Ion Formation Process Research Articles

Formation of ions under the action of cosmic rays in humid air

The processes of ion formation in humid tropospheric air under the action of cosmic rays are considered. In this case, positive and negative cluster ions appear. For this analysis, a kinetic model was developed that includes 55 components and 161 reactions. The calculation was carried out using the KINET software package. It is shown that the ionization of air by cosmic rays at altitudes of 5–35 km leads to the formation of plasma consisting mainly of NH4+⋅NH3⋅H2O, H+⋅(H2O)4 and O2−⋅(H2O)2 ions. The maximum concentrations of ions under conditions of minimum magnetic rigidity are observed at altitudes from 10 to 18 km. These results differ sharply from the calculation results obtained for the dry air model.

Hydrated Formic Acid Clusters and their Interaction with Electrons.

We investigate ion formation in hydrated formic acid (FA) clusters upon collision with electrons of variable energy, focusing on electron ionization at 70 eV (EI) and low-energy (1.5-15 eV) electron attachment (EA). To uncover details about the composition of neutral clusters, we aim to elucidate the ion formation processes in FAM ⋅ WN clusters initiated by interaction with electrons and determine the extent of cluster fragmentation. EI predominantly produces protonated [FAm+H]+ ions, and in FA-rich clusters, the stable ring structures surrounding H3O+ ions are formed. In contrast, EA leads to a competition between the formation of intact [FAm ⋅ Wn]- and dissociated [FAm ⋅ Wn-H]- fragment ions, influenced by the cluster size, level of hydration, and electron energy. Our findings reveal a predisposition of low-energy EA towards forming [FAm ⋅ Wn]-, while higher electron energies tend to favor the formation of [FAm ⋅ Wn-H]- due to intracluster ion-molecule reactions. The comparison of positive and negative ion spectra suggests that the mass spectra of FA-rich clusters may indicate their actual size and composition. On the other hand, the more weakly bound water evaporation from the clusters depends strongly on the ionization. Thus, for the hydrated clusters, the neutral cluster size can hardly be estimated from the mass spectra.

On the Selectivity in the Synthesis of 3-Fluoropiperidines Using BF3-Activated Hypervalent Iodine Reagents.

Fluorinated piperidines find wide applications, most notably in the development of novel therapies and agrochemicals. Cyclization of alkenyl N-tosylamides promoted by BF3-activated aryliodine(III) carboxylates is an attractive strategy to construct 3-fluoropiperidines, but it suffers from selectivity issues arising from competitive oxoaminations and the inability to easily modulate the reactions diastereoselectivity. Herein, we report an itemized optimization of the reaction conditions carried out on both cyclic and acyclic substrates and outline the origins of substrate- and reagent-based stereo-, regio-, and chemoselectivity. Extensive mechanistic studies encompassing multinuclear NMR spectroscopy, deuterium labeling, rearrangements on stereodefined substrates, and careful structural analyses (NMR and X-ray) of the reaction products are performed. This revealed the processes and interactions crucial for achieving controlled preparation of 3-fluoropiperidines using I(III) chemistry and has provided an advanced understanding of the reaction mechanism. In brief, we propose that BF3-coordinated I(III) reagents attack C═C to produce the corresponding iodiranium(III) ion, which then undergoes diastereodetermining 5-exo-cyclization. Transiently formed pyrrolidines with an exocyclic σ-alkyl-I(III) moiety can further undergo aziridinium ion formation or reductive ligand coupling processes, which dictate not only the final product's ring size but also the chemoselectivity. Importantly, the selectivity of the reaction depends on the nature of the ligand bound to I(III) and the presence of electrolytes such as TBABF4. Reported findings will facilitate the usage of ArI(III)-dicarboxylates in the reliable construction of fluorinated azaheterocycles.

