Dissociative Electron Capture Research Articles

Exploring Negative Chemical Ionization of Per- and Polyfluoroalkyl Substances via a Liquid Electron Ionization LC-MS Interface.

Per- and polyfluoroalkyl substances (PFASs) are a class of aliphatic manufactured compounds comprising fluoro-chemicals with varied functional groups and stable carbon-fluorine bonds. They are defined as "forever chemicals" due to their persistent and bioaccumulative character. These substances have been detected in various environmental samples, including water, air, soil, and human blood, posing significant health hazards. High-performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC-ESI-MS) is typically employed for the analysis of PFASs. Negative chemical ionization (NCI) is generally coupled to gas chromatography (GC) and offers high selectivity and sensitivity for compounds containing electronegative atoms, such as PFASs. The liquid electron ionization (LEI) interface is an efficient mechanism developed to robustly couple a liquid flow rate from an LC system to an EI or a CI source. This interface has been successfully utilized for pesticide determination in UHPLC-LEI-CI in negative ion mode (NCI). This work aims to evaluate different parameters involved in the ionization of PFASs analyzed in LC-LEI-NCI and subsequently develop a method for their detection in real samples. The parameters considered for this study include (i) a comparison of different CI reagent gases (methane, isobutane, and argon); (ii) the use of acetonitrile as both the chromatographic solvent and CI reagent gas; (iii) the presence of water and formic acid as chromatographic mobile phase components; and (iv) the mobile phase flow rate. The optimal combination of these parameters led to promising results. Tentative fragmentation pathways of PFASs in NCI mode are proposed based on the dissociative electron capture mechanism.

Non-Covalent Structures of Negative Ions Formed during the Dissociative Capture of Electrons by Molecules

Non-Covalent Structures of Negative Ions Formed during the Dissociative Capture of Electrons by Molecules

Non-covalent structures of negative ions formed during the dissociative capture of electrons by molecules

The method of dissociative electron attachment (DEA) spectroscopy was used to study the attachment of electrons to 1-chloronaphthalene molecules. It has been established that the dominant channel for the decay of molecular ions is the formation of Cl- ions in three resonances at 0.7, 1.5, and 3.0 eV. Ions [M-H]- and [M-Cl]- are observed at energies from 3.5 to 8.5 eV and have two to three orders of magnitude lower formation cross sections. Long-lived molecular ions were not registered. Calculations in the DFT CAM B3LYP/6-311+G(d,p) approximation predict the presence of six stable anionic structures in which the chlorine anion is coordinated to the neutral residue via noncovalent H-Cl^--H bonds. The electron affinity of the most stable of these structures coincides with the experimentally measured value EA_a=0.2771±0.003 eV. These results agree with the previously obtained data on the DEA of molecules of bromine-substituted biphenyls, naphthalenes, and anthracenes and confirm the existence of anionic structures with non-covalent H-Hal-H bonds. Such non-covalent anion structures should be extremely reactive, which makes them promising for the synthesis of self-assembling hydrocarbon nanomembranes. Keywords: Attachment of electrons to molecules, electron affinity, potential surface, DFT calculations.

Studies on the Processes of Electron Capture and Clustering of Benzyl Chloride by Ion Mobility Spectrometry.

This paper presents the results of the study on the course of the benzyl chloride (BzCl) ionization process in a drift tube ion mobility spectrometer (DT IMS) in which nitrogen was used as the carrier gas. BzCl ionization follows the dissociative electron capture mechanism. The chloride ions produced in this process take part in the formation of cluster ions. Using DT IMS allows for estimation of the value of the electron attachment rate for BzCl and the equilibrium constant for the cluster ion formation. The basic experimental method used in this work was to analyze drift time spectra obtained for the introduction of the sample to the spectrometer with the drift gas. The theoretical interpretation of the results is based on the mathematical description of the ion transport. This description takes into account the phenomenon of diffusion, as well as the processes of formation and dissociation of ionic clusters occurring during the movement of ions in the drift section.

Ionization and electron capture for H+ collisions at low keV energy with a water-vapor target

We have investigated the total relative ionization and dissociative electron capture cross section for proton collisions with water-vapor molecules within the energy range of 2.5–10 keV. Time of flight mass spectroscopy was employed in the measurements of the atomic and molecular singly charged particles H+, H2+, O+, OH+ and H2O+ as well as the doubly charged particles O++, H2O++, OH++, and the contribution of the different channels was evaluated. Ionization was found to be the dominant reaction channel as expected. The contribution of the singly charged particles was found to be one order of magnitude higher than that of the doubly charged particles.

Dissociative electron attachment studies with hyperthermal Rydberg atoms.

