Parent Ion Research Articles

VUV photoionization and dissociative photoionization of o-xylene: experimental and theoretical insights

The photoionization and dissociative photoionization of o-xylene have been studied using synchrotron radiation vacuum ultraviolet light with the photon energy in the range of 8.0–13.0 eV. The ionization energy of o-xylene and the appearance energies for the main fragment ions $$\hbox {C}_{\mathrm {8}}\hbox {H}_{\mathrm {9}}^{+}$$ , $$\hbox {C}_{\mathrm {7}}\hbox {H}_{\mathrm {7}}^{+}$$ and $$\hbox {C}_{\mathrm {6}}\hbox {H}_{\mathrm {6}}^{+}$$ were determined to be 8.45, 11.08, 11.47 and 11.17 eV, respectively, with photoionization efficiency spectra (PIES). Three dissociation pathways of parent ions are investigated by experimental photoionization and the quantum-chemical calculations. The fragment ions $$\hbox {C}_{\mathrm {7}}\hbox {H}_{\mathrm {7}}^{\mathrm {+}}$$ derived from parent ions involved hydrogen atom transfer rather than direct dissociation. Particularly, detailed mechanisms of dissociation $$\hbox {C}_{\mathrm {6}}\hbox {H}_{\mathrm {6}}^{+}$$ are presented for the first time in this work. In addition, the geometries of the intermediates (INTs), transition states (TSs) and products have been performed at B3LYP/6-311 $$+ \,+$$ G (d, p) level. The mechanisms of dissociative photoionization of o-xylene and the INTs and TSs involved are discussed in detail.

Identification and characterization of new impurities in zopiclone tablets by LC-QTOF-MS

Zopiclone, a non-benzodiazepine hypnotic, is the first-line treatment for insomnia. The quality and stability of zopiclone tablets directly affects its efficacy and safety. However, the impurity investigation in zopiclone tablets remain incomplete. In this study, the accelerated and long-term stabilities of zopiclone tablets, as well as the stability characteristics under thermal and photolytic conditions were evaluated according to the ICH guidelines. In addition, a sensitive and specific LC-QTOF-MS method was developed for the separation and identification of all the impurities in zopiclone tablets and its stability test samples. Nine impurities were found in the test samples, five among them have not been reported before. Based on the accurate mass and elemental compositions of the parent and product ions obtained, the structures of all the detected impurities were identified. Combined with the formulation composition analysis and stability studies, the origins and the formation mechanisms of these impurities were elucidated. The obtained results are useful for the establishment of the optimum formulation, storage condition, manufacturing processes and quality control of zopiclone tablets.

Rapid screening of 70 colorants in dyeable foods by using ultra-high-performance liquid chromatography–hybrid quadrupole–Orbitrap mass spectrometry with customized accurate-mass database and mass spectral library

A rapid screening method of 70 colorants for regulatory control in dyeable foods was established using ultra-high-performance liquid chromatography–hybrid quadrupole–Orbitrap mass spectrometry (UHPLC-Q/Orbitrap MS) with customized accurate-mass database and mass spectral library. A rapid, high-throughput, and simple sample pretreatment condition with low reagent consumption and high recovery was developed on the basis of ultrasound-assisted extraction and dispersion solid-phase extraction. Rapid screening was conducted by comparing the experimentally measured exact mass of the parent and fragment ions, the isotope pattern, and the retention time with the accurate-mass database and by matching the acquired MS/MS spectra against the mass spectral library. The performance of the method was evaluated in terms of linearity, limits of detection, limits of quantitation, recovery, repeatability, reproducibility, and matrix effect. The proposed method was applied for simultaneous analysis of 70 colorants in seven kinds of dyeable foods, and it exhibited great potential for broad, sensitive, and reliable.

Capillary Zone Electrophoresis-Electron-Capture Collision-Induced Dissociation on a Quadrupole Time-of-Flight Mass Spectrometer for Top-Down Characterization of Intact Proteins.