Thymidine and stavudine molecules in reactions with low-energy electrons

The negative ion formation and fragmentation processes in thymidine and stavudine were studied by means of resonant electron capture mass spectrometry in the electron energy range 0–14 eV. Stavudine is a nucleoside analog of thymidine, that differs from the latter by the presence of double bond in the deoxyribose moiety. In the energy range below 5 eV transient negative ions have been found to arise via shape resonances resulting in electron attachment into the empty π*-orbitals. The main fragmentation channels for molecular ions are the H-atom loss and N-glycosidic bond cleavage between sugar and a nucleobase moieties. The dissociative electron attachment cross sections for the fragment negative ions were measured. The structures for some observed anions and neutral counterparts were suggested basing on the thermochemical approach for dissociative reactions energy balance under the support of quantum chemical calculations. The processes of ion formation in thymidine were found to be similar to those revealed previously for deoxyuridine nucleoside. In contrast, very strong dissociative channel was found for stavudine yielding [M–R]– ions (where R is a sugar moiety) in the energy range <3 eV. The efficient fragmentation found for stavudine in the low energy electron attachment reactions may have implications for the design of nucleoside radiosensitizers for cancer therapy.

Removal of optimal cutting temperature (O.C.T.) compound from embedded tissue for MALDI imaging of lipids.

Matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) is a common molecular imaging modality used to characterise the abundance and spatial distribution of lipids in situ. There are several technical challenges predominantly involving sample pre-treatment and preparation which have complicated the analysis of clinical tissues by MALDI-MSI. Firstly, the common embedding of samples in optimal cutting temperature (O.C.T.), which contains high concentrations of polyethylene glycol (PEG) polymers, causes analyte signal suppression during mass spectrometry (MS) by competing for available ions during ionisation. This suppressive effect has constrained the application of MALDI-MSI for the molecular mapping of clinical tissues. Secondly, the complexity of the mass spectra is obtained by the formation of multiple adduct ions. The process of analyte ion formation during MALDI can generate multiple m/z peaks from a single lipid species due to the presence of alkali salts in tissues, resulting in the suppression of protonated adduct formation and the generation of multiple near isobaric ions which produce overlapping spatial distributions. Presented is a method to simultaneously remove O.C.T. and endogenous salts. This approach was applied to lipid imaging in order to prevent analyte suppression, simplify data interpretation, and improve sensitivity by promoting lipid protonation and reducing the formation of alkali adducts.

Online Characterization of Organic Aerosol by Condensational Growth into Aqueous Droplets Coupled with Droplet-Assisted Ionization.

Online analysis of ultrafine (<100 nm diameter) particles was performed by sending the aerosol through a condensation growth chamber (CGC) to create micrometer-size aqueous droplets that were subsequently analyzed by mass spectrometry with droplet-assisted ionization (DAI). Three experiments are reported which illustrate key performance characteristics of the method and give insight into the ion formation process: size-selected cortisone particles, size-selected secondary organic aerosol (SOA) particles, and freshly nucleated SOA under atmospherically relevant conditions. In each case, SOA was produced by α-pinene ozonolysis. For size-selected cortisone particles between 30 and 90 nm diameter and SOA particles between 30 and 70 nm, the ion signal intensity was found to be approximately independent of particle size. This observation is attributed to the formation of aqueous droplets in the CGC whose size distribution is independent of the original particle size. A consequence of this behavior is that the sensitivity of molecular detection increases as the particle size decreases, and the method is particularly well suited for new particle formation studies under atmospherically relevant conditions. This aspect of the CGC-DAI method was illustrated by the online analysis of freshly nucleated SOA samples with median diameters, number concentrations, and mass concentrations on the order of 25 nm, 104 cm-3, 0.2 μg m-3, respectively. Mass spectra of freshly nucleated SOA could be explained by condensation of highly oxidized molecules (HOMs) that subsequently reacted in the particle phase. Size-selected SOA showed increasing oligomerization with increasing particle size, which is consistent with established particle growth mechanisms.

