Substochastic semigroups and densities of piecewise deterministic Markov processes

Necessary and sufficient conditions are given for a substochastic semigroup on L¹ obtained through the Kato-Voigt perturbation theorem to be either stochastic or strongly stable. We show how such semigroups are related to piecewise deterministic Markov process, provide a probabilistic interpretation of our results, and apply them to fragmentation equations.     

Properties in Reactive Structure Materials: ZrW2 and HfW2 – High Melting Temperatures,Densities, Hardness,and Exothermic Ignition Energies

ZrW₂ and HfW₂ were prepared by melting the constituent elements in an arc-furnace. By cooling through perictectic phase lines above approximately 2000 °C, samples of the nominal 1:2 composition were obtained as products consisting of two phases, of which approximately 95 % adopts the MgCu₂ type structure and 5 % the W structure. In ZrW₂ and in HfW₂, the MgCu₂ type structure enables efficient topological compact spacefilling of the smaller W atoms into the cavities of the three-dimensional net constructed by the larger tetravalent metals Zr and Hf. Correspondingly, very dense and hard alloys are obtained which burn at a crash in air in a strong exothermic ignition reaction. These materials are non-toxic.     

Identification of pentacyclic triterpenes by GC-MS-MS

The development of tandem mass spectrometry (MS-MS) and its combination with gas chromatography (GC-MS-MS) has greatly enhanced our ability to analyze exceedingly complex mixtures from a wide variety of sources, including fossil fuels. This paper describes a comparison of the conventional electron impact (El) mass spectral fragmentation patterns, obtained in the MS-MS precursor – product mode, of three oleanane isomers and one ursene, all of which are used as indicators of higher plant input to the sedimentary environment Results show that El and collision-activated dissociation (CAD) spectra are most similar at collision energies between ?15 and ?20 eV.The presence of methyl substituents in these pentacyclic terpenes plays an important role in the process of ring fragmentation: the daughter spectra of the M – 15 fragments can be used to obtain structural information, such as the positions of the double bonds.     

Fragmentation of pulverized coal in a laminar drop tube reactor: Experiments and model

Fragmentation during pulverized coal particles conversion shifts the particle size distribution of the fuel towards smaller particle sizes, affecting both conversion rates and heat release. After pyrolysis of a high volatiles Colombian coal in CO₂ atmosphere in a drop tube reactor at 1573?K, solid carbonaceous particles of different size, from 100?µm of the particle feed down to the nanometric size, have been observed. A fragmentation model has been used to predict the fate of Colombian coal particles under the experimental conditions of the drop tube experiment and predict the particle size distribution (PSD). Model and experimental results are in very good agreement and indicate that in the DTR experiment the coal underwent almost complete pyrolysis and that fragmentation generated a 36?wt% population of particles with size close to 30?µm. The close match between the PSDs obtained from experiments and from the fragmentation model is an important novelty. It demonstrates that fragmentation occurs not only under fluidized bed conditions but also under the conditions of pulverized coal combustion. Experimentalists are warned against the fact that the fine particulate sampled at the outlet of laminar flow reactors and boilers is not always composed of soot only. Char fragments can be misidentified as soot. The implementation of fragmentation submodels in pulverized fuel combustion and gasification codes is highly recommended.     

Comparative TOF-SIMS and MALDI TOF-MS analysis on different chromatographic planar substrates

A comparison is made between two high resolution, surface-based, mass spectrometric methods: time-of-flight secondary ion mass spectrometry (TOF-SIMS) and matrix-assisted laser desorption/ionisation mass spectrometry (MALDI TOF-MS) in indication of abietic and gibberellic acids molecular profiles on different chromatographic thin layers. The analytes were applied to silica gel chromatographic thin layers with SIMS on-line interfacing channel, monolithic silica gel ultra-thin layers, and thin layers specifically designed for direct Raman spectroscopic analysis. Two MALDI matrices were used in this research: ferulic acid and 2,5-dihydroxybenzoic acid. The silica gel SIMS-interfacing channel strongly supported formation of numerous different MALDI MS fragments with abietic and gibberellic acids, and ferulic acid matrix. The most intense fragments belonged to [M-OH](+) and [M](+) ions from ferulic acid. Intense conjugates were detected with gibberellic acid. The MALDI MS spectrum from the monolithic silica gel surface showed very low analyte signal intensity and it was not possible to obtain MALDI spectra from a Raman spectroscopy treated chromatographic layer. The MALDI TOF MS gibberellic acid fragmentation profile was shielded by the matrix used and was accompanied by poor analyte identification. The most useful TOF-SIMS analytical signal response was obtained from analytes separated on monolithic silica gel and a SIMS-interfacing modified silica gel surface. New horizons with nanostructured surfaces call for high resolution MS methods (which cannot readily be miniaturised like many optical and electrochemical methods) to be integrated in chip and nanoscale detection systems.     

