Secondary Ion-molecule Reactions Research Articles

Investigation of neuroleptics and other aromatic compounds by laser-based ion mobility mass spectrometry

Laser-based ion mobility (IM) spectrometry was used for the detection of neuroleptics and PAH. A gas chromatograph was connected to the IM spectrometer in order to investigate compounds with low vapour pressure. The substances were ionized by resonant two-photon ionization at the wavelengths λ = 213 and 266 nm and pulse energies between 50 and 300 μJ. Ion mobilities, linear ranges, limits of detection and response factors are reported. Limits of detection for the substances are in the range of 1-50 fmol. Additionally, the mechanism of laser ionization at atmospheric pressure was investigated. First, the primary product ions were determined by a laser-based time-of-flight mass spectrometer with effusive sample introduction. Then, a combination of a laser-based IM spectrometer and an ion trap mass spectrometer was developed and characterized to elucidate secondary ion-molecule reactions that can occur at atmospheric pressure. Some substances, namely naphthalene, anthracene, promazine and thioridazine, could be detected as primary ions (radical cations), while other substances, in particular acridine, phenothiazine and chlorprothixene, are detected as secondary ions (protonated molecules). The results are interpreted on the basis of quantum chemical calculations, and an ionization mechanism is proposed.

Electron attachment to chlorine azide at 298 and 400 K

Electron attachment to chlorine azide (ClN(3)) was studied using a flowing-afterglow Langmuir-probe apparatus. Electron attachment rates were measured to be 3.5x10(-8) and 4.5x10(-8) cm(3) s(-1) at 298 and 400 K, respectively, with an estimated 35% absolute accuracy. Cl(-) was the sole ion product of the attachment reaction; weak ion signals were observed for other anions and attributed to impurities and secondary ion-molecule reactions. Assuming a relative uncertainty of +/-10% for these data, an activation energy for the attachment reaction may be given as 24+/-10 meV.

Laser Desorption/Ablation Plumes from Capillary-Like Restricted Volumes

Laser desorption/ionization from structured surfaces has been the object of recently renewed interest. Conditions in the plume of material ablated from such surfaces may differ from those of a sample which is ablated in bulk. Since recombination and secondary ion-molecule reactions in the plume play a major role in determining the types and quantities of ions observed at the detector, these differences are analytically relevant. Desorption/ionization substrates with channels of high aspect ratio are modeled as capillary nozzles, from which free jets are emitted. A previously developed matrix-assisted laser desorption/ionization ablation/ionization model is adapted for these jets. More primary ions reach the detector when ablated from a capillary orifice, but fewer analye ions are created in secondary reactions. These differences in ion yield can persist for arrays of capillaries on the surface, depending on the ratio of their diameter to spacing.

Positive/negative ion ratios and in-plume reaction equilibria in MALDI

Recent experimental data on positive vs negative ion ratios in MALDI mass spectra are compared with predictions of the thermodynamic model of secondary ion–molecule reactions. Taking mass balance into account, the ion ratios are predicted to remain close to 1, as long as the forward reactions are favorable. This is remains true for both high and low analyte concentrations, and high or low laser intensities. The data support approach to local thermodynamic equilibrium in the MALDI plume.

Functional Screening of Serine Protease Inhibitors in the Medical Leech Hirudo medicinalis Monitored by Intensity Fading MALDI-TOF MS

The blood-feeding invertebrates are a rich biological source of drugs and lead compounds to treat cardiovascular diseases because they have evolved highly efficient mechanisms to feed on their hosts by blocking blood coagulation. In this work, we focused our attention on the leech Hirudo medicinalis. We performed, by "intensity fading" MALDI-TOF mass spectrometry, a comprehensive detection and functional analysis of pre-existent peptides and small proteins with the capability of binding to trypsin-like proteases related to blood coagulation. Combining "intensity fading MS" and off-line LC prefractionation allowed us to detect more than 75 molecules present in the leech extract that interact specifically with a trypsin-like protease over a sample profile of nearly 2,000 different peptides/proteins in the 2-20-kDa range. Moreover we resolved 232 individual components from the complex mixture, 13 of which have high sequence homology with previously described serine protease inhibitors. Our findings indicate that such extracts are much more complex than expected. Additionally, intensity fading MS, when complemented with LC separation strategies, seems to be a useful tool to investigate complex biological samples, establishing a new bridge between profiling, functional peptidomics, and subsequent drug discovery.

EPR studies of amine radical cations, part 1: thermal and photoinduced rearrangements of n-alkylamine radical cations to their distonic forms in low-temperature freon matrices.