A deep insight into the ion foreshock with the help of test particle two-dimensional simulations

Abstract. Two-dimensional (2D) test particle simulations based on shock profiles issued from 2D full particle-in-cell (PIC) simulations are used in order to analyze the formation processes of ions back streaming within the upstream region after interacting with a quasi-perpendicular curved shock front. Two different types of simulations have been performed based on (i) a fully consistent expansion (FCE) model, which includes all self-consistent shock profiles at different times, and (ii) a homothetic expansion (HE) model in which shock profiles are fixed at certain times and artificially expanded in space. The comparison of both configurations allows one to analyze the impact of the front nonstationarity on the back-streaming population. Moreover, the role of the space charge electric field El is analyzed by either including or canceling the El component in the simulations. A detailed comparison of these last two different configurations allows one to show that this El component plays a key role in the ion reflection process within the whole quasi-perpendicular propagation range. Simulations provide evidence that the different field-aligned beam (FAB) and gyro-phase bunched (GPB) populations observed in situ are essentially formed by a Et×B drift in the velocity space involving the convective electric field Et. Simultaneously, the study emphasizes (i) the essential action of the magnetic field component on the GPB population (i.e., mirror reflection) and (ii) the leading role of the convective field Et in the FAB energy gain. In addition, the electrostatic field component El is essential for reflecting ions at high θBn angles and, in particular, at the edge of the ion foreshock around 70∘. Moreover, the HE model shows that the rate BI% of back-streaming ions is strongly dependent on the shock front profile, which varies because of the shock front nonstationarity. In particular, reflected ions appear to escape periodically from the shock front as bursts with an occurrence time period associated to the self-reformation of the shock front.

Ultraviolet-Laser-Induced Electron Transfer from Peptides to an Oxidizing Matrix: Study of the First Step of MALDI In-Source Decay Mass Spectrometry.

Although the N-H bond in peptide backbones is stronger than the C-H bond, hydrogen abstraction from the amide nitrogen is considered to be the initial step in the Cα-C bond cleavage of peptide backbones by matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) when using an oxidizing matrix. MALDI-ISD induces Cα-C bond cleavage in most amino acid residues, whereas the N-terminal sides of proline (Pro) residues preferentially undergo peptide bond cleavage, which cannot be explained by the previously proposed mechanism involving hydrogen abstraction from peptides. To explain the whole MALDI-ISD process, electron abstraction from peptides by the oxidizing matrix is proposed as the initial step in the MALDI-ISD process. The electron abstraction occurs from either nitrogen or oxygen in the peptide backbone and induces the cleavage of both Cα-C and N-H bonds in most amino acid residues, except for those on the N-terminal sides of Pro residues. Electron abstraction from the Pro residues induces the cleavage of both peptide and Cα-C bonds, which is consistent with MALDI-ISD experimental results. The electron transfer from the peptide to the oxidizing matrix occurs simultaneously with the formation of matrix ions, which is considered to be the initial ion formation process in MALDI. The resultant peptide radical cation produces protonated and neutral molecules/radicals, which undergo subsequent ion-molecule reactions in the MALDI plume, finally yielding the ions that are observed in MALDI-ISD spectrum. As a result, the fragment ions formed by MALDI-ISD are observed as both positive and negative ions.

Ionization probability of sputtered indium atoms under impact of slow highly charged ions

In order to investigate the different role of kinetic and potential projectile energy for secondary ion formation, the authors have measured the ionization probability of indium atoms sputtered from a clean indium surface under irradiation with rare gas (Xeq+) ions of different charge states q at the same kinetic impact energy of 20 keV. In this energy range, the kinetic energy of the projectile is predominantly deposited via nuclear stopping, leading to a collision-dominated sputtering process. The authors find that the ionization probability increases significantly if a highly charged ion is used as a projectile, where the ionization energy becomes comparable to or even exceeds the kinetic energy, indicating that a higher level of electronic substrate excitation induced by the potential energy stored in the projectile can boost the secondary ion formation process. This experimental result is discussed in terms of microscopic model calculations describing the secondary ion formation process. At the same time, the authors observe a significant change of the emission velocity distribution of the sputtered particles, leading to a pronounced low-energy contribution at higher projectile charge states. It is shown that this “potential sputtering” contribution strongly depends on surface chemistry even under conditions where the surface is dynamically cleaned by interleaved 5 keV Ar+ ion bombardment.

