P. Demirev

Photochemical versus thermal mechanisms in matrix-assisted laser desorption/ionization probed by back side desorption

The contribution of photochemical and thermal effects in the formation and ejection of biomolecular ions in matrix-assisted laser desorption/ionization; (MALDI) is studied by comparing desorption from one-and-the-same sample in two incident laser geometries—irradiating the sample front, or irradiating the back side through a 2000u2009A gold layer. Such a set-up results in the efficient decoupling of photoexcitation from thermal processes. Our results indicate that laser-induced photochemistry, e.g. charge transfer reactions between electronically-excited matrix molecules and the analyte, plays a major role in the MALDI process.

Formation of fullerenes in MeV ion track plasmas

Abstract MeV primary 127 I 14+ ions from the Uppsala EN-tandem accelerator were used to bombard a polymer — poly (vinylidene difluoride). Positive ions of even numbered carbon clusters (C + n , n =40−120) are ejected as a result of the interaction of fast MeV ions with the organic solid. The distribution of cluster sizes suggests that stable, closed carbon-cage structures — fullerenes — are formed. Initial radial velocity distributions of the desorbed carbon species are consistent with preferential ejection backward into the direction of the incoming MeV ions. Correlation between the directions of incoming particles and ejected clusters suggests that the latter (including C + 50 and C + 60 ) originate in the infratrack plasma produced by the fast primary ions interaction with the sample. A main conclusion resulting from this study is that carbon cluster ions form as a result of a single primary ion impact and that they are ejected from an axially expanding infratrack plasma region.

HEAVY-ION-INDUCED SPUTTERING AND CRATERING OF BIOMOLECULAR SURFACES

We measured the electronic-stopping-power dependences of mega-electronvolt atomic-ion-induced sputtering and cratering of bio-organic targets. We employed a collector method to study relative total ...

Damaging of C60 films by MeV heavy ions

Abstract Thin films of C60 (99.99% purity) have been irradiated with 55 MeV 127I10+ ions in a fluence range from 2.5 × 109 to 3 × 1012 ions/cm2. Two methods have been employed to assess the modifications induced by the MeV ion irradiation: in situ plasma desorption mass spectrometry analysis and off-line micro-Raman spectroscopy. The yields of secondary low mass carbon cluster ions and intact C60 ions have been determined at different fluences. Damage cross-sections for the C60 molecules have been extracted from the ion yield curves as a function of MeV ion fluence. The secondary ion spectra of the C60 targets also show peaks corresponding to larger fullerenes with masses at least up to 2500 u. The evolution of the ion yields of these higher mass carbon clusters gives an evidence that they are not originally present in the target. They may be formed by coalescence reactions of C60 molecules as a result of an individual MeV ion impact.

MeV ion sputtering of polymers: correlation between secondary ion radial velocity distributions and heavy ion track structure

Abstract Systematic investigations of the initial radial velocity distributions of low mass positive and negative secondary ions, sputtered electronically from thin films of polyvinylidene fluoride and polystyrene, are reported. 72.3 MeV 127 I 13+ primary ions bombard the targets at 45° angle of incidence. Sputtered secondary ions in an individual MeV ion impact are analysed in a high resolution time-of-flight mass spectrometer. The accurate mass measurements of all ion peaks in the range from 1 to 100 m/z provide unequivocal determination of the chemical composition of these ions, forming homologous series, C n H ± m and C n H m F ± p . Plots of both the initial mean radial velocity (〈 v x 〉) and kinetic energy (α 〈 v 2 x 〉) as a function of the ion m / z results in a periodic pattern. Ions with lower hydrogen content exhibit wider velocity distributions (i.e. higher 〈 v 2 x 〉) and (〈 v x 〉) directed towards the primary ion trajectory. Ions with higher hydrogen content have lower mean kinetic energies and 〈 v x 〉 directed away from the incident ion trajectory. We argue that the 〈 v x 〉 and 〈 v 2 x 〉 periodic behaviour, connected to the chemical constitution of the ions, reflects the radial profile of the deposited energy density in the heavy ion track.

