Threshold electron impact ionization studies of uracil

Electron impact ionization of 5- and 6-chlorouracil: appearance energies

Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

A method 1 for the computerized acquisition of ionization efficiency data followed by mathematical analysis using the energy-distribution difference technique,2 which is a considerable improvement on conventional mass spectrometric techniques, has been applied to the determination of the ionization and appearance potentials of a variety of compounds and processes. Comparisons of the electronimpact ionization potential data with photoelectron spectroscopic measurements show that it is possible to measure adiabatic ionization potentials by electron impact in favourable cases and indicates a reason for the consistently high values found by other methods. The accuracy of measurement of appearance potentials approaches that of photoionization methods and in one case evidence for a preionization process is found. A high activation energy process is shown to occur without the expected large kinetic shift in appearance potential, suggesting that this computerized method is very sensitive. The measurements also yield a heat of formation and ionization potential for the "phenyl" fragment ion from monosubstituted benzenes.

Dissociative-ionization cross sections, fragment appearance potentials, and fragment kinetic energies were measured for electron-impact excitation of jet-cooled NH3, hydrazine (N2H4), and monomethyl hydrazine (MMH) over an energy range of 10–270 eV. A data base of 35 parent and fragment ions is reported. All measurements were made in a crossed electron–molecular beam apparatus using pulsed extraction and time-of-flight mass detection to ensure field-free excitation and high collection efficiency for energetic ions. Cross sections for NH3 ionization are in good agreement with previous measurements except for ions with high kinetic energy (KE). These discrepancies are attributed to instrument-dependent KE detection efficiencies in the previous results. Cross section data have not been previously reported for N2H4 and MMH. The measured cross sections for total ionization at 70 eV are 2.35 Å2 (NH3), 3.76 Å2 (N2H4), and 4.20 Å2 (MMH). KE distributions were measured by an ion deflection method and gave results consistent with time-of-flight peak-shape analysis. Mean KE values <εt≳ are reported for all fragment ions studied. For 170-eV excitation of NH3, <εt≳ varied from 0.026 eV (NH2+) to 1.4 eV (H+). The kinetic energies for N2H4 and MMH fragment ions at similar excitation energies are typically much lower than for fragment ions from NH3, conforming to statistical arguments based on density of internal states. High resolution mass spectra were recorded for MMH in order to distinguish different fragment ions of the same unit mass. Substantial rearrangement is evident for N2H4 and MMH dissociative ionization based on the appearance of ions such as NH3+ and NH4+ (the latter for MMH ionization only) and the magnitude of <εt≳ for certain ions. The role of electronic structure and geometry on dissociation is explored using a molecular orbital analysis to predict product correlations for the excited states of N2H4+.

Protonated molecules of 2,5-di-R1,R2-substituted-1,3,4-oxadiazoles lose isocyanic acid (loss of mass 43) via skeletal rearrangement. This was observed in low-energy CID mass spectra recorded by using electrospray ionisation (ESI). On electron ionisation induced decomposition these compounds also reveal complex skeletal rearrangement, consisting on N2 and CO elimination, leading to the formation of fluorene or indene type ions. It was found that abundances of fragment ions formed in these processes are in good agreement with their theoretically calculated stabilities. In the case of ESI use the fragment ion abundances were calculated in relation to [M+H]+ ion abundances and for EI use, where extensive fragmentations proceed, the sums of the abundances of [M-N2-CO]+. ions and abundances of fragment ions derived from them, were expressed as a percentage of total-ion current.

We previously discovered that squalene monohydroperoxide (SQ-OOH) was produced on human forehead skin and suggested that skin squalene (SQ) may be the principal target lipid for oxidative stress (e.g., sunlight exposure). Because of its six double bonds, SQ peroxidation can yield various positional hydroperoxide isomers. However, the structural characterization of skin SQ-OOH isomers has never been reported. Here, we prepared pure SQ-OOH isomers and developed an analytical method for SQ-OOH isomers using a quadrupole/linear ion-trap mass spectrometer (QTRAP) MS/MS system. Collision-induced dissociation produced specific fragment ions for each SQ-OOH isomer, which permitted discrimination between SQ-OOH isomers by multiple reaction monitoring (MRM). When lipid extract from human forehead skin was subjected to LC-MS/MS with MRM, individual SQ-OOH isomers could be separated and detected with a sensitivity of 0.05 ng/injection. The total concentration of SQ-OOH isomers in forehead skin was approximately 956 microg/g skin lipids, but it increased up to 2,760 microg/g skin lipids after 3 h of sunlight exposure. The LC-MS/MS method was useful for investigating the peroxidation mechanisms of SQ as well as SQ-OOH-mediated skin disorders.

The retarding potential difference method has been used to measure ionization efficiency curves of CH3X+ (X=H, F, Cl, Br, I), CH3+, and X+ from methane and the methyl halides in a time-of-flight mass spectrometer. Discontinuities between linear segments of the ionization efficiency curves are interpreted as appearance potentials (A.P.) of electronic states or processes. For all CH3+, A.P.'s occur at 1.1, 1.7, 2.7, and about 3.4 eV above onset in the ion pair region. Empirical knowledge of this structure makes it possible to identify the A.P. for the ion—neutral pair, CH3+—X, in their ground states. Combining these data and the A.P.'s for X+/CH3X gives the ionization potential for CH3 and the bond dissociation energies, D(CH3—X).

