoint, an excess production of superoxide radical anions could indirectly favor triuret formation; however very little is known about the potential in vivo roles of this metabolite. Triuret’s structure is suggestive of its ability to adopt various conformations and act as a flexible ligand for metal ions. In the current study, HPLC–MS/MS, energy-resolved mass spectrometry, selected ion monitoring, collision-induced dissociation, IRMPD spectroscopy, Fourier transform-ion cyclotron resonance mass spectrometry and computational methods were employed to characterize the structure of triuret and its metal complexes, to determine the triuret–alkali metal binding motif, and to evaluate triuret affinity toward alkali metal ions, as well as its affinity for Na + and K + relative to other organic ligands. The most favored binding motif was determined to be a bidentate chelation of triuret with the alkali metal cation involving two carbonyl oxygens. Using the complexation selectivity method, it was observed that in solution triuret has an increased affinity for potassium ions, compared to sodium and other alkali metal ions. We propose that triuret may act as a potential hypokalemic agent under pathophysiological conditions conducive to its excessive formation and thus contribute to electrolyte disorders. The collision- or photo-induced fragmentation channels of deprotonated and protonated triuret, as well as its alkali metal adducts, are likely to mimic the triuret degradation pathways in vivo.

Supramolecular Aggregates in Vacuum: Positively Mono-Charged Sodium Alkanesulfonate Clusters

The formation and structural features of positively monocharged aggregates of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium methane--(MetS), butane--(ButS) and octane--(OctS) sulfonate molecules in gas phase have been investigated by electrospray ionization mass spectrometry, energy resolved mass spectrometry and density functional theory (DFT) calculations. The experimental results show that the center-of-mass collision energy required to dissociate 50% of these monocharged aggregates scantly depends on the length of the alkyl chain as well as on the aggregation number. This, together with the large predominance of monocharged species in the mass spectra, was rationalized in terms of an aggregation pattern mainly driven by the counter ions and head groups electrostatic interactions while minor effects were attributed to the steric hindrance caused by the size of the surfactant head group and alkyl chain. DFT calculations show that the most favoured structural arrangement of these aggregates is always characterized by an internal polar core constituted by the sodium counter ions and surfactant head groups surrounded by an external layer composed by the surfactant alkyl chains.

Differentiation of lisinopril and its RSS diastereomer by liquid chromatography combined with collision‐induced dissociation mass spectrometry

A simple and sensitive liquid chromatography tandem multiple-stage mass spectrometry (HPLC/MS/MS) method suitable for bulk lisinopril analysis was developed, by which lisinopril and its RSS isomer were separated and differentiated. In the collision-induced dissociation (CID) mass spectra of the [M + H](+) ions, the abundance of the fragment ion of m/z 246 for lisinopril was about two times higher than the ion of m/z 245; however, the former fragment ion was noted to be a little lower than the latter for RSS isomer at all collision energies. In the CID mass spectra of the [M + Li](+) ion, the abundance of the rearrangement ion of m/z 315 for the RSS isomer was about three times higher than that for lisinopril. Furthermore, the difference was supported by the results of energy-resolved mass spectrometry (ERMS) in the test range of collision energies. Similar differences were also observed between the CID mass spectra of lisinopril and RSS isomer methylester, which indicated that the RSS isomer could be rapidly characterized by the CID mass spectra of both the protonated and lithium adduct ion. Elemental compositions of all the ions were confirmed by Fourier Transform ion cyclotron resonance ESI mass spectrometry (FT-ICR-ESI/MS). In addition, theoretical computations were carried out to support the experimental results.