Sample exposure effects in matrix-assisted laser desorption—ionization mass spectrometry of large biomolecules

Abstract Effects of sample exposure to UV laser irradiation on the matrix-assisted laser desorption—ionization (MALDI) mass spectra of different proteins are reported. The exposure is varied by irradiating the same sample spot with a differing number of UV laser pulses. The ion yield, mass resolution and internal energy content of ejected molecular ions are monitored as a function of the sample exposure. Other parameters that influence the MALDI spectra (and related to sample exposure) such as laser fluence, sample thickness, matrix-to-protein molar ratio, total deposited amount, and molecular mass of the protein, are also examined.

Isotope Depletion of Large Biomolecules: Implications for Molecular Mass Measurements

Isotope depletion (or enrichment) of large biomolecules is a procedure already used in high resolution Fourier transform ion cyclotron resonance mass spectrometry for improving the reliability and accuracy of biomolecular mass characterization. In this work, effects of isotope depletion on a number of mass spectrometric parameters are systematically studied. Implementation of the isotope depletion techniques in conjunction with lower resolution mass analyzers is discussed as well. We investigate theoretically the position of the centroid of the isotopic mass distributions (centroid mass) and the shift between the monoisotopic and the centroid masses of biopolymers as a function of the isotope abundance (e.g., 12C:13C ratio). The behaviour of other additive mass parameters, like the ratio between the monoisotopic and the first isotopic peak, is also discussed. We address by computer simulations the effects of different instrumental parameters like mass resolution and ion statistics as a function of isotope abundances and from there the achievable mass accuracy for high-mass biopolymers. We assess some of the practical issues of the isotope depletion technique, viz., to what degree and with what accuracy the depletion procedure should be performed for achieving the desired mass accuracy.

Hydrogenation of solid C60 by atomic hydrogen

We report a mass spectrometric study of the hydrogenation of pure C60 in thin solid films. The reactions were performed in vacuum by exposing C60 films to atomic hydrogen, which covalently binds to individual fullerene molecules, resulting in C60Hn (n=2 to > 24), with a wide distribution in n for a given exposure. Fragmentation of the fullerene cages, which for pure C60 proceeds by losses of multiples of two C atoms, is redistributed for C60Hn to include losses of odd numbers of C atoms, though the total degree of fragmentation is unchanged.

On a model of fullerene formation from polymers under MeV ion impact

Abstract Fullerenes — large even-numbered carbon clusters with a closed three-dimensional structure — can be formed in the gas phase by different methods, including laser ablation of graphite and other carbon containing materials. A method for production of macroscopic amounts of predominantly C60 by electric arc graphite vaporization in He atmosphere has lead to an explosive growth in fullerene research. Recently it has been demonstrated that fullerenes are also formed in the electronic sputtering of a specific polymer — poly(vinylidenedifluoride), PVDF - by swift MeV atomic ions. In a model for formation of fullerenes in MeV ion impact on polymer surfaces we assume that fullerenes result from carbon atom condensation in the dense axially expanding plasma in the MeV ion infratrack. Gas-flow concepts are employed to explain the ejection of the fullerenes. The temporal and radial dependence of the temperature in the track region is described on the basis of the linear thermoconductivity theory for thermal sources with cylindrical symmetry. The dependence of the fullerene yield on the MeV ion stopping power and on the number of atoms in the cluster is calculated by taking into account a requirement of minimal plasma temperature for fullerene formation. Calculations for the average cluster velocity are also performed and compared to experimental data. Finally we argue that both the model and the experimental findings bear implications for the mechanism of fullerene formation in general, providing estimates of the required time frame and spatial domain.

A plasma desorption time-of-flight mass spectrometer with a single-stage ion mirror: improved resolution and calibration procedure

Abstract A first order electrostatic ion mirror has been constructed and mounted on a time-of-flight spectrometer coupled to the Uppsala EN-tandem accelerator at The Svedberg Laboratory. In electronic sputtering of CsBr with MeV 127 I ions a highest mass resolution around 10500 at a full-width at half maximum for the cluster ions (CsBr) 2 Cs + has been obtained. The resolution for organic molecules is somewhat lower than this value but still significantly better than in a linear system. The mirror also provides great simplification of the calibration procedure and an improvement in mass accuracy compared with a linear spectrometer. For stable ions, and with the help of an einzel lens, the transmission approaches 100% in the mirror. The mirror is equipped with two stop detectors which make it possible to study the decay of metastable ions.