Effects of electronic stopping power on fast-ion-induced secondary ion emission from methanol microdroplets

The formation processes of secondary ions in liquid materials were studied for methanol microdroplets bombarded by carbon ions with incident energies of 0.4–4.0 MeV, where the corresponding electronic stopping power ranged 300–800 eV nm−1. Positive and negative secondary ions including molecular fragments, methanol clusters, and reaction products were investigated, and each ion yield was examined as a function of electronic stopping power Se. We observed different Se-dependence on the emission yields between positive and negative ions. For positive cluster ions [(CH3OH)n + H]+ (n = 2−10), the yield nonlinearly increases and follows the power-law Seα with α = 3. For negative secondary ions, the value of α varies according to secondary ion species or ion mass: α ≈ 0 for fragments with small mass (CH−, CH2−, and OH−), α = 0.5–1.5 for reaction products with medium mass(C2−, C2H−, C2HO−, and C2H5O−), and α = 1.2−1.5 for clusters with large mass [(CH3OH)n – H]− (n = 1−25). The latter finding implies that the value of α is a quantity related to the electronic energy density depending on the distance from the ion trajectory.

Investigation of middle school students' understanding of external representations for atom and ion-formation process

Investigation of middle school students' understanding of external representations for atom and ion-formation process

Kinetic resolution of indolines through reductive amination of aldehydes by chiral Brønsted acid

Abstract We have developed a highly efficient and practical strategy for the kinetic resolution of indoline derivatives, involving a chiral Bronsted acid-catalyzed iminium ion formation and asymmetric transfer hydrogenation cascade process. The kinetic resolution allows the synthesis of 2-substituted N -benzylindolines in good yields with moderate to excellent enantioselectivities.

Thin Film Electrochemical Capacitors Based on Organolead Triiodide Perovskite

Organometal halide perovskites exhibit not only extraordinary electronic properties but also interesting ionic responses due to the ease of forming and moving native defects in the perovskite lattice. In this work such dual (electronic and ionic) conduction property is exploited to fabricate a thin film electrochemical capacitor (EC) based on organolead triiodide perovskite. The EC cells show excellent cyclability with stable capacitance output beyond 10 000 cycles. By performing electrochemical characterizations on the cells with different electrolytes and perovskite thickness, it is found that the perovskite layer serves not only as an electrode but also a solid electrolyte. Through circuit modeling of the electrochemical impedance spectra, the density of mobile ions (i.e., ionic native defects) in the perovskite films is extracted and is found to be lower than those estimated from perovskite electronic devices, e.g., solar cells. The result suggests that the ion formation and transport processes in organometal halide perovskites may largely be activated by the current of free charge carriers as well as specific interface defects.

Changes in the molecular ion yield and fragmentation of peptides under various primary ions in ToF-SIMS and matrix-enhanced ToF-SIMS.