Site-Preferential Dissociation of Peptides with Active Chemical Modification for Improving Fragment Ion Detection

Multiple reaction monitoring tandem mass spectrometry becomes an important strategy for measuring protein targets in complex biomatrixes. Active chemical modification of peptides like phenylthiocarbamoylation has unique potential for improving the measurement. This potential is enabled by active participation of a modifying group in site-preferential dissociation of modified peptides, which produces certain fragment ions at very high yields and in a sequence-independent manner. In this work, a novel combination of energy-resolved mass spectrometry with substituent effect investigation is used to analyze important factors that control the specificity of the site-preferential dissociation of phenylthiocarbamoyl peptides. On the basis of the linear correlation between collision energy and the Hammett constant as well as computational studies, it is found that the initial enhanced capture of a mobile proton and the subsequent, site-directed intramolecular proton transfer are important to the high yields (approximately 70-90%) for producing two types of fragment ions of phenylthiocarbamoyl peptides: the modified b(1) ion and the complementary y(n-1) ion. This understanding will help the design of new modification reagents. When integrated with the throughput and the signal-enhancing potential of peptide modification, active chemical modification of peptides will significantly advance mass spectrometry-based, targeted proteome analysis.

Analysis of a series of isomeric oligosaccharides by energy‐resolved mass spectrometry: a challenge on homobranched trisaccharides

Glycans exist as part of glycoproteins and glycolipids, which are involved in a variety of biological functions. The analysis of glycan structures, particularly that of structural isomers, is fundamentally important since isomeric glycans often show distinct functions; however, a method for their structural elucidation has not yet been established. Anomeric configurations, linkage positions and branching are the major factors in glycans and their alteration results in a large diversity of glycan structures. The analysis of vicinally substituted oligosaccharides is extremely difficult because the product ions formed in tandem mass spectrometry (MS/MS) often have the same m/z values. In our endeavor to address the issue, we analyzed a series of homo-substituted trisaccharides consisting only of glucose by collision-induced dissociation (CID), especially energy-resolved mass spectrometry (ERMS). It was found that these structurally related glycans could be distinguished by taking advantage of differences in their activation energies in ERMS.

Modeling the flux of high energy negative ions during reactive magnetron sputtering

The negative ion flux during reactive sputtering from planar and rotating cylindrical magnetrons has been studied. Energy resolved mass spectrometry was used to measure the energy and mass distribution of the negative ions. Also the angular distribution of the high energy ions was characterized for planar as well as for rotating cylindrical magnetrons. Besides these measurements, a binary collision Monte Carlo simulation code, SiMTRA, was adapted in order to simulate the energy, mass, and angular distribution of the high energy negative ions. All simulated distributions, for both planar and rotating cylindrical magnetrons, were in excellent correspondence with the experimental observations. Also a model for the amount of high energy negative O− ions was proposed. Indeed, the logarithm of the amount of high energy negative O− ions is shown to be related to the secondary electron emission yield of the oxide target, and these two parameters are known to be related to the work function. The SiMTRA simulations, in combination with knowledge of the work function or secondary electron emission yield of the target, allow modeling the flux of high energy negative ions during reactive magnetron sputtering.

Time evolution of ion energies in HIPIMS of chromium plasma discharge

High power impulse magnetron sputtering (HIPIMS) discharges produce metal ions with energies up to 100 eV and the degree of metal ionization can be up to 90%. The ion energy distribution function (IEDF) of ions in HIPIMS of a chromium target in an argon atmosphere was measured using plasma sampling energy resolved mass spectrometry. The time resolved measurements show that in the pulse-on phase the IEDF of Cr1+ comprises a single high energy group of ions, whereas at the end of the pulse, ions are thermalized due to collisions with Ar and the IEDF comprises both a low energy group of ions and a high energy group of ions. Finally, after 320 µs from the start of the pulse only a main low energy peak is present. The IEDF of Ar1+ comprises an intense low energy peak and a high energy shoulder, the latter appearing simultaneously with the high energy Cr ions and probably created due to collisions with the hot metal ions. After the pulse only a low energy group of ions of Ar1+ IEDF was detected. The temporal evolution of the total ion density extracted from Langmuir probe measurements is linked with metal ion-to-gas ion ratios giving an insight into the time evolution of deposition fluxes in the plasma. The influence of the time evolution of IEDF and plasma composition on the thin film growth is discussed.