Mass spectrometry (MS)-based denaturing top-down proteomics (dTDP) requires high-capacity separation and extensive gas-phase fragmentation of proteoforms. Herein, we coupled capillary zone electrophoresis (CZE) to electron-capture collision-induced dissociation (ECciD) on an Agilent 6545 XT quadrupole time-of-flight (Q-TOF) mass spectrometer for dTDP for the first time. During ECciD, the protein ions were first fragmented using ECD, followed by further activation and fragmentation by applying a CID potential. In this pilot study, we optimized the CZE-ECciD method for small proteins (lower than 20 kDa) regarding the charge state of protein parent ions for fragmentation and the CID potential applied to maximize the protein backbone cleavage coverage and the number of sequence-informative fragment ions. The CZE-ECciD Q-TOF platform provided extensive backbone cleavage coverage for three standard proteins lower than 20 kDa from only single charge states in a single CZE-MS/MS run in the targeted MS/MS mode, including ubiquitin (97%, +7, 8.6 kDa), superoxide dismutase (SOD, 87%, +17, 16 kDa), and myoglobin (90%, +16, 17 kDa). The CZE-ECciD method produced comparable cleavage coverage of small proteins (i.e., myoglobin) with direct-infusion MS studies using electron transfer dissociation (ETD), activated ion-ETD, and combinations of ETD and collision-based fragmentation on high-end orbitrap mass spectrometers. The results render CZE-ECciD a new tool for dTDP to enhance both separation and gas-phase fragmentation of proteoforms.

Recognizing Contamination Fragment Ions in Liquid Chromatography-Tandem Mass Spectrometry Data.

Tandem mass spectral (MS/MS) data in liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis are often contaminated as the selection of precursor ions is based on a low-resolution quadrupole mass filter. In this work, we developed a strategy to differentiate contamination fragment ions (CFIs) from true fragment ions (TFIs) in an MS/MS spectrum. The rationale is that TFIs should coelute with their parent ions, but CFIs should not. To assess coelution, we performed a parallel LC-MS/MS analysis in data-independent acquisition (DIA) with all-ion-fragmentation (AIF) mode. Using the DIA (AIF) data, peak-peak correlation (PPC) score is calculated between the extracted ion chromatogram (EIC) of the fragment ion using the MS/MS scans and the EIC of the precursor ion using the MS1 scans. A high PPC score is an indication of TFIs, and a low PPC score is an indication of CFIs. Tested using metabolomics data generated by high resolution QTOF and Orbitrap MS from various vendors in different LC-MS configurations, we found that more than 70% of the fragment ions have PPC scores < 0.8 and identified three common sources of CFIs, including (1) solvent contamination, (2) adjacent chemical contamination, and (3) undetermined signals from artifacts and noise. Combining PPC scores with other precursor and fragment ion information, we further developed a machine learning model that can robustly and conservatively predict CFIs. Incorporating the machine learning model, we created an R program, MS2Purifier, to automatically recognize CFIs and clean MS/MS spectra of metabolic features in LC-MS/MS data with high sensitivity and specificity.

Strong Field Ionization-Photofragmentation on Ultrafast Evolution of Electronic States of Toluene Cations.

Ultrafast time-resolved strong field ionization-photofragmentation (SFI-PF) has emerged as a useful method for investigation of dynamics of molecular cations. Here we perform a SFI-PF study on the electronic states of toluene cations. By measuring the ion yields as a function of delay time, we obtain the transients of both parent and daughter ions, which show ultrafast decays and out-of-phase oscillations. The results provide the first experimental evidence of D1-D0 ultrafast relaxation of toluene cations occurring in about 530 fs and indicate coincident resonance between the vibrational states in D1 and D0 leading to oscillations with a period of about 2.05 ps. Our study should shed some light on the ultrafast photochemistry involving complex molecular cations.

Radiation for laser-driven rescattering near the ultimate high-energy cutoff: The single-atom response and bremsstrahlung limit

The discovery of laser-driven rescattering and high harmonic radiation out to a maximum photon energy of 3.17 times the ponderomotive energy laid the groundwork for attosecond pulse generation and coherent x rays. As the laser field drives the interaction to higher energies, relativity and the Lorentz force from the laser magnetic field enter into the dynamics. Recent studies of laser rescattering have included these effects to give a quantitative description of rescattering dynamics in the high-energy limit, i.e., recollision energies of order 1,000 hartree (27 keV). In this high-energy limit, we treat the emitted high harmonic radiation from rescattering as a bremsstrahlung process. We find the highest-energy photons in the radiated energy spectra follow the rescattering electron energy near the ultimate cutoff. The effective charge from the ion core shielding does not affect the radiation yield at cutoff; however, the overall energy radiated does scale quadratically with the atomic number of the parent ion.

