Showing papers by "Tilmann D. Märk published in 2007"

Rapid and non-destructive identification of strawberry cultivars by direct PTR-MS headspace analysis and data mining techniques

Abstract: Proton transfer reaction-mass spectrometry (PTR-MS) is a spectrometric technique that allows direct injection and analysis of mixtures of volatile compounds. Its coupling with data mining techniques provides a reliable and fast method for the automatic characterization of agroindustrial products. We test the validity of this approach to identify samples of strawberry cultivars by measurements of single intact fruits. The samples used were collected over 3 years and harvested in different locations. Three data mining techniques (random forests, penalized discriminant analysis and discriminant partial least squares) have been applied to the full PTR-MS spectra without any preliminary projection or feature selection. We tested the classification models in three different ways (leave-one-out and leave-group-out internal cross validation, and leaving a full year aside), thereby demonstrating that strawberry cultivars can be identified by rapid non-destructive measurements of single fruits. Performances of the different classification methods are compared.

Rapid white truffle headspace analysis by proton transfer reaction mass spectrometry and comparison with solid‐phase microextraction coupled with gas chromatography/mass spectrometry

Abstract: The gastronomic relevance and high price of white truffle are related mainly to its unique aroma. Here we evaluate, for the first time, the possibility of characterizing in a rapid and non-destructive way the aroma of white truffles based on proton transfer reaction mass spectrometry (PTR-MS). We indicate that anonymous PTR-MS fingerprinting allows sample classification and we also compare qualitatively and quantitatively PTR-MS data with measurements made by solid-phase microextraction gas chromatography (SPME-GC) of the same samples under the same conditions. PTR-MS fragmentation data of truffle-relevant compounds are also published here for the first time. Most of the sulfur-containing compounds detected by GC and relevant for white truffle aroma have a high positive correlation with single PTR-MS peaks. Our work indicates that, after preliminary comparison with GC data, PTR-MS is a new tool for the rapid, quantitative and non-invasive characterization of white truffle by direct headspace injection without any pre-concentration.

PTR-MS Assessment of Photocatalytic and Sorption-Based Purification of Recirculated Cabin Air during Simulated 7-h Flights with High Passenger Density

Abstract: Four different air purification conditions were established in a simulated 3-row 21-seat section of an aircraft cabin: no air purifier; a photocatalytic oxidation unit with an adsorptive prefilter; a second photocatalytic unit with an adsorptive prefilter; and a two-stage sorption-based air filter (gas-phase absorption and adsorption). The air purifiers placed in the cabin air recirculation system were commercial prototypes developed for use in aircraft cabin systems. The four conditions were established in balanced order on 4 successive days of each of 4 successive weeks during simulated 7-h flights with 17 occupants. Proton-transfer reaction mass spectrometry was used to assess organic gas-phase pollutants and the performance of each air purifier. The concentration of most organic pollutants present in aircraft cabin air was efficiently reduced by all three units. The photocatalytic units were found to incompletely oxidize ethanol released by the wet wipes commonly supplied with airline mealsto produce unacceptably high levels of acetaldehyde and formaldehyde.

Dissociative electron attachment to gas phase glycine: exploring the decomposition pathways by mass separation of isobaric fragment anions.

Abstract: Dissociative electron attachment to gas phase glycine generates a number of fragment ions, among them ions observed at the mass numbers 15, 16 and 26 amu. From stoichiometry they can be assigned to the chemically rather different species NH−/CH3− (15 amu), O−/NH2− (16 amu) and CN−/C2H2− (26 amu). Here we use a high resolution double focusing two sector mass spectrometer to separate these isobaric ions. It is thereby possible to unravel the decomposition reactions of the different transient negative ions formed upon resonant electron attachment to neutral glycine in the energy range 0–15 eV. We find that within the isobaric ion pairs, the individual components generally arise from resonances located at substantial different energies. The corresponding unimolecular decompositions involve complex reaction sequences including multiple bond cleavages and substantial rearrangement in the precursor ion. To support the interpretation and assignments we also use 13C labelling of glycine at the carboxylic group.

Multiply charged neon clusters: failure of the liquid drop model?

Abstract: We have analyzed the stability and fission dynamics of multiply charged neon cluster ions. The critical sizes for the observation of long-lived ions are ${n}_{2}=284$ and ${n}_{3}=656$ for charge states 2 and 3, respectively, a factor 3 to 4 below the predictions of a previously successful liquid-drop model. The preferred fragment ions of fission reactions are surprisingly small ($2\ensuremath{\le}n\ensuremath{\le}5$); their kinetic energy distributions peak at 200 meV or below. The size of these fragments and their average kinetic energies are much less than predicted by the liquid-drop model.

Identification of isomers of nitrotoluene via free electron attachment.