Spatially ordered surfactant assemblies in the gas phase: negatively charged bis(2‐ethylhexyl)sulfosuccinate‐alkaline metal ion aggregates

The formation and structural features of negatively charged aggregates of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) surfactant molecules in the gas phase have been investigated by electrospray ionization mass spectrometry (ESI-MS) and density functional theory calculations. Mainly driven by the interactions of alkali metal ions both with the oxygen atoms of the sulfonate group and with the succinate moiety of the AOT- anion, spatially ordered supramolecular assemblies, characterized by an internal core composed of surfactant counterions and hydrophilic head groups surrounded by the surfactant alkyl chains pointing outwards, are formed. Calculations have shown that surfactant self-organization in the gas phase is energetically favoured, the energy of formation of negatively charged aggregates from isolated AOTNa and AOT- being linearly related to the aggregation number. Information on the chelating properties of AOTNa towards clusters of inorganic salts was achieved by infusion of solutions at various AOTNa/metal salt (NaCl, NaBr, NaI, LiI, KCl, CsI, RbI) ratios in the ESI source of a mass spectrometer. A wide variety of negatively charged AOT-metal aggregates, some of them also incorporating halide (X-) ions, has been observed. Calculations have shown that the capture of a halide anion to give the AOTMX- species is favoured but the energetics of the process depends on the alkali metal and halide types. The use of energy-resolved mass spectrometry has allowed us to evaluate the stability of different complexes and to evaluate the role played by the metal ion. Overall, the present investigation supports the idea that, in the gas phase, mainly driven by electrostatic interactions, surfactant molecules are present as molecular aggregates characterized by a reverse micelle-like organization with an internal core formed by the surfactant counterions and head groups surrounded by the surfactant alkyl chains. These peculiar aggregates are able to incorporate ionic clusters in their hydrophilic core.

Simulation of ion energy distributions in Ar/CH4 rf discharges with ion extraction

Capacitive radio-frequency discharges in argon–methane gas mixture are used for hydrocarbon deposition on substrates. Ion energy distributions (IEDs) are important for the physics in such discharges. One diagnostics for obtaining IEDs is the energy resolved mass spectrometry. A one-dimensional particle-in-cell code has been used to give better insight into the principle of operation of the ion extraction to interpret more exactly the measured IEDs at the powered electrode. The effects of the ion transfer optics and the modulation effects of the potential between the aperture and ion extractor lens have been studied. A better match between simulation and experiment is achieved by introducing an effective drift length for the ion optics. However, problems remain for reactive species like hydrocarbons indicating more complex plasma reactions within the mass spectrometer.

Observations on the detection of b‐ and y‐type ions in the collisionally activated decomposition spectra of protonated peptides

Tandem mass spectrometric data from peptides are routinely used in an unsupervised manner to infer product ion sequence and hence the identity of their parent protein. However, significant variability in relative signal intensity of product ions within peptide tandem mass spectra is commonly observed. Furthermore, instrument-specific patterns of fragmentation are observed, even where a common mechanism of ion heating is responsible for generation of the product ions. This information is currently not fully exploited within database searching strategies; this motivated the present study to examine a large dataset of tandem mass spectra derived from multiple instrumental platforms. Here, we report marked global differences in the product ion spectra of protonated tryptic peptides generated from two of the most common proteomic platforms, namely tandem quadrupole-time-of-flight and quadrupole ion trap instruments. Specifically, quadrupole-time-of-flight tandem mass spectra show a significant under-representation of N-terminal b-type fragments in comparison to quadrupole ion trap product ion spectra. Energy-resolved mass spectrometry experiments conducted upon test tryptic peptides clarify this disparity; b-type ions are significantly less stable than their y-type N-terminal counterparts, which contain strongly basic residues. Secondary fragmentation processes which occur within the tandem quadrupole-time-of-flight device account for the observed differences, whereas this secondary product ion generation does not occur to a significant extent from resonant excitation performed within the quadrupole ion trap. We suggest that incorporation of this stability information in database searching strategies has the potential to significantly improve the veracity of peptide ion identifications as made by conventional database searching strategies.