High-order harmonic generation of benzene molecules irradiated by circularly polarized laser pulses

High-order harmonic generation is investigated by using time-dependent density functional theory to numerically simulate the interaction of the benzene molecule with a few-cycle circularly polarized laser pulse. Compared with the harmonics from atoms and diatomic molecules with similar ionization energy irradiated by the same laser pulse, the harmonics from benzene molecules possess higher efficiency and cut-off energy. Further research reveals that the harmonic efficiency and cut-off energy of the benzene molecule increase gradually as the wavelength of the driving laser decreases. Through the analysis of the time-dependent evolution of electronic wave packets, it is found that the harmonic generation can be attributed to the recombination of the ionized electrons and their parent ions. By optimizing the driving laser pulse, we demonstrated that high-order harmonic generation in the benzene molecule can be used to produce isolated circularly polarized attosecond pulse.

Formation of negative and positive ions in the radiosensitizer nimorazole upon low-energy electron collisions.

A comprehensive investigation of low-energy electron attachment and electron ionization of the nimorazole radiosensitizer used in cancer radiation therapy is reported by means of a gas-phase crossed beam experiment in an electron energy range from 0 eV to 70 eV. Regarding negative ion formation, we discuss the formation of fifteen fragment anions in the electron energy range of 0 eV-10 eV, where the most intense signal is assigned to the nitrogen dioxide anion NO2 -. The other fragment anions have been assigned to form predominantly from a common temporary negative ion state close to 3 eV of the nitroimidazole moiety, while the morpholine moiety seems to act only as a spectator in the dissociative electron attachment event to nimorazole. Quantum chemical calculations have been performed to help interpreting the experimental data with thermochemical thresholds, electron affinities, and geometries of some of the neutral molecules. As far as positive ion formation is concerned, the mass spectrum at the electron energy of 70 eV shows a weakly abundant parent ion and C5H10NO+ as the most abundant fragment cation. We report appearance energy (AE) measurements for six cations. For the intact nimorazole molecular cation, the AE of 8.16 ± 0.05 eV was obtained, which is near the presently calculated adiabatic ionization energy.

Composition of Ultrafine Particles in Urban Beijing: Measurement Using a Thermal Desorption Chemical Ionization Mass Spectrometer.

Ultrafine particles (UFPs) dominate the particle number population in the urban atmosphere and revealing their chemical composition is important. The thermal desorption chemical ionization mass spectrometer (TDCIMS) can semicontinuously measure UFP composition at the molecular level. We modified a TDCIMS and deployed it in urban Beijing. Radioactive materials in the TDCIMS for aerosol charging and chemical ionization were replaced by soft X-ray ionizers so that it can be operated in countries with tight regulations on radioactive materials. Protonated N-methyl-2-pyrrolidone ions were used as the positive reagent ion, which selectively detects ammonia and low-molecular weight-aliphatic amines and amides vaporized from the particle phase. With superoxide as the negative reagent ion, a wide range of inorganic and organic compounds were observed, including nitrate, sulfate, aliphatic acids with carbon numbers up to 18, and highly oxygenated CHO, CHON, and CHOS compounds. The latter two can be attributed to parent ions or the decomposition products of organonitrates and organosulfates/organosulfonates, respectively. Components from both primary emissions and secondary formation of UFPs were identified. Compared to the UFPs measured at forest and marine sites, those in urban Beijing contain more nitrogen-containing and sulfur-containing compounds. These observations illustrate unique features of the UFPs in the urban environment and provide insights into their origins.

Simultaneous determination of DL-cysteine, DL-homocysteine, and glutathione in saliva and urine by UHPLC-Q-Orbitrap HRMS: Application to studies of oxidative stress

A high-sensitivity and -selectivity mass spectrometry derivatization reagent, (R)-(5-(3-isothiocyanatopyrrolidin-1-yl)-5-oxopentyl) triphenylphosphonium (NCS-OTPP), was developed for the enantiomeric separation of chiral thiol compounds as prospectively important diagnostic markers for oxidative stress-related diseases. Complete separation of GSH, DL-Cys, and DL-Hcy was achieved. The parent ions of all derivatives had a fragment of m/z 473.18 and a structure of m/z 75.95 (R-S = C-S-R’), conducive to qualitative and quantitative analysis. Good linear relationships were obtained for all analytes (R2≥ 0.9995). The intra-day and inter-day precision were 0.82–5.16 % and 1.02–4.18 % in saliva, and 0.81–3.45 % and 0.99–6.47 % in urine, with mean recoveries of 83.31–105.66 % and 84.09–101.11 %, respectively. The limit of detection (S/N = 3) was 19.20–57.60 nM. Free and total GSH, DL-Cys, and DL-Hcy were detected simultaneously in saliva and urine from 10 volunteers in the normal, stressed, and stable states by UHPLC-Q-Orbitrap HRMS. The thiol compounds were quantitatively related to oxidative stress state changes.