Abstract: Free electron attachment to the three different isomers of mononitrotoluene molecules in the gas phase is studied using a crossed electron−molecule beams technique. In contrast to previous studies for a large number of negative ions, the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO2-, remarkable differences for the three isomers are observed. In almost all fragment anion efficiency curves, the 2-nitrotoluene exhibits pronounced differences from the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other.

Electron impact ionization of thymine clusters embedded in superfluid helium droplets

Abstract: Embedding molecules in helium clusters has become a powerful technique for the preparation of cold targets for spectroscopy experiments, as well as for the assembly of complex, fragile molecular species. We have recently developed a helium cluster source and a pick-up cell to produce neutral beams of doped helium droplets, to be used as targets in studies on electron collisions with molecules of biological relevance. In the present work we present the results of a series of experiments on electron-impact ionization of helium clusters doped with thymine and 1-methylthymine, where several interesting phenomena were observed, i.e., (i) electron impact ionization of molecular clusters inside the helium droplets leads predominantly to protonated clusters; (ii) the appearance energies are close to the ionization threshold of the helium atom but ionization efficiency curves in addition extend down by several eV; (iii) ionized molecular clusters can undergo metastable decay via the loss of one neutral monomer.

Absolute cross sections and kinetic energy release distributions for electron impact ionization and dissociation of CD

Abstract: Absolute cross sections for electron impact dissociative excitation and ionization of CD+ leading to the formation of ionic products ( D+, C+, C2+ and C3+) are reported in the energy range from their respective thresholds to 2.5 keV. Around the maximum, cross sections are found to be ( 10.5 +/- 1.0) x 10(-17) cm(2), ( 20.6 +/- 3.5) x 10(-17) cm(2), ( 1.20 +/- 0.11) x 10(-17) cm(2) and ( 8.2 +/- 1.5) x 10(-20) cm(2) for D+, C+, C2+ and C3+, respectively. In the very low- energy region, dissociative excitation leading to the C+ formation dominates over the D+ one. The cross section for dissociative ionization ( C+ + D+ formation) is found to be ( 6.9 +/- 1.3) x 10(-17) cm(2) around 105 eV and the corresponding threshold energy is ( 22.1 +/- 0.5) eV. The animated crossedbeams method is used and the analysis of ionic product velocity distributions allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 15 eV both for C+ and for D+, and up to 40 eV both for C2+ and for C3+. For singly charged products, the comparison of the present energy thresholds and kinetic energy release with published data allows the identification of states contributing to the observed processes. In particular, contributions from primary ions formed in the a(3)Pi metastable state are perceptible. At fixed electron energy, the cross sections for the various ionization channels are seen to reduce exponentially with the potential energy of each dissociated ion pair. Anisotropies are estimated to be in the range 8 +/- 2% for both C+ and D+. The total CD+ single ionization cross section calculated by application of the Deutsch - Mark formalism is found to be in good agreement with experimental results.

Dissociative electron attachment to HBr: A temperature effect

Abstract: The effects of rovibrational temperature on dissociative electron attachment to hydrogen bromide has been investigated from the experimental and theoretical point of view. Theoretical calculations based on the nonlocal resonance model predict a strong temperature effect on the Br{sup -} fragment ion yield due to population of higher vibrational and rotational states. A crossed beam experimental setup consisting of a temperature controlled effusive molecular beam and a trochoidal electron monochromator has been used to confirm this prediction. The high degree of agreement between experiment and theory indicates the validity of the theoretical model and its underlying physical picture.

Absolute cross sections and kinetic energy release for doubly and triply charged fragments produced by electron impact on CO

Abstract: Absolute cross sections for electron impact ionization of CO+ leading to the formation of doubly and triply charged products (CO2+, C2+ and O2+, C3+ and O3+) are reported in the energy range from their respective thresholds to 2500 eV. Around the maximum, cross section values are found to be ( 13.36 +/- 0.56) x 10(-18) cm(2), ( 5.58 +/- 0.55) x 10(-18) cm(2) and (1.37 +/- 0.14) x 10(-18) cm(2) for CO2+, C2+ and O2+, respectively, and (28.3 +/- 7.0) x 10(-21) cm(2) and (2.4 +/- 0.7) x 10(-21) cm(2) for C3+ and O3+, respectively. The analysis of ionic product velocity distributions, obtained by means of a crossed electron-ion beam set-up, allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 50 eV both for C2+ and O2+. The mean kinetic energy releases for C3+ and O3+ are found to be 33 +/- 5 eV and 39 +/- 9 eV, respectively. The cross sections are seen to depend exponentially on the potential energy of each dissociated ion pair. The total single CO+ ionization cross section calculated by application of the Deutsch-Mark ( DM) formalism is found to be in good agreement with the experimental results.

