Charged Parent Ions Research Articles

Electron impact fragmentation of adenine: partial ionization cross sections for positive fragments

Using computer-controlled data acquisition we have measured mass spectra of positive ions for electron impact on adenine, with electron energies up to 100 eV. Ion yield curves for 50 ions have been obtained and normalized by comparing their sum to the average of calculated total ionization cross sections. Appearance energies have been determined for 37 ions; for 20 ions for the first time. All appearance energies are consistent with the fragmentation pathways identified in the literature. Second onset energies have been determined for 12 fragment ions (for 11 ions for the first time), indicating the occurrence of more than one fragmentation process e.g. for 39 u (C2HN+) and 70 u (C2H4N 3 + ). Matching ion yield shapes (118–120 u, 107–108 u, 91–92 u, and 54–56 u) provide new evidence supporting closely related fragmentation pathways and are attributed to hydrogen rearrangement immediately preceding the fragmentation. We present the first measurement of the ion yield curve of the doubly charged parent ion (67.5 u), with an appearance energy of 23.5 ± 1.0 eV.

Fragmentation of multiply ionized biomolecules induced by MeV heavy ions

SynopsisCollision-induced ionization and fragmentation of gas-phase adenine molecules (C5N5H5) have been investigated for single electron capture collisions of 1.2 MeV C2+ ions. Coincidence measurements between the number of ejected electrons and the time-of-flight of fragment ions allows us to determine charge states of intermediate parent ions r. Highly charged parent ions are observed up to r ∼ 6 and the production of lighter fragment ions becomes predominant with increasing r. We found that kinetic energies of H+ are significantly larger than those of other fragment ions and become nearly equivalent irrespective of r for r > 1.

Observation of hydrogen migration in cyclohexane under an intense femtosecond laser field.

We experimentally demonstrate hydrogen migration in photoionization and photofragmentation processes of cyclohexane (C6H12) under a near-infrared (800 nm) intense femtosecond laser field by the dc-slice imaging technique, and the observation of fragment ions CH3(+), C2H5(+), and C3H7(+) can be regarded as direct evidence that chemical bond rearrangement processes associated with hydrogen migration occur in the dissociative ionization process of cyclohexane. We measure the sliced images of fragment ions CH3(+), C2H5(+), and C3H7(+) and calculate their corresponding kinetic energy release (KER) and angular distributions, and it is confirmed that high-KER components of these fragment ions result from two-body Coulomb explosion of the doubly charged parent ion C6H12(2+) whereas low-KER components result from dissociative ionization of the singly charged parent ion C6H12(+). Moreover, we measure the maximally attainable relative yields of fragment ions CH3(+), C2H5(+), and C3H7(+), which approach 4.5%, 4.0%, and 3.0%, respectively.

Correlation between multiple ionization and fragmentation ofC2H6in charge-changing collisions with580−keVC+

We investigate correlations between multiple ionization and fragmentation processes of the ethane molecule in collisions with $580\text{\ensuremath{-}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}{\mathrm{C}}^{+}$ ions under single-electron capture and loss conditions. Employing an electron counting technique, we directly obtain number distributions of ionized electrons, which correspond to distributions of multiple ionization probabilities of ethane. In addition, fragmentation patterns as a function of the charge state $r$ of intermediate parent ions ${\mathrm{C}}_{2}{{\mathrm{H}}_{6}}^{r+*}$ are obtained from coincidence measurements between the time of flight of the product ions and the number of electrons emitted. Fragmentation patterns in the different charge-changing conditions reveal a crucial role of the internal excitation in the fragmentation processes. Also, we provide clear evidence of strong selectivity on the parent charge state for formation of the ${{\mathrm{H}}_{3}}^{+}$ ion, which is exclusively generated through doubly charged parent ions ${\mathrm{C}}_{2}{{\mathrm{H}}_{6}}^{2+*}$.

Photodissociation ofBr2molecules in an intense femtosecond laser field

We experimentally demonstrate the photodissociation process of ${\mathrm{Br}}_{2}$ molecules in the intense femtosecond laser field by a dc-sliced ion velocity map imaging technique. We show that four fragment ions $\mathrm{B}{\mathrm{r}}^{n+}\phantom{\rule{0.16em}{0ex}}\left(n=1--4\right)$ are observed, and their kinetic energy increases while their angular distribution decreases with the increase of the charge number. We prove that the low (or high) charged fragment ions result from the photodissociation of the low (or high) charged parent ions. We explain the changes of the kinetic energy and angular distribution in these fragment ions by considering the potential energy curves of these parent ions that involve both the interaction of the Coulomb repulsive energy and chemical bonding energy. We also explain the experimental observation that the measured kinetic energy release in the experiment is much smaller than the theoretical calculation by enhanced ionization at a critical distance.

