Degradation of vitamin C by low-energy electrons

We report on the degradation of gas phase vitamin C (ascorbic acid, AA) induced by low-energy electrons. In the energy range of (0–12) eV, different negatively charged fragments, attributed to the dehydro-ascorbic acid anion ((AA–H)⁻), OH⁻, O⁻ and H⁻, are observed. The yield functions indicate that these ions are formed via dissociative electron attachment, DEA. While the formation of (AA–H)⁻ is exclusively observed at sub-excitation energies (<1.5 eV), the other fragments arise from resonance features at higher energies. Possible implications of these observations for radiation damage and food treatment by high energy radiation are considered.     

Interaction of low-energy electrons with linear diphenylethynyl derivatives in the gas phase

The gas phase electron impact spectroscopy has been used to study the relative efficiency of excitation into singlet states and energies of singlet-triplet transitions for two electroactive organic materials, anthracene and biphenyl-containing diphenylethynyl derivatives. The probability of the lowest singlet-triplet transition in anthracene-containing molecule was found to be much higher than that in anthracene which is connected with triple bonds. No noticeable contribution of the triple bonds into singlet spectra of the studied molecules was observed. There are a number of intense transitions in the range higher than 10 eV. The optical spectrum calculated using the density functional theory is in good agreement with experimental electron energy loss and optical absorption spectra.     

Photocatalytic debromination of decabromodiphenyl ether by graphitic carbon nitride

The graphitic carbon nitride(g-C3N4) is found to be an efficient photocatalyst for the reductive degradation of decabromodiphenyl ether(BDE209) under UV irradiation(>360 nm).g-C3N4 was prepared by heating dicyandiamide.X-ray diffraction,X-ray photoelectron spectroscopy,and UV-vis spectra were used to characterize the properties of as-prepared catalysts.The photoreductive degradation kinetics of BDE209 was further investigated under different reaction conditions.The degradation of BDE209 is a stepwise process,and the bromines at meta positions are much more susceptible to remove than those at the ortho and para positions.A possible photoreductive mechanism was also proposed.     

Inelastic scattering of low-energy electrons by metastable atoms

Cross sections for inelastic collisions of low-energy electrons with metastable atoms in the case of close coupling between different atomic states are obtained in the quasi-statistical approximation. Processes with metastable C, N, O and rare-gas atoms are considered. The calculated cross sections and rate constants are compared with experimental and numerical calculations.     

Glycosidic bond cleavage of thymidine by low-energy electrons

Thymidine was exposed to low-energy electrons (LEE) as a thin solid film under a high vacuum. Nonvolatile radiation products, remaining on the irradiated surface, were analyzed by HPLC/UV and GC/MS. Here, we show that exposure of thymidine to 3-100 eV electrons gives thymine as a major product with a yield of 3.2 x 10-2 per electron (about one-third of the total decomposition of thymidine). The formation of thymine indicates that LEE induces cleavage of the glycosidic bond separating the base and sugar moieties, suggesting a nonionizing resonant process involving dissociative attachment (<15 eV). In contrast, this reaction is not very efficient by DNA base ionization and does not occur by the reaction of solvated electrons with DNA. These studies introduce a new mechanism of DNA damage involving the interaction of LEE.     

Surface analysis with the aid of low-energy electrons

A comparison is made between methods for surface analysis which use low-energy electron beams, and methods such as those using surface conductance and field effect. The distinction between these two groups of methods is relevant to the case of semiconductors. Surface conductance and field effect measurements have a high sensitivity, 10⁸–10¹⁰ excess charges per cm² near the surface being detectable, but they do not reveal the chemical composition of the surface. On the other hand, Auger electron spectroscopy (AES), which uses low-energy electrons, is less sensitive, 10¹²–10¹³ foreign atoms per cm² being detectable, but this method is highly specific with respect to the sort of atoms (and in some cases molecules) present on the surface. A number of applications of AES are given. Another method, that of low-energy electron diffraction (LEED), can only be used for surfaces of single crystals. With LEED, values of repeat distances at the surface can be found. In some cases these are very sensitive to the concentrations of foreign atoms.     