Time of flight secondary ion mass spectrometry (ToF-SIMS) is a powerful technique for the nanoanalysis of biological samples, but improvements in sensitivity are needed in order to detect large biomolecules, such as peptides, on the individual cell level at physiological concentrations. Two promising options to improve the sensitivity of SIMS to large peptides are the use of cluster primary ions to increase desorption of intact molecules or the use of matrix-assisted laser desorption/ionization (MALDI) matrices to increase the ionization probability. In this paper, the authors have combined these two approaches in order to improve understanding of the interaction between ionization and fragmentation processes. The peptides bradykinin and melittin were prepared as neat monolayers on silicon, in a Dextran-40 matrix and in two common MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxy cinnamic acid (HCCA). ToF-SIMS spectra of these samples were collected using a range of small Bi cluster primary ions and large Ar cluster primary ions. The trends observed in the molecular ion yield and the [M+H](+)/C4H8N(+) ratio with primary ion cluster size were sample system dependent. The molecular ion yield of the bradykinin was maximized by using 30 keV Bi3 (+) primary ions in a DHB matrix but in the HCCA matrix, the maximum molecular ion yield was obtained by using 30 keV Bi7 (+) primary ions. In contrast, the molecular ion yield for melittin in both matrices was greatest using 20 keV Ar2000 (+) primary ions. Improvements in the molecular ion yield were only loosely correlated with a decrease in small fragment ions. The data indicate a complex interplay between desorption processes and ion formation processes which mean that the optimal analytical conditions depend on both the target analyte and the matrix.

ChemInform Abstract: Synthesis of Azabicycles via Cascade Aza‐Prins Reactions: Accessing the Indolizidine and Quinolizidine Cores.

The first detailed studies of intramolecular aza-Prins and aza-silyl-Prins reactions, starting from acyclic materials, are reported. The methods allow rapid and flexible access toward an array of [6,5] and [6,6] aza-bicycles, which form the core skeletons of various alkaloids. On the basis of our findings on the aza-Prins and aza-silyl-Prins cyclizations, herein we present simple protocols for the intramolecular preparation of the azabicyclic cores of the indolizidines and quinolizidines using a one-pot cascade process of N-acyliminium ion formation followed by aza-Prins cyclization and either elimination or carbocation trapping. It is possible to introduce a range of different substituents into the heterocycles through a judicial choice of Lewis acid and solvent(s), with halo-, phenyl-, and amido-substituted azabicyclic products all being accessed through these highly diastereoselective processes.

Metal oxide collectors for storing matter technique applied in secondary ion mass spectrometry

We present results of the use of metal and metal oxide substrates that serve as collectors in ‘storing matter’, the quantitative technique of secondary ion mass spectrometry (SIMS). This technique allows separating the two base processes of secondary ion formation in SIMS. Namely, the process of ion sputtering is separated from the process of ionisation. The technique allows sputtering of the analysed sample and storing the sputtered material, with sub-monolayer coverage, onto a collector surface. Such deposits can be then analysed by SIMS, and as a result, the so called ‘matrix effects’ are significantly reduced. We perform deposition of the sputtered material onto Ti and Cu substrates and also onto metal oxide substrates as molybdenum, titanium, tin and indium oxides. The process of sputtering is carried within the same vacuum chamber where the SIMS analysis of the collected material is performed. For sputtering and SIMS analysis of the deposited material we use 5keV Ar+ beam of 500nA. The presented results are obtained with the use of stationary collectors. Here we present a case study of chromium. The obtained results show that the molybdenum and titanium oxide substrates used as collectors increase useful yield by two orders, with respect to such pure elemental collectors as Cu and Ti. Here we define useful yield as a ratio of the number of detected secondary ions during SIMS analysis and the number of atoms sputtered during the deposition process.

Electrochemical Formation and Phase Control of Mg–Cu Alloys

AbstractMg–Cu alloys with different phases were electrodeposited in a molten LiCl–KCl eutectic mixture containing a small amount (0.5 mol %) of MgCl2 at 673 K. The electrochemical behavior of the MgII ion and alloy formation processes were studied at inert molybdenum (Mo) and active copper (Cu) electrodes in the molten salts, respectively. Different electrochemical techniques, such as cyclic voltammetry, square‐wave voltammetry, chronoamperometry, and open‐circuit potentiometry were carried out to investigate the electrochemical formation mechanism of Mg–Cu alloys. Three signals, corresponding to the formation of metal Mg and two different Mg–Cu alloy phases, are observed when using cyclic voltammetry, square‐wave voltammetry, and open‐circuit potentiometry. Chronoamperometry studies indicate the different interfacial properties of the magnesium metal versus the Mg–Cu alloys. The results of X‐ray diffraction and scanning electron microscopy show that the MgCu2 and Mg2Cu phases could be obtained at −2.30 and −2.38 V (vs. Pt), respectively.