Comparison of the double-photoionization processes in triphenylene and corannulene with coronene

We have studied the production of doubly charged molecular ions by a single photon for the aromatic molecules triphenylene (C18H12) and corannulene (C20H10) using monochromatized synchrotron radiation from 18 eV to 270 eV. We compare our results with previously published data for partially deuterated benzene (C6H3D3), pyrene (C16H10), and coronene (C24H12). The question that we address in this paper is how the different but similar molecular structures of coronene, corannulene, and triphenylene affect the photon-energy dependence of the ratio of doubly to singly charged parent ions. A theoretical analysis of the main features in terms of independent molecular subsystems will be discussed.

Quantitative retrieval of the angular dependence of laser-induced electron rescattering in molecules

We demonstrate a time-domain method to quantitatively retrieve the angular dependence of laser-induced electron rescattering from the measured rotational half-revival signals of molecules with a reaction microscope. The method is based on the measured channel-resolved ion yields of single ionization and electron-rescattering-induced double ionization and the in situ measured alignment distribution of molecules. From the measured data for ${\mathrm{CO}}_{2}$, we retrieve the angular dependence of electron rescattering in nonsequential double ionization for both nondissociative and dissociative cases. The results imply that the angular dependence of electron rescattering is affected by the state of the parent ion populated during the first ionization step. The method demonstrated here opens up the possibility to quantitatively retrieve from measured data which molecular orbitals are involved in the interaction of strong laser fields with molecules.

Dissociative Photoionization of Methyl Vinyl Ketone-Thermochemical Anchors and a Drifting Methyl Group.

The dissociative photoionization of methyl vinyl ketone (MVK), an important intermediate in the atmospheric oxidation of isoprene, has been studied by photoelectron photoion coincidence spectroscopy. In the photon energy range of 9.5-13.8 eV, four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C2H3CO+) is formed from ionized MVK by direct methyl loss, while breaking the C-C bond on the other side of the carbonyl group results in the acetyl cation (CH3CO+, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO-loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C2H3+ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C2H3CO+. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed herein for gaseous methyl vinyl ketone (ΔfH0K = -94.3 ± 4.8 kJ mol-1; ΔfH298K = -110.5 ± 4.8 kJ mol-1), together with cationic heats of formation and bond dissociation energies.

Direct measurement of Coulomb-laser coupling

The Coulomb interaction between a photoelectron and its parent ion plays an important role in a large range of light-matter interactions. In this paper we obtain a direct insight into the Coulomb interaction and resolve, for the first time, the phase accumulated by the laser-driven electron as it interacts with the Coulomb potential. Applying extreme-ultraviolet interferometry enables us to resolve this phase with attosecond precision over a large energy range. Our findings identify a strong laser-Coulomb coupling, going beyond the standard recollision picture within the strong-field framework. Transformation of the results to the time domain reveals Coulomb-induced delays of the electrons along their trajectories, which vary by tens of attoseconds with the laser field intensity.

Dissociative Photoionization of Chloro-, Bromo-, and Iodocyclohexane: Thermochemistry and the Weak C-Br Bond in the Cation.

Double-imaging photoelectron photoion coincidence spectroscopy (i2PEPICO) with tunable synchrotron vacuum ultraviolet radiation was used to record threshold ionization mass spectra of the halocyclohexanes C6H11X (X = Cl, Br, and I). Calculations show that experimental dissociative ionization thresholds correspond to thermochemical limits. Among the processes observed (X loss, followed by C2H4 or C3H6 loss; C2H3Cl loss; HCl loss, followed by CH3 or C2H4 loss), halogen atom loss can be used to derive enthalpies of formation and C-X bond energies in the cation. As an ancillary value, we propose a new proton affinity for cyclohexene at PA298K(c-C6H10) = 771.5 ± 1.7 kJ mol-1. The halogen loss onsets 10.74 ± 0.06 eV, 10.125 ± 0.005, and 9.474 ± 0.005 eV thus yield ΔfHo298K(C6H11X (g)) = -164.4 ± 6.2, -114.4 ± 2.3, and -56.3 ± 2.3 kJ mol-1 for X = Cl, Br, and I, respectively. The last two agree with DFT-calculated isodesmic reaction energies very well, as opposed to G4 theory for X = Br. The C-X bond energy in the cation is the lowest for X = Br. This is the sum result of the weakening C-X bond in the neutral and the increasing stabilization of the parent ion with increasing halogen size.