Interaction of toluene with two-color asymmetric laser fields: controlling the directional emission of molecular hydrogen fragments.

The interaction of toluene with strong asymmetric two-color laser irradiation of 40 fs duration is studied by means of Time of flight mass spectrometry. Highly energetic H2(+) and H3(+) fragment ions are produced through an isomerization process taking place within transient multiply charged parent ions. Comparative study of deuterium labeled toluene isotopes enables the discrimination between molecular hydrogen fragments formed exclusively within the CH3- part from those that require hydrogen atom exchange between the former and the phenyl moiety. It is demonstrated that by manipulating the relative phase of the ω/2ω field components the selective ionization of oriented toluene molecules can be used as a tool to control the directional emission of the H2(+), H3(+) species.

Concerted elimination of Br2+ resulting from the Coulomb explosion of 1,2-dibromoethane in an intense femtosecond laser field

Concerted elimination pathway leading to Br2+ and C2H4+ from 1,2-dibromoethane molecule has been investigated in 800nm femtosecond laser field by dc-slice imaging technology. The kinetic energy release and angular distributions of Br2+ and C2H4+ demonstrate that Br2+ results from the two-body Coulomb explosion of the doubly charged parent ion. Ab initio calculations show that the doubly charged precursor overcomes a small energy barrier (0.18eV) and then dissociates into Br2+ and C2H4+ through a synchronous concerted elimination mechanism. Moreover, the relative yield of the Br2+ channel is obtained, and it remains about 3.4% when the laser intensity exceeds 1.0×1014W/cm2.

Double photoionization of pyrene and other aromatic hydrocarbons

We have measured the ratio of doubly to singly charged parent ions of pyrene, an aromatic hydrocarbon, for photon energies ranging from threshold to the carbon K edge. Monochromatized synchrotron radiation of the Synchrotron Radiation Center has been used in combination with an ion time-of-flight spectrometer for this investigation. We compared these results with our previous measurements on benzene, coronene and pyrrole. We will discuss differences and similarities in the double photoionization for these aromatic hydrocarbons.

Ion–ion coincidence imaging of dissociative ionization dynamics of formic acid in intense laser fields

We have studied the dissociative ionization dynamics of formic acid molecules with intense femtosecond laser pulses centered at 800nm, employing the ion–ion coincidence imaging technique. Fragmentation reactions originating from both singly and doubly charged parent formic acid ions are observed. In this paper the kinetic energy distributions of several different ionic fragments will be presented. A clear difference in the kinetic energy distributions is observed between ionic fragments originating from the dissociation of the singly charged parent ion and those from the Coulomb explosion of the doubly charged parent ion. Channels exhibiting the emission of neutral hydrogen and channels exhibiting intra molecular hydrogen migration will also be discussed. We present a combined theoretical and experimental study to get to an understanding of the hydrogen migration mechanism at the molecular level.

The dissociation behaviour of a singly and doubly charged (cationised) aza-crown ether/metalloporphyrin-conjugate: Metal-induced reactivity and Coulomb explosions

Fe(III) and Mn(III) complexes of an aza-crown ether/porphyrin conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-5,10,15,20-tetraphenylporphyrin, with Cl− and Br− as respective counter ions, are studied by electrospray ionisation (ESI)-mass spectrometry (MS). Fragmentation pathways are established by MS2 and MS3 collision-induced dissociation (CID) experiments performed with a quadrupole ion trap (QIT) and energy-dependent CID is studied with a quadrupole/time-of-flight (q-ToF) instrument. Singly charged ions are produced by selective protonation, sodiation or potassiation of the crown ether moiety, and dications possess one additional charge on the halide-released porphyrin metal cation. Singly charged parent ions undergo a two-step decay. Firstly, an “ion pair” is lost, composed of the cationic charge carrier of the crown ether and the anionic counter ion at the porphyrin. Secondly, the intact crown ether is released. The threshold of this process is dependent on the nature of the central porphyrin metal, with Fe(III) dramatically facilitating the reaction. The dications undergo two different Coulomb explosions: crown/porphyrin-fission and cation loss from the crown ether, the ratio of which is dependent on the affinity of the cation to the crown ether.

Isomer effects in the double-to-single photoionization ratio of aromatic hydrocarbons

We have measured the ratios of doubly to singly charged molecular parent ions of azulene and phenanthrene, which are isomers of naphthalene and anthracene, respectively. The photon-energy dependences of the ratio curves have been analyzed and compared to each other from threshold to the carbon $K$ edge. Qualitatively, the ratio curves for azulene and naphthalene as well as phenanthrene and anthracene are very similar, but we have found a distinct quantitative difference in both cases for the two isomers.