The influence of geometrical isomerism on the behaviour of cis and trans hexahydroindanes under electron impact appears in two different ways.Firstly, mass spectra, although qualitatively identical, show important differences between abundances of several main fragment ions, especially C5H7+ and C6H9+. Relative abundances of molecular ions also differ, by about 15%, and indicate a greater stability of ionized trans isomer.Secondly, from the standpoint of energetics, two results are to be noticed. (a) Ionization potentials of cis and trans hexahydroindanes are the same within experimental uncertainties, which means that an effect due to isomerism appears in both ionized molecules. (b) Appearance potentials of the main fragment ions show that a lower energy is required to dissociate ionized cis hexahydroindane into fragments, which has to be related to the energy provided by steric conformation in this molecule.These results are in perfect agreement with those previously obtained in a study of cis and trans decalines.

We present results on the valence level excitation, ionization and dissociation of adenine, using time-of-flight mass spectrometry and synchrotron radiation, in the vacuum ultraviolet (VUV) range of 12-21 eV. The measurements were performed using a gas-phase (Ne) harmonics filter in order to eliminate contributions from higher-order harmonics. Mass spectra were obtained using the photoelectron-photoion coincidence technique (PEPICO). The relative abundances for each ionic fragment and their mean kinetic energy release have been determined from the analysis of the corresponding peak shapes in the mass spectra. Comparison with the available photoelectron spectra and previous measurements allowed the assignment of the main features in the spectra. A discussion on the dissociative photoionization channels of this molecule has also been included. Due to our harmonics-free incident photon beam we were able to propose new appearance energy (AE) for the most important ionic channels in this energy range. The precursor ion, C(5)H(5)N(5)+, is the most abundant species (40% at 15 eV and 20% at 20 eV), which confirms the high stability of adenine upon absorption of VUV photons. We have observed other intense fragment ions such as: C(4)H(4)N(4)+, C(3)H(3)N(3) (+), C(2)H(2)N(2)+ and HCNH+. The production of the neutral HCN fragment represents up to 40% of the dissociative channels for this molecule as induced by VUV photons.

Photon-induced dissociation with a four-sector tandem mass spectrometer

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for mandelic acid (MA) and mandelic acid methyl ester (MAME) . An ionization energy of 8.55 eV was obtained for both compounds. For the formation of C 6 H 5 + ions, appearance energies of 14.25 eV or 14.40 eV were determined for MA or MAME, respectively. On the main fragmentation route from molecular ions to these C 6 H 5 + ions, an activation energy of (1.1±0.1)eV in excess of the calculated total reaction enthalpy was found for the elimination of CO from the prominent C 7 H 7 O + ions to C 6 H 7 + ions, involving the transfer of two H-atoms. The second (minor) fragmentation route via C 7 H 6 O + ions resembles the benzaldehyde decomposition pathway and the individual steps were found to be near to their calculated thermochemical reaction enthalpies. Gas-phase enthalpies of formation of (–4.652±0.3 eV) [≡ (–448.8±30) kJ mol −1 ] for MA and (–4.243±0.3) eV [≡ (–409.4±30) kJ mol −1 ] for MAME are derived, based on the appearance energy of their C 7 H 7 O + ions at m/z 107 and the formation enthalpy adopted from protonated benzaldehyde. The results are compared with data for benzylalcohol.

When collisionally activated dissociation (CAD) of glycerophosphocholine (GPC) species is examined using quadrupole ion trap mass spectrometry (QITMS), the spectral patterns differ from those obtained using sector or quadrupole mass spectrometry. Methods employed in the structural analysis of GPCs using a sector or quadrupole mass spectrometers are not necessarily useful for an ion trap mass spectrometer. A novel method is presented for structurally analyzing GPCs that involves the CAD of trifluoroacetic acid (TFA) adducts of kaliated GPCs. Solutions of GPCs in 0.1% TFA/methanol were electrosprayed to produce precursor ions by attaching a trifluoroacetic acid (TFA) molecule to a kaliated GPC molecule. The CAD-MS/MS spectra obtained by QITMS revealed a dramatic increase in the abundance of fragment ions, corresponding to the losses of sn-1 and sn-2 fatty acyl substituents. A preferential loss of the sn-1 fatty acyl group over the loss of the sn-2 fatty acyl group was observed among the GPC standards examined. A GPC extract from egg yolk was directly analyzed by this method without prior separation. The identities and positions of fatty acyl substituents of over 20 GPC species were identified. Some isomers present in very low relative abundance, which could not be analyzed by QITMS/MS using other ions as precursors, were identified by the TFA attachment method.

The ionization potentials of methyl ethers of D‐homoequilenine, its 14ß‐isomer, D‐homoestrone and its 8°‐isomer, as well as appearance potentials of certain fragment ions have been determined by means of electron‐impact. The dissociation energies of the bonds which are split in the course of formation of these ions have been calculated.

Highly branched xyloglucan oligosaccharides were analyzed by the post-source decay (PSD) fragmentation method of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The ratio of [M-Xyl]+ and [M-Gal]+ fragment ion intensities could be used to characterize the degree of Gal substitution at the non-reducing end, because the number of possible chemical species was directly related to their relative ion intensity. The intensity of the [M-Fuc]+ ion was predominantly strong in the fragment spectrum of fucosyl oligosaccharides as the first fragmentation, indicating the fucosyl linkage to be much weaker than the other glycosidic linkages in the MALDI-PSD fragmentation. Setting fragment ion [M-Fuc]+ to the pseudo precursor ion [MF]+, the second fragmentation ions were produced from [MF]+ in the drift region in PSD fragmentation of fucosyl oligosaccharides.

Optimization of experimental parameters for electron capture dissociation of peptides in a Fourier transform mass spectrometer