Earlier studies of the velocity distributions of heavy-Rydberg ion-pair states formed in collisions between potassium Rydberg atoms with low-to-intermediate values of n, 10 ≲ n ≲ 15, and targets that attach free low-energy electrons have shown that such measurements can provide a window into the dynamics of dissociative electron capture. Here we propose that the reaction dynamics can be explored in much greater detail through studies using hyperthermal Rydberg atoms. This is demonstrated using, as an example, helium Rydberg atoms and a semi-classical Monte Carlo collision code developed specifically to model the dynamics of Rydberg electron transfer in collisions between Rydberg atoms and attaching targets. The simulations show that the outcome of collisions is sensitive not only to the lifetime and decay energetics of the excited intermediate negative ion formed upon initial Rydberg electron capture but also to the radial electron probability density distribution in the Rydberg atom itself, i.e., to its ℓ value.

Generation and Fragmentation of Phthalide Derivative Negative Ions

Molecules of diphenylphthalide and 3,3-diphenylphthalide-4′,4′-dicarbonic acid have been studied using the method of dissociative electron capture negative ion mass spectrometry. The mean lifetimes of negative molecular ions relative to the electron detachment have been measured. The electron affinity has been estimated in the Arrhenius approximation and calculated by the DFT B3LYP/6-31G+(d) method.

Destruction of Peptides and Nucleosides in Reactions with Low-Energy Electrons

Mass-spectrometry of negative ions is used to study dissociative electron capture by molecules of several nucleosides, simplest di- and tripeptides, and modified dipeptides. Energy domains and efficiencies of dissociative capture are determined for the objects under study, and threshold energies of several fragmentation processes are estimated. It is shown that cytidine and peptides are stable against fragmentation due to simple bond breaking at electron energies ranging from ~0 to 1 eV.

Ionization and Electron Capture for H+ Collisions with CO at low keV Energy

We have investigated the total relative ionization and dissociative electron capture cross section for proton collisions with CO within the energy range of 2 keV to 10 keV. Time of flight mass spectroscopy was employed in the measurements of CO+, O+ and C+ and the contribution of the different channels was evaluated. Ionization was found to be the dominant reaction channel as expected, while the C+ ionic fragments intensity was found to be higher than those of O+.

Resonant electron capture by aspartame and aspartic acid molecules

The processes for dissociative electron capture are the key mechanisms for decomposition of biomolecules, proteins in particular, under interaction with low-energy electrons. Molecules of aspartic acid and aspartame, i.e. modified dipeptides, were studied herein to define the impact of the side functional groups on peptide chain decomposition in resonant electron-molecular reactions. The processes of formation and decomposition of negative ions of both aspartame and aspartic acid were studied by mass spectrometry of negative ions under resonant electron capture. The obtained mass spectra were interpreted under thermochemical analysis by quantum chemical calculations. Main channels of negative molecular ions fragmentation were found and characteristic fragment ions were identified. The СООН fragment of the side chain in aspartic acid is shown to play a key role like the carboxyl group in amino acids and aliphatic oligopeptides. Copyright © 2016 John Wiley & Sons, Ltd.

Probing dissociative electron attachment through heavy-Rydberg ion-pair production in Rydberg atom collisions.

Electron transfer in collisions between low-n, n = 12, Rydberg atoms and targets that attach low-energy electrons can lead to the formation of heavy-Rydberg ion-pair states comprising a weakly-bound positive-negative ion pair that orbit each other at large separations. Measurements of the velocity and angular distribution of ion-pair states produced in collisions with 1,1,1-C2Cl3F3, CBrCl3, BrCN, and Fe(CO)5 are used to show that electron transfer reactions furnish a new technique with which to examine the lifetime and decay energetics of the excited intermediates formed during dissociative electron capture. The results are analyzed with the aid of Monte Carlo simulations based on the free electron model of Rydberg atom collisions. The data further highlight the capabilities of Rydberg atoms as a microscale laboratory in which to probe the dynamics of electron attachment reactions.

Gas phase ion chemistry of coumarins: ab initio calculations used to justify negative chemical ionization trends

The gas phase ion chemistry of coumarins using electron ionization (EI), positive chemical ionization (PCI) and negative chemical ionization (NCI) in a time of flight and quadrupole mass spectrometer (qMS) coupled to a gas chromatograph is outlined. The observations in NCI mode were complimented with Ab initio calculations. This was because NCI gave enhanced signals in contrast to EI and PCI and therefore could serve as a potential ionization method for quantitative analysis. For the two classes of analytes that were examined here, i.e. methyl and acetyl coumarin derivatives, experimental data showed that the methyl derivatives underwent dissociative electron capture to produce ions of the type [M–H]¯ while the acetyl derivatives underwent resonance electron capture to produce ions of the type M˙¯. The M˙¯ type of ions were more intense than the earlier. Ab initio calculations showed higher electron affinities in the acetyl than the methyl derivatives. KEY WORDS : Coumarins, Ab Initio calculations, Electron ionization, Positive chemical ionization, Negative chemical ionization Bull. Chem. Soc. Ethiop. 2015 , 29(3), 473-484. DOI: http://dx.doi.org/10.4314/bcse.v29i3.15