Photo-double-ionization mechanisms in aromatic hydrocarbons

We have measured the ratios of doubly to singly charged molecular parent ions of benzene (partially deuterated), naphthalene, anthracene, pentacene, pyrrole, furan, selenophene, and coronene for photon energies ranging from threshold to the carbon $K$ shell. The photon-energy dependence of the ratio curves has been analyzed and compared to each other for the above molecules. We conclude that two---and in some cases three---different photo-double-ionization mechanisms exist for aromatic hydrocarbons. One of the mechanisms is the formation of a two-electron pseudoparticle. This finding may be useful in the quest for understanding high-temperature superconductivity.

Tandem DMA Generation of Strictly Monomobile 1–3.5 nm Particle Standards

Generation of monomobile molecular standards by electrospray (ES) followed by classification in a differential mobility analyzer (DMA) fails at diameters above ∼2 nm because many clusters in different charge states z crowd in a narrow mobility range. Use of a second DMA (DMA2) in series (tandem) with DMA1 is very helpful because, unexpectedly, many multiply charged ions selected in DMA1 undergo spontaneous transitions, appearing as pure species at different mobilities in DMA2. Remarkably, for salt clusters of composition (CA) n (C+ ) z carrying z elementary charges and n neutral ion pairs, (i) ion evaporation (CA) n (C+ ) z →(CA) n –1(C+ ) z– 1+(CA)C+ and (ii) neutral evaporation transitions (CA) n (C+ ) z →(CA) n –1(C+ ) z+CA affect a substantial fraction of the clusters. Neutral evaporation (fueled by the Kelvin effect) is effective in isolating singly charged clusters, yielding mobility standards easily exceeding 2 nm. Ion evaporation (fueled by large electric fields) produces even larger well-resolved standards. Singly charged clusters of up to 2.5 nm rising in isolation result from metastable doubly charged parent ions (z = 2→1 transition). Isolated doubly charged ions of up to 3.5 nm arise from the z = 3→2 transition, but are harder to resolve from the products of higher initial charge states. We report tandem DMA measurements for electrosprayed nanodrops of two ionic liquids: EMI-Im and EMI-Methide, both based on the small cation EMI+ (1-Ethyl-3-methylimidazolium+) and two relatively large anions: Im− = (CF3SO2)2N−; Methide− = (CF3SO2)3C−. Some exploration on the effect of actively reducing the charge on the clusters as they pass between both analyzers is also included. Copyright 2013 American Association for Aerosol Research

Photoemission of Cooper Pairs from Aromatic Hydrocarbons

We report the discovery of the formation of an electron Cooper pair approximately 40 eV above the double-ionization threshold in benzene, naphthalene, anthracene, and coronene after absorption of a single photon. We have measured the ratios of doubly to singly charged parent ions of the above mentioned molecules as well as pyrrole and furan by using monochromatized synchrotron radiation up to 100 eV above the corresponding thresholds. We also recorded photoelectron spectra of benzene and naphthalene at selected energies. The electron-pair formation is based on the specific structure of the molecules and does not exist for pyrrole and furan.

Large Molecules Reveal a Linear Length Scaling for Double Photoionization

We have measured the ratio of doubly to singly charged parent ions of benzene, naphthalene, anthracene, and pentacene using monochromatized synchrotron radiation up to 30 eV above the corresponding threshold. Our measurements show a striking similarity between the ratio of doubly charged to all parent ions and the ratio for helium. Moreover, the magnitudes of the ratios for these molecules scale linearly with their lengths with an amazing accuracy. A high ratio, i.e., a high relative double-photoionization probability, makes a molecule an important source of low-energy electrons that can promote radiation damage of biomolecules [B. Boudaïffa et al., Science 287, 1658 (2000)].

Dissociative double ionization of 1-bromo-2-chloroethane irradiated by an intense femtosecond laser field

The dissociative double ionization and multi-photon ionization of 1-bromo-2-chloroethane (BCE) irradiated by the 800 nm femtosecond laser field have been investigated by dc-slice imaging technology. The charged parent ion ratio [BCE(2+)]/[BCE(+)] was measured, and the corresponding ionization process including non-sequential double ionization and sequential double ionization was analyzed. The sliced images of different photo-dissociated ions were detected, and the corresponding kinetic energy release (KER) distributions were calculated and extracted. Furthermore, the dissociative double ionization channels, attributed to the cleavage of the C-C, C-Br, and C-Cl bonds by the Coulombic repulsive forces, were discussed, and the revised equilibrium distance R(e)*, the energy ratio E(exp)/E(coul), and the value a=√(R(e)*)/(E(exp)/E(coul)) were calculated.