Interaction of low-energy electrons with surface lattice vibrations

In the present article, we examine the interaction of low-energy electrons near the crystal surface with lattice vibrations of the surface region. Our primary attention is devoted to the use of the inelastic scattering of low-energy electrons to elucidate the nature of this interaction, and to obtain information about the nature of atomic vibrations in and near the surface. We review both the theoretical and the experimental literature in this area, within the framework of a discussion of the concepts which are required to interpret the data. We also present a discussion of a number of closely related topics, such as the origin of the image potential in ionic crystals, the properties of the surface polaron, the surface dynamic effective charge and its possible relation to the phenomenon of surface reconstruction, and the scattering by electronic excitations of low-energy electrons incident on the surface.     

Chemical basis of DNA sugar-phosphate cleavage by low-energy electrons

DNA damage by low-energy electrons (LEE) was examined using a novel system in which thin solid films of oligonucleotide tetramers (CGTA and GCAT) were irradiated with monoenergetic electrons (10 eV) under ultrahigh vacuum. The products of irradiation were examined by HPLC. These analyses permitted the quantitation of 16 nonmodified nucleobase, nucleoside, and nucleotide fragments of each tetramer resulting from the cleavage of phosphodiester and N-glycosidic bonds. The distribution of nonmodified products suggests a mechanism of damage involving initial electron attachment to nucleobase moieties, followed by electron transfer to the sugar-phosphate backbone, and subsequent dissociation of the phosphodiester bond. Moreover, virtually all the nonmodified fragments contained a terminal phosphate group at the site of cleavage. These results demonstrate that the phosphodiester bond breaks by a distinct pathway in which the negative charge localizes on the phosphodiester bond giving rise to nonmodified fragments with an intact phosphate group. Conversely, the radical must localize on the sugar moiety to give as yet unidentified modifications. In summary, the reaction of LEE with simple tetramers involved dissociative electron attachment leading to phosphodiester bond cleavage and the formation of nonmodified fragments.     

Determination of decabromodiphenyl ether in water samples by single-drop microextraction and RP-HPLC

This paper describes the development of a new method using single-drop microextraction (SDME) and RP-HPLC for the determination of decabromodiphenyl ether (BDE-209) in water samples. The effects of SDME parameters such as extraction solvent, microdrop volume, extraction time, stirring speed, salt concentration, and sample pH on the extraction performance are investigated. Under optimal extraction conditions (extraction solvent, toluene; solvent drop volume, 3.0 microL; extraction time, 15 min; stirring speed, 600 rpm; no addition of salt and change of sample pH), the calibration curve was drawn by plotting peak area against a series of BDE-209 concentrations (0.001-1 microg/mL) in aqueous solution; the correlation coefficient (r) was 0.9998. The limit of detection was 0.7 ng/mL. The enrichment factor was 10.6. The precision of this method was obtained by six successive analyses of a 100 ng/mL standard solution of BDE-209, and RSD was 4.8%. This method was successfully applied to the extraction of BDE-209 from tap and East Lake water samples with relative recoveries ranging from 92.5 to 102.8% and from 91.5 to 96.2%, respectively, and the relative standard deviations (n = 3) were 4.4 and 2.2%. The proposed method is acceptable for the analysis of BDE-209 in water samples.     

Microencapsulation of decabromodiphenyl ether by in situ polymerization: Preparation and characterization

Melamine-formaldehyde (MF) resin microcapsules containing decabromodiphenyl ether (DBDPO) with better thermal stability were successfully prepared by in situ polymerization, DBDPO being the core material and MF resins being the wall materials. Chemical structure of the prepared microcapsules was characterized by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Morphologies and thermal properties were also investigated by scanning electron microscopy (SEM) and thermogravimetric analysis (TGA), respectively. The results indicated that MF microcapsules with DBDPO particles prepared in this study showed better thermal stability, and could be used as effective flame retardant even for the resins which should be processed at temperatures higher than 350 °C.     

Damage induced by low-energy electrons in solid films of tetrahydrofuran

We report on the low-energy electron-induced production of aldehydes within thin solid films of tetrahydrofuran (THF) condensed on a solid Kr substrate. The aldehyde fragments, which remain trapped within the bulk of the THF film, are detected in situ via their 3,1(n-->pi*) and 3(pi-->pi*) electronic transitions and vibrational excitations in the ground state using high-resolution electron-energy-loss spectroscopy. The production of aldehyde is studied as a function of the electron exposure, film thickness, and incident electron energy between 1 and 18.5 eV. The aldehyde production is calibrated in terms of an electron scattering cross section, which is found to be