Hydrated electron-based degradation of atenolol in aqueous solution

Abstract Photoactivation of SO 3 2− with UV 254 irradiation (UV/SO 3 2− ) was used to generate hydrated electron (e aq − )-based process to degrade atenolol (ATL) in drinking water. The decontamination mechanism of UV/SO 3 2− process was investigated by means of competitive kinetic analysis and identification of transformation products, followed by the assessment of potential for drinking water treatment through evaluating the influence of operational parameters (dosage of SO 3 2− , solution pH, HCO 3 − , dissolved organic matters (DOM), and temperature), detoxification efficiency, and technical economy. The kinetic and transformant observations lead to the suggestion that e aq − -induced dissociative electron capture and addition reactions be involved in the ATL degradation mechanism. The results show a positive correlation between the ATL removal efficiency and dosage of SO 3 2− (0.5–2.0 mM). The process is also pH-dependant. Increasing solution pH from 5 to 9 promoted the elimination of ATL due to the pH-dependent SO 3 2− photoactivity. For the pH increase from 9 to 11, deprotonation of ATL caused further enhancement of ATL destruction. 1–4 mM HCO 3 − exerted little inhibition effect on the ATL destruction, while the presence of 1 and 4 mg L −1 L DOM deteriorate the ATL degradation due to the effective UV fluence decrease. In addition, for every 10 °C the solution temperature rose, the reaction rate increased 1.3–1.6 times. The toxicity evaluation further suggests that ATL degradation and detoxification do not occur simultaneously. Compared to UV/H 2 O 2 or UV/OXONE process, UV/SO 3 2− process was more energy-consuming.

Gas chromatography plasma-assisted reaction chemical ionization mass spectrometry for quantitative detection of bromine in organic compounds.

We have recently introduced plasma-assisted reaction chemical ionization mass spectrometry (PARCI-MS) for elemental analysis of halogens in organic compounds. Here, we utilize gas chromatography (GC) coupled to PARCI-MS to investigate the mechanism of Br(-) ion generation from organobromines and to evaluate analytical performance of PARCI for organobromine analysis. Bromine atoms in compounds eluting from GC are converted to HBr in a low-pressure microwave induced helium plasma with trace amounts of hydrogen added as a reaction gas. Ionization is achieved by introducing nitrogen into the afterglow region of the plasma, liberating electrons via penning ionization and leading to formation of negative ions. We demonstrate that N2 largely affects the ionization process, whereas H2 affects both the ionization process and in-plasma reactions. Our investigations also suggest that dissociative electron capture is the main ionization route for formation of Br(-) ions. Importantly, GC-PARCI-MS shows a uniform response factor for bromine across brominated compounds of drastically different chemical structures, confirming PARCI's ability to quantify organobromines in the absence of compound-specific standards. Over 3 orders of magnitude linear dynamic range is demonstrated for bromine quantification. We report a detection limit of 29 fg of bromine on-column, ~4-fold better than inductively coupled plasma-MS.

Resonant dissociative electron capture by simple tripeptides

The processes of resonant electron capture by tripeptide molecules of diglycylalanine and trialanine were studied in the electron energy range 0–14 eV. The experimentally observed H-shift rearrangement reactions, metastable dissociations of fragmentation ions, and electron autodetachment processes by the fragmentation ions indicate that short-lived molecular negative ions are formed in vibrationally excited states in the whole energy range and their subsequent fragmentation occurs predominantly via nonradiative transitions. The characteristic reactions in negative ions of peptides were identified and considered to be model reactions in protein decomposition processes through electron-molecular interactions.

Dissociation of Single 2-Chloroanthracene Molecules by STM-Tip Electron Injection

We have studied the adsorption and tip-induced chemistry of 2-chloroanthracene on TiO2(110). STM images show that at 135 K and low coverage, i.e., ∼0.1 ML, these molecules are physisorbed along the five-coordinated titanium rows on the rutile(110) surface as a result of electrostatic interaction. Applying electric pulses >2.5 V from the STM tip to individual molecules causes either desorption or dissociation of the molecules, as indicated by the changes in the STM images. We have observed dissociative electron capture of a single 2-chloroanthracene molecule, which leaves behind a surface chlorine atom adsorbed in the on-top configuration on a surface Ti atom. The threshold energy required for the dissociation was found to be ∼2.7 eV.