Profiling Cys34 Adducts of Human Serum Albumin by Fixed-Step Selected Reaction Monitoring

A method is described for profiling putative adducts (or other unknown covalent modifications) at the Cys34 locus of human serum albumin (HSA), which represents the preferred reaction site for small electrophilic species in human serum. By comparing profiles of putative HSA-Cys34 adducts across populations of interest it is theoretically possible to explore environmental causes of degenerative diseases and cancer caused by both exogenous and endogenous chemicals. We report a novel application of selected-reaction-monitoring (SRM) mass spectrometry, termed fixed-step SRM (FS-SRM), that allows detection of essentially all HSA-Cys34 modifications over a specified range of mass increases (added masses). After tryptic digestion, HSA-Cys34 adducts are contained in the third largest peptide (T3), which contains 21 amino acids and an average mass of 2433.87 Da. The FS-SRM method does not require that exact masses of T3 adducts be known in advance but rather uses a theoretical list of T3-adduct m/z values separated by a fixed increment of 1.5. In terms of added masses, each triply charged parent ion represents a bin of ±2.3 Da between 9.1 Da and 351.1 Da. Synthetic T3 adducts were used to optimize FS-SRM and to establish screening rules based upon selected b- and y-series fragment ions. An isotopically labeled T3 adduct is added to protein digests to facilitate quantification of putative adducts. We used FS-SRM to generate putative adduct profiles from six archived specimens of HSA that had been pooled by gender, race, and smoking status. An average of 66 putative adduct hits (out of a possible 77) were detected in these samples. Putative adducts covered a wide range of concentrations, were most abundant in the mass range below 100 Da, and were more abundant in smokers than in nonsmokers. With minor modifications, the FS-SRM methodology can be applied to other nucleophilic sites and proteins.

Multiple ionization of oxygen studied by coincident measurement

We experimentally study double and triple ionization of oxygen using a reaction microscope. The kinetic energy releases (KERs) and angular distributions are obtained through coincidentally measuring the ionic fragments of doubly or triply charged parent ions. The pathway O(2+)2 ? O (+) O(+)?proceeds through some excited electronic states. The KERs exhibit definite structures independent of the laser intensity and the pulse duration. However, the angular distribution of coincident O(+) reflects the symmetry of the highest occupied molecular orbital (HOMO) only for few-cycle laser pulses at low intensity. The pathways O(2+) 2 --> O(2+) +O and O(3+) 2 --> O(2+) + O(+) occur through some repulsive states. The KERs show a single broad peak and decrease with increasing the pulse duration. The decrease of KER comes from the stretch of the internuclear distance in intense laser fields.

Analysis of Large Peptides by MALDI Using a Linear Quadrupole Ion Trap with Mass Range Extension

Analysis of large peptides can be used to discover or to monitor biomarkers for various diseases. For example, the levels of such peptides can determine the effectiveness of an experimental drug or the progress of a disease. Many mass spectrometric methods for monitoring these peptides use MALDI-ToF instruments because of their high molecular weights, although such instruments typically lack MS/MS or MS(n) capabilities. Here, the m/z range of a MALDI-LIT instrument was extended to m/z 5500 for the MS or MS(n) analysis of large peptides. Instrument performance was examined using amyloid beta 1-40 and 1-42 (avg. MW 4330.8 and 4515.0, respectively), large peptides that comprise the bulk of neuritic plaques and are potential biomarkers for Alzheimer's Disease. The amyloid beta 1-40 was detected in the full-scan mass spectrum with sufficient resolution to distinguish and match the expected isotopic pattern. The MS/MS product ion spectra of both peptides matched the expected fragmentation patterns; up to MS(4) experiments were performed to verify the identity of the peptides. These experiments clearly demonstrate the advantages of this approach, including MS(n) experiments for structural elucidation and simplified spectra due to singly charged parent ions, for large peptides.

IONIZATION AND FRAGMENTATION OF ANTHRACENE UPON INTERACTION WITH keV PROTONS AND α PARTICLES

The interaction of keV ions with polyaromatic hydrocarbons is dominated by charge exchange and electronic stopping. We have studied the response of the polyaromatic hydrocarbon anthracene (C14H10) upon keV H+ and He2+ impact using high-resolution time-of-flight mass spectrometry. Extensive fragmentation into small C n H q+ m as well as formation of up to triply charged parent ions is observed. Ab initio electron densities are used to calculate the molecular excitation due to electronic stopping. Fragment yields increase with the increase of electronic stopping as a function of projectile velocity. For equal electronic stopping, He2+ is found to induce more fragmentation than H+. The difference in fragmentation is concluded to be due to two electron processes, which are relevant channels only for He2+.