Unusual Thionation via Fragmentation

Reaction of 4,4‐dichloro‐2,6‐diphenyl‐4,4‐dihydropyrane (2) with thiols in the presence of triethylamine gives interesting results. The exclusive formation of 2,6‐diphenylpyran‐4‐thione (3) in the reaction with PhSH through cleavage of C_Ar‐S bond is remarkable.     

Effects of an arginine-selective tagging procedure on the fragmentation behavior of peptides studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS)

The presence of arginine as the naturally occurring amino acid with the highest gas-phase basicity strongly influences the fragmentation behavior of peptides undergoing collision-induced dissociation. Using a derivatization procedure recently developed in our group, based on a reversible reaction of the guanidino group with 2,3-butanedione and an arylboronic acid, we examined how this label affects the fragmentation patterns of labeled versus unlabeled peptides in MS/MS experiments. As part of this fundamental study, two groups of model peptides (angiotensins and bradykinins) as well as tryptic peptides were labeled according to our protocol and subjected to collision-induced dissociation (CID) in both a triple quadrupole and a quadrupole ion trap instrument. It was found that for angiotensins containing an AspArg sequence, C-terminal cleavage at Asp that occurs for native peptides was completely inhibited in Arg-labeled peptides. For bradykinins and peptides obtained from tryptic digests of standard proteins, some sample peptides were little affected by the tagging of arginine residues. Others, in contrast, exhibited an almost total loss of nonspecific backbone cleavage and their fragment ion spectra were dominated by loss of the arginine tag. These and other experimental results are discussed in view of the nature of the arginine tag and the concept of proton mobility.     

Electrospray Ionization Mass Spectra of Amino Acid Methyl Ester 5 ＇-Phosphoramidates of 2＇, 3＇-Isopropylideneuridine

Amino acid methyl ester phosphates were synthesized and determined by using positive-ion mode dectrospmy ionization mass spectrometry（ESIMS） in combination with multistage tandem mass spectrometry. The fragmentation pathways were investigated, and it was observed that most fragment ions contained the phosphoryl group. It was interesting to observe that the fragmentation pathways of the protonated molecule show some differences when compared with those of the sodium ion adduct. The methoxy group of amino acid methyl ester can migrate from the carbonyl group to the phosphoryl group in the sodium ion adduct.     

Mass Spectroscopy of the Antileukemic Drug MGBG and Related Bis(Amidinohydrazones) [‘Bis(Guanylhydrazones)’]

The first study on the mass spectroscopy of various bis(amidinohydrazones) is reported. The compounds studied included the investigational antileukemic drugs methylglyoxal bis(amidino-hydrazone) [‘methylglyoxal bis(guanylhydrazone)’, MGBG] and glyoxal bis(amidinohydrazone), as well as seven mono- and dialkylglyoxal analogs thereof. The results indicate that the free bases of these high-melting compounds are volatilized well enough to allow a facile detection of the molecular ions and to make mass spectroscopy of the underivatized compounds a suitable method for the verification of the identity of the substances. This result is of importance considering the development of novel analogs and derivatives. A compilation of electron-impact mass spectra is reported and possible fragmentation routes are outlined. The fragmentation of the various congeners appears to occur essentially similarly, the main paths involving breakage of the carbon-carbon single bond in the glyoxal moiety or breakage of either one of the nitrogen-nitrogen single bonds.     

Fragmentation experiment and model for falling mercury drops

The experiment consists of counting and measuring the size of the many fragments observed after the fall of a mercury drop on the floor. The size distribution follows a power-law for large enough fragments. We address the question of a possible crossover to a second, different power-law for small enough fragments. Two series of experiments were performed. The first uses a traditional film photographic camera, and the picture is later treated on a computer in order to count the fragments and classify them according to their sizes. The second uses a modern digital camera. The first approach has the advantage of a better resolution for small fragment sizes. The second, although with a poorer size resolution, is more reliable concerning the counting of all fragments up to its resolution limit. Both together clearly indicate the real existence of the quoted crossover.