(S)‐(−)‐N‐(Pentafluorobenzylcarbamoyl)prolyl chloride: a chiral derivatisation reagent designed for gas chromatography/negative ion chemical ionisation mass spectrometry of amino compounds

The synthesis of a novel chiral derivatisation reagent, (S)-(-)-N-(pentafluorobenzylcarbamoyl)prolyl chloride is described which is preferably useful for negative-ion chemical ionisation mass spectrometry. Preparation of the reagent followed a general strategy used to prepare enantioselective reagents based on the N-substitution of L-proline. Pentafluorobenzyl chloroformate smoothly reacted with L-proline to give the desired derivatisation reagent after conversion into the acyl chloride. The product was sufficiently pure to be used in the following steps without any additional purification. The reagent was tested against selected chiral and non-chiral analytical targets. Chromatographic enantioseparation was at least equal to the commonly used (S)-(-)-N-(heptafluorobutyryl)prolyl derivatives. The derivatives exhibit excellent mass spectral properties under negative ion chemical ionisation, i.e. reduced fragmentation and thus high ion current for the targeted m/z during analysis. With electron ionisation, the fragmentation that occurs is mainly directed by the introduced group. Enantioseparation with gas chromatography/negative-ion chemical ionisation mass spectrometry of the derivatives was demonstrated for the enantiomers of amphetamine, α-aminocaprylic acid methyl ester and threo-methylphenidate. The new derivatisation reagent shows highly improved mass spectral properties for negative-ion chemical ionisation mass spectrometry and is thus suitable for sensitive chiral detection of amino compounds. The reagent extends the applicability of dissociative resonance electron capture using pentafluorobenzyl derivatives to chiral analysis.

Photoemission ambient pressure ionization (PAPI) with an ultraviolet light emitting diode and detection of organic compounds

The development of compact, rugged and low-power ion sources is critical for the further advancement of handheld mass analyzers. Further, there is a need to replace the common (63)Ni source used at atmospheric pressure with a non-radioactive substitute. We present here a description of a light emitting diode (LED) photoemission ionization source for use in mass spectrometry for the detection of volatile organic compounds. This technique relies upon the generation of photoelectrons from a low-work function metal via low-energy ultraviolet (UV) light (280 or 240 nm) generated by a single LED in air at atmospheric pressure. These low-energy photoelectrons result in either direct electron capture by the analyte or chemical ionization. Currently, only negative ions are demonstrated due to operation at atmospheric pressure. Ion generation occurs without use of high electric fields such as those found in corona discharge or electrospray ionization. This source is effective for measuring organic vapors from gases, liquids and surface residues via atmospheric pressure chemical ionization, initiated by photoemission off a conductive surface. Several classes of organic vapors are analyzed and found to be effectively detected, including compounds that ionize via electron attachment, dissociative electron capture, proton abstraction, adduct formation and replacement ionization.

Radiation-induced decomposition of cyclotrimethylenetrinitramine in aqueous solutions

Radiation-induced decomposition of cyclotrimethylenetrinitramine, a representative of heterocyclic nitramines, in dilute aqueous solutions saturated with argon, air, and N2O has been investigated. Dose dependence curves have been obtained, and the initial yields of formation of nitrite ions and hydrogen peroxide have been measured, making up 0.122 and 0.049 in deaerated solutions, 0.050 and 0.065 in aerated solutions, and 0 and 0.074 μmol/J in N2O-saturated solutions, respectively. The main radiolysis initial steps involve the reaction of dissociative electron capture with nitro-group elimination in the form of nitrite ion and the reaction of hydrogen atom abstraction from the methylene group by hydroxyl radicals and atomic hydrogen.

A versatile electrophoric derivatisation reagent for negative ion chemical ionisation mass spectrometry: o-(pentafluorobenzyloxycarbonyl)-2,3,4,5-tetrafluorobenzoyl chloride

The synthesis of a novel electrophoric derivatisation reagent, o-(pentafluorobenzyloxycarbonyl)-2,3,4,5-tetrafluorobenzoyl chloride, is described. The reagent was tested against selected primary and secondary amino compounds, as well as phenolic and aliphatic hydroxyl compounds as analytical targets. The derivatives exhibit excellent mass spectral properties under negative ion chemical ionisation, i.e. reduced fragmentation and thus high ion current for the targeted m/z during analysis. Since the reagent bears a pentafluorobenzyl ester group, resulting negative ion chemical ionisation mass spectra were expectedly dominated by dissociative resonance electron capture typically observed with these compounds, additionally showing neutral loss of carbon dioxide and ammonia (in the case of primary amines). The reagent is suitable for detecting the target compounds with high sensitivity, as exemplified for the analysis of amphetamine and methylphenidate from human plasma where chromatographic background is drastically reduced by a shift in detected m/z and retention time and lower limits of quantification at 7.8 pg/mL (amphetamine) and 4.5 pg/mL (methylphenidate) can be obtained. The choice of two or three target quantification masses allows selective detection and adjustment of lowest background interference. No carryover effect was observed for the derivatives of amphetamine and methylphenidate.