The thermal and photochemical transformations of primary amine radical cations (n-propyl 1.+, n-butyl 5.+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spectroscopy. Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterioamines. The identifications were supported by quantum chemical calculations on the geometry, electronic structure, hyperfine splitting constants and energy levels of the observed transient radical species. The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of 1.+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation .C(2CH2CH2NH3+. By comparison, the formation of the corresponding .CH2CH2CH2CH2NH3+ species from 5.+ is considerably more facile and already occurs within the short irradiation time. These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines, a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines. The greater ease of the thermal rearrangement of 5.+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hydrogen shifts, the lower barrier for the former being associated with minimisation of the ring strain in a six-membered transition state. For 1.+, the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to approximately 350 nm light, although there is also evidence for the 1,3-hydrogen shift requiring a higher energy. A more surprising result is the photochemical formation of the H2C=N. radical as a minor product under hard-matrix conditions in which diffusion is minimal. It is suggested that this occurs as a consequence of the beta-fragmentation of 1.+ to the ethyl radical and the CH2=NH2+ ion, followed by consecutive cage reactions of deprotonation and hydrogen transfer from the iminonium group. Additionally, secondary ion-molecule reactions were studied in CFCl2CF2Cl under matrix conditions that allow diffusion. The propane-1-iminyl radical CH3CH2CH=N. was detected at high concentrations of the n-propylamine substrate. Its formation is attributed to a modified reaction sequence in which 1.+ first undergoes a proton transfer within a cluster of amine molecules to yield the aminyl radical CH3CH2CH2N.H. A subsequent disproportionation of these radicals can then yield the propane-1-imine precursor CH3CH2CH=NH, which is known to easily undergo hydrogen abstraction from the nitrogen atom. The corresponding butane-1-iminyl radical was also observed.

Thermodynamic control of final ion distributions in MALDI: in-plume proton transfer reactions

Gas-phase thermochemical data on matrix-assisted laser desorption/ionization (MALDI) matrix species were used to calculate the energetics of possible proton transfer reactions. These were investigated for the MALDI matrices 2,4,6-trihydroxyacetophenone and sinapic acid, with the tripeptide glycyl–glycyl–histidine as analyte. MALDI proton transfer product ion distributions were found to be predicted by the energetics of possible secondary ion–molecule reactions, at all laser fluences sufficient to generate a dense plume. Near the ion generation fluence threshold, the mass spectra deviate from the thermodynamic predictions. This shows that the MALDI plume exhibits both thermodynamically and kinetically controlled regimes, depending on desorption conditions. Possible thermal ground state proton transfer primary ionization pathways were also considered, and found to be inconsistent with the data.

Chemiionization and secondary ion-molecule reactions in CH 3OH and CH 3OD

Ions formed in primary and secondary processes on impact of 2 3,1S helium metastable atoms with CH 3OH and CH 3OD have been studied by measuring the composition of the ionic products as a function of CH 3OH and CH 3OD pressure using a single-source mass spectrometer. None of the ions observed contained helium. The primary ion spectrum for the chemiionization of CH 3OH was found to be very similar to that obtained by electron impact. Cross-sections have been calculated for the secondary ion-molecule reactions observed and these are compared with previous measurements in which the primary ions are produced by electron impact.

Chemiionization and secondary ion reactions in H 2O and D 2O

Ions formed in primary and secondary processes on impact of 3.1S helium and 3P 2.0 neon metastable atoms with H 2O and D 2O have been studied by measuring the composition of the ionic products as a function of H 2O and D 2O pressure uzing a single-source mass spectrometer. No associative ionization and very little rearrangement ionization is observed. Cross-sections for several secondary ion—molecule reactions were measured. Comparisons of these results with ion—molecule reaction cross-section measurements using electron impact ionization are made, and possible reasons for the differences are discussed. Within the experimental error, isotope effects are not observed for the secondary-ion reactions.

Identification of the primary chemi-ion in hydrocarbon oxidations

The ion CHO + has been identified as the primary chemi-ion in the reaction of oxygen atoms with acetylene, ethylene and isobutane. Low concentrations of reactants in helium were mixed in the annular region between two concentric stainless steel cylinders. The chemi-ions were sampled through a pinhole in the outer cylinder and analyzed by a quadrupole mass spectrometer. By applying an electric field between the two cylinders, the residence time of the ions could be decreased, thereby reducing the importance of secondary ion—molecule reactions. CHO + was the only ion that increased in importance as the electric field was applied, becoming more than 90% of the total ions at high fields. This observation provides strong support for the proposal of Calcote that the reaction, O + CH → CHO + + e −1 is the source of chemi-ionization in hydrocarbon flames.

Ion Cyclotron Resonance Study of the Secondary Ion-Molecule Reactions in Hexafluoroethane

The mechanism of secondary ion production in the electron impact decomposition of C2F6 has been determined using ion cyclotron double-resonance techniques. The secondary formation of CF3+ occurs primarily from the decomposition of C2F5+ and charge transfer from CF2+. The reaction CF3++C2F6→CF2++ neutral products is postulated to account for secondary CF2+ production.