Isotope effects in the metastable decay of Ne 2

Abstract: Neon dimer ions undergo spontaneous dissociation (metastable decay) several microseconds after formation by electron impact ionization of neon clusters. In this contribution we compare the kinetic energy release distribution (KERD) of the previously reported isotopomer 20Ne2 + with that of 22Ne2 +. The heavy isotopomer shows the same two components in the KERD as the lighter ones. However, the high-energy component that is due to electronic pre-dissociation is reduced in intensity. The decrease is attributed to a reduced predissociation rate from the II(1/2u) state into I(3/2u).

Electron Impact Ionization/Dissociation of Molecules: Production of Energetic Radical Ions and Anions

Abstract: In order to provide quantitative information on electron collision processes involving various plasma constituents (in particular hydrocarbons) and to elucidate the properties of cations and anions produced we have carried out the past years a series of studies with a variety of techniques in our laboratory in Innsbruck. In the present review we will present some recent results on electron impact ionization and attachment in order to illustrate recent progress in this field in particular concerning the production of energetic fragment cations for hydrocarbons and differences in the attachment of isomers for nitro-organics. Using a Nier type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene, propene and other hydrocarbons have been measured for electron energies up to 1000 eV. Discrimination factors for ions have been determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover charge separation reactions (for instance in case of acetylene the Coulomb explosion of the doubly-charged parent ions C2H2++ into the fragment ions C2H+ and H+) are investigated by means of a number of metastable mass spectrometry methods and the associated mean kinetic energy release is deduced. Free electron attachment to the three different isomers of mono-nitrotoluene molecules in the gas phase is studied using two different crossed electron-molecule beams technique. In contrast to previous studies for a large number of negative ions in nitro-organic compounds the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO2− remarkable differences for the three isomers are observed. For almost all fragment anion efficiency curves the 2-nitrotoluene exhibits pronounced differences compared to the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other.

Dissociative electron attachment to unstable molecules and slow fragmentation of metastable molecular anions

Abstract: Recent experimental investigations of dissociative electron attachment to unstable molecules and free radicals are reviewed along with observations of metastable negatively charged ions. Measurements have been made with a time-of-flight mass spectrometer in Belfast and also, for metastable ions, with a double focussing twin field mass spectrometer in Innsbruck. Electron attachment to unstable CS, for example, was found to be similar to electron attachment to the valence isoelectronic CO molecule with observation of S- and C- ions just above the thermodynamic threshold for S- + C (3P) at 5.43 eV, C-+ S at 6.40 eV and S-+ C(1D) at 6.70 eV with peak cross sections of ~ 0.025 ?2, 0.002 ?2 and 0.003 ?2 respectively. Slow fragmentation of metastable SF-*6 formed in low energy electron attachment to SF6 has been observed on microsecond timescales in competition with autodetachment; processes SF-*6 ? SF-5 + F and SF-*6 ? SF6 + e- respectively. Fragmentation of metastable anions of benzene derivatives, such as 2,4-dinitro-toluene [CH3.C6H3(NO2)2], has also been observed on microsecond timescales.

Sticking Coefficient and SIMS of Hydrocarbons on Fusion Relevant Plasma-Sprayed Tungsten Surfaces

Abstract: In this work we concentrate on the quantification of the sticking coefficient of CD 3 + on fusion relevant plasma-sprayed tungsten (PSW) surfaces in the collision energy range from about 0 up to 100 eV. PSW samples were cut from ASDEX Upgrade tiles and both untreated and electropolished surfaces are investigated. A collision energy, Ecoll, selected CD 3 + ion beam deposits hydrocarbon layers onto the target surfaces which are ex-situ analyzed by nuclear reaction analysis (NRA) via D(3He, p)4He at 800 kV and 2.5 MeV, respectively. The sticking coefficient, S, for deuterium has been found to be collision energy dependent, decreasing with increasing energy. It is in the order of S ∼ 0.4 for PSW and S ∼ 0.1 for electro polished PSW at Ecoll = 10 eV. Secondary ion mass spectra of charged particles are recorded after ion–surface collisions with the polished PSW surface for collision energies up to 100 eV. While at very low collision energies below about 10 eV only simple reflection of CD 3 + is observed, at higher collision energies both fragmentation of the projectile and fragmentation of ion–surface reaction products are observed. Concerning the stability of the layers we prove that hydrogen exchange plays a role on the time scale of weeks.

Dissociative electron attachment to nitroaromatic compounds – resonances as fingerprints for isomers

Abstract: We present detailed free electron attachment measurements on nitroaromatic compounds in the gas phase. It turns out that dissociative electron attachment can act as a selective and sensitive probe for the identification of isomeric forms of nitrotoluene and dinitrobenzene. Rich fragmentation patterns have been observed for both nitroaromates and all fragments above the detection limit of our instrument have been investigated with a high energy resolution electron monochromator in the energy range of about 0 to 15 eV. It is shown that relative attachment cross section curves can act as fingerprints for the particular molecule and its isomers. Additionally it has been observed that numerous fragments arise from surprisingly complex structural and electronic rearrangements.