The Coulomb effects in laser-assisted single photoionization

Attosecond photoionization time delays reveal information about an outgoing electron getting away from the Coulomb potential. In this study, we improve the eikonal approximation model that overestimates the laser-Coulomb coupling effect in the laser-assisted photoionization of atoms. The improved eikonal approximation model can be accurate on about the 2 asec level relative to the time-dependent Schrödinger equation results. The improvement of photoionization time delays demonstrates that the electrons ionized by extreme ultraviolet attosecond pulse possesses momentum larger than the asymptotical momentum due to the attractive Coulomb potential of the parent ion. These results will give a deeper insight into understanding the laser-assisted single photoionization.

Where do the counterions go? Tip-induced dissociation of self-assembled triazatriangulenium-based molecules on Au(111).

Chemical coupling of functional molecules on top of the so-called platform molecules allows the formation of functional self-assembled monolayers (SAMs). An often-used example of such a platform is triazatriangulenium (TATA), which features an extended aromatic core providing good electronic contact to the underlying metal surface. Here, we present a study of the SAM formation of a TATA platform on Au(111) employing scanning tunneling microscopy (STM) under ambient atmospheric conditions. In solution, the TATA platform is stabilized by BF4 counterions, while after deposition on a gold substrate, the localization of the BF4 counterions remains unknown. We used 1,2,4-trichlorobenzene as a solvent of TATA-BF4 to induce SAM formation on a heated (∼50 °C) Au substrate. We show by STM how to detect and distinguish TATA-BF4 from TATA platforms, which lost their BF4 counterions. Finally, we observe a change of the counterion position on the SAM during the STM scanning, which we explain by an electric-field-induced decrease of the electrostatic interaction in TATA-BF4 on the surface. We applied DFT calculations to reveal the influence of the gold lattice and the electric field of the STM tip on the stability of TATA-BF4 physisorbed on the surface.

Ionization and fragmentation of fluorene upon 250 keV proton impact

We have investigated the ionization and fragmentation of a polycyclic aromatic hydrocarbon molecule, fluorene ( $$\hbox {C}_{{13}}\,\hbox {H}_{{10}}$$ ), upon proton impact with energy 250 keV. Various ionization and fragmentation products have been identified. Hydrogen-loss process from singly and doubly ionized parent molecules has been found to be a dominating channel. In case of singly ionized molecule, up to three H-losses have been observed. However, two H-loss ion has not been observed. For the doubly ionized molecule up to four H-losses have been observed. The ratio of doubly to singly charged parent ions are found to be quite high compared to the atomic targets, such as, He and Ne.

Ultrafast dynamics of electron excited states of phenylacetylene

Interaction of light with matter has always been important in the field of natural science. Particularly, the ultrafast radiationless relaxation induced by UV light of molecular electronic excited states accompanied by ultrafast energy transfer plays an important role in the natural photophysical, photochemical and biological reactions. Generally, the molecular electronic excited state can be deactivated through a variety of decay channels, including dissociation, isomerization, internal conversion, intersysterm crossing, vibrational energy redistribution, and autoionization. This complexity of relaxation channels brings about a wide variety of deactivation mechanisms. The ultrafast nonadibatic relaxation dynamics of the excited state of phenylacetylene is studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S&lt;sub&gt;2&lt;/sub&gt; of phenylacetylene is excited by 235 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The time-dependent curves of parent ions include two exponential curves. One is the fast component with a time constant of 116 fs, and the other is the slow component with a time constant of 106 ps. The time-resolved photoelectron kinetic energy distribution is obtained from the time-resolved photoelectron images. Combined with the time-resolved photoelectron spectroscopy data, the fast component with a time constant of 116 fs is found to reflect the internal conversion process from S&lt;sub&gt;2&lt;/sub&gt; state to S&lt;sub&gt;1&lt;/sub&gt; state. The experimental results also show that S&lt;sub&gt;1&lt;/sub&gt; state is arranged by internal conversion, and the inter system jump process to T&lt;sub&gt;1&lt;/sub&gt; state is an important attenuation channel. This work provides a clearer physical picture for S&lt;sub&gt;1&lt;/sub&gt; state nonadibatic relaxation dynamics of phenylacetylene.