Atom probe tomography (APT) of carbonate minerals

Atom probe tomography (APT) combines the highest spatial resolution with chemical data at atomic scale for the analysis of materials. For geological specimens, the process of field evaporation and molecular ion formation and interpretation is not yet entirely understood. The objective of this study is to determine the best conditions for the preparation and analysis by APT of carbonate minerals, of great importance in the interpretation of geological processes, focusing on the bulk chemical composition. Results show that the complexity of the mass spectrum is different for calcite and dolomite and relates to dissimilarities in crystalochemical parameters. In addition, APT bulk chemistry of calcite closely matches the expected stoichiometry but fails to provide accurate atomic percentages for elements in dolomite under the experimental conditions evaluated in this work. For both calcite and dolomite, APT underestimates the amount of oxygen based on their chemical formula, whereas it is able to detect small percentages of elemental substitutions in crystal lattices. Overall, our results demonstrate that APT of carbonate minerals is possible, but further optimization of the experimental parameters are required to improve the use of atom probe tomography for the correct interpretation of mineral geochemistry.

Synthesis of Azabicycles via Cascade Aza-Prins Reactions: Accessing the Indolizidine and Quinolizidine Cores.

The first detailed studies of intramolecular aza-Prins and aza-silyl-Prins reactions, starting from acyclic materials, are reported. The methods allow rapid and flexible access toward an array of [6,5] and [6,6] aza-bicycles, which form the core skeletons of various alkaloids. On the basis of our findings on the aza-Prins and aza-silyl-Prins cyclizations, herein we present simple protocols for the intramolecular preparation of the azabicyclic cores of the indolizidines and quinolizidines using a one-pot cascade process of N-acyliminium ion formation followed by aza-Prins cyclization and either elimination or carbocation trapping. It is possible to introduce a range of different substituents into the heterocycles through a judicial choice of Lewis acid and solvent(s), with halo-, phenyl-, and amido-substituted azabicyclic products all being accessed through these highly diastereoselective processes.

Using the partial least squares method to model the electrospray ionization response produced by small pharmaceutical molecules in positive mode.

The signal intensity in electrospray ionization mass spectrometry (ESI-MS) positive mode is affected by parameters that are related to the physicochemical properties and structural features of a molecule. Accordingly, the combined interactions of an analyte and the mobile phase used is still an area that demands further clarifications since there is no general pattern regarding the nature of a molecule and the mechanism by which vapor-phase ions are produced. A multivariate analysis method, such as Partial Least Squares (PLS), provides the opportunity to correlate the effect of a large number of parameters interpreting this complex procedure with the use of appropriate mathematical algorithms. This work involves the development of models containing up to 84 X variables which characterize the analytes studied (99) focusing on their positive or negative effect on the vapor-phase ion formation process. These descriptors are correlated with the signal response of the positively charged analyte ions which corresponds to the Y variable. The results showed that parameters referring or directly related to the ionization percentage of basic or acidic groups of an analyte can be used to determine the signal response on positive ESI-MS mode. Structural characteristics, polar surface area, lipophilicity, the ability of analytes to acts as hydrogen bond donors or acceptors, water solubility, density of a solid, surface tension of the substance and the number of free rotatable bonds are descriptors which are of secondary importance, still they cannot be considered negligible. The models derived are proved to be reliable for the investigation of such mechanisms, with a small number of components and good linearity (R(2) >83%, Q(2) >70%).