Effect of pulse frequency on the ion fluxes during pulsed dc magnetron sputtering

The ion fluxes and energies which impinge on the substrate during the deposition of chromium nitride by asymmetric bipolar pulsed dc reactive magnetron sputtering have been analyzed using energy resolved mass spectrometry. It has been found that there is a remarkable increase in ion flux at higher pulse frequencies and that the peak ion energy is directly related to the positive voltage overshoot of the target voltage. The magnitude of the metal flux depositing on the substrate is consistent with a “dead time” of ∼0.7μs at the start of the on period. The variation of the ion flux with pulse frequency has been explained by a simple model in which the ion density during the on period has a large peak which is slightly delayed from the large negative voltage overshoot which occurs at the start of the on pulse due to increased ionization at that time. This is consistent with the previously observed phenomena in pulsed sputtering.

Analysis of behavior of sodiated sugar hemiacetals under low-energy collision-induced dissociation conditions and application to investigating mutarotation and mechanism of a glycosidase.

Analysis of anomericity is one of the most important issues in the structure elucidation of carbohydrates. Mass spectrometry (MS)-based methods are of particular interest and important to address the issue related to resolving anomericity of monosaccharide units in a glycan. However, direct analysis of hemiacetals has not been possible by MS because of the nonavailability of information regarding the gas-phase behavior of such ion species. We addressed this issue by using stage-discriminated energy-resolved mass spectrometry (ERMS) at the stages of MSn and MSn+1 and showed that such analysis can be made. This was achieved by proving that individual anomers can be identified and that the equilibrium of sodium adducted ion species of α- and β-anomers can be negated in the gas phase under collision-induced dissociation (CID) conditions. On the basis of these results, we could 1) observe the mutarotation of lactose and 2) speculate the hydrolysis mechanism of endo-glycosylceramidase by using mass spectrometry.

Ion-trap mass spectrometry unveils the presence of isomeric oligosaccharides in an analyte: stage-discriminated correlation of energy-resolved mass spectrometry

Mass spectrometry, especially tandem mass spectrometry, has been widely used in the field of analytical sciences for handling biological and chemical samples. The technique resolves molecular and fragment ions based on the mass to charge ratio. Energy-resolved mass spectrometry (ERMS) further provides an activation energy-related factor in the dissociation reaction. Therefore, it is a very powerful technique that can discriminate isomeric compounds. Despite the power of ERMS, useful information cannot be obtained when an analyte contains structural isomers. Carbohydrates carry multiple chiral centers, thus oligomers of monosaccharides can form a vast number of structural isomers. We decided to use such species in our endeavors to establish a method of identifying the ‘purity’ of an analyte solely based on mass spectrometry. In the present paper, we describe a stage-discriminated spectral correlation of ERMS, which not only enables identification of the presence of contaminants in an analyte, but also provides information regarding the ‘purity’ of fragment ions.

Energy-Resolved Structural Details Obtained from Gangliosides

Gangliosides, a family of glycosphingolipids (GSLs) that comprise sialic acid residue(s), are an important class of molecules that exist on the outer surface of the plasma membrane. To assess the functions of a particular series of gangliosides that play important roles in brain functions, their structures and localizations need to be investigated. We studied the structures of these gangliosides by collision-induced dissociation using quadrupole ion-trap mass spectrometry. The dissociation processes were investigated in detail based on energy-resolved mass spectrometry using sodiated molecules. The decision of utilization of the positive mode was based on the assumption that it was the generally applicable method for GSLs, including neutral ones. In this investigation, sialic acid residues were esterified to stabilize the linkages and to generate multiple fragment ions for successful structural investigations. A detailed analysis of a series of sodiated species of gangliosides based on energy-resolved mass spectrometry revealed that the GM1-equivelent fragments generated from the precursor ions under low energy CID conditions had the structural characteristics of their individual precursors. It was suggested that this information will be useful in determining the structures of their precursor gangliosides.

Quantification of the ion and momentum fluxes toward the substrate during reactive magnetron sputtering

The ion and momentum fluxes toward the growing film during reactive magnetron sputtering of a Ti target in a mixture of Ar and N2 are determined. For the ion flux and ion energy distribution a retarding field energy analyzer has been employed. The results were confronted with planar and cylindrical probe measurements, two more common used techniques. For the momentum flux, energy resolved mass spectrometry and simulations with the binary collision Monte Carlo code SIMTRA were performed to determine the contribution to this flux by the impact of ions and sputtered and reflected particles. Based on the quantification of both fluxes, it can be concluded that there is a relation between the hardness and elastic modulus of the TiN films and the momentum flux.

Noble gas retention in the target during rotating cylindrical magnetron sputtering

A rotating cylindrical magnetron with a Ti target was sputtered in pure Xe or in a mixture of Xe and N2. The atomic composition of the target surface during sputtering has been investigated by in situ Rutherford backscattering spectrometry. The noble gas atomic ratio at the target surface is around 3.4% or 9.8% for sputtering in pure Xe and with 10% N2 addition, respectively. Energy resolved mass spectrometry reveals that some of the implanted Xe atoms are sputtered from the target. A radiation enhanced diffusion/detrapping/sputtering mechanism is proposed to model the flux of noble gas leaving the target during sputtering.

Determination of apparent decomposition threshold energies of lithium adducts of acylglycerols using tandem mass spectrometry and a novel derived effective reaction path length approach

Apparent decomposition threshold energies for the fragmentation pathways of lithiated acylglycerols were experimentally determined by collisional activation in a quadrupole-hexapole-quadrupole (QhQ) mass spectrometer. A previously developed 'derived effective reaction path length' approach for predicting bond dissociation energies (BDEs) of simple dissociations of electrostatic complexes such as alkali metal adducts (Li+), or halide adducts (Cl(-)) of acylglycerols, was extended to predict covalent bond apparent decomposition threshold energies of lithium adducts of a mono-acylglycerol, a 1,2-diacylglycerol, and a 1,3-diacylglycerol. The ability of the model to treat relatively large ionic systems (e.g. more than 100 atoms) represents a huge advantage of this approach. The model's calculated apparent decomposition threshold energies (Ea) are used in conjunction with the method of energy-resolved mass spectrometry, employing breakdown graphs, to give a more complete quantitative description of the fragmentation processes. Calculated Ea values allowed ranking of the 1,2-diacylglycerol as more reactive than the 1,3-diacylglycerol; the mono-acylglycerol was ranked the least reactive. The method was applied to the low molecular weight product ions generally associated with the hydrocarbon series CnH2n+1+, where two separate pathways are deduced as contributing to the production of the abundant m/z 81 fragment ion. The favored ranking of the neutral losses of fatty acyl substituents for the 1,2-diacylglycerol was determined as: loss of lithium fatty acetate > loss of fatty acid > loss of fatty acyl chain as ketene. For the 1,3-diacylglycerol, the descending order of ease of neutral loss was: loss of fatty acyl ketene > loss of lithium fatty acetate > loss of fatty acid. The results of this study demonstrate that the newly developed method is general in nature, and it can be used for the measurement of covalent bond decomposition threshold energies, as well as for the previously documented electrostatic (noncovalent) bond energies.

The influence of non-uniform target poisoning on the energy distributions of atoms sputtered in a reactive dc magnetron discharge

The lateral variation of the energy distributions of sputtered Ti atoms has been investigated by energy resolved mass spectrometry during reactive dc magnetron sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture. The mass spectrometer was placed at the substrate position and scanned across the target surface of a 2 inch planar circular magnetron. The energy distribution of sputtered particles is influenced by their origin, showing significant differences between the center and the erosion zone of the target. The results are interpreted in terms of laterally different states of target poisoning, which results in a variation of the surface binding energy. The reactive gas target coverage as derived from the sputtered energy distributions is in reasonable agreement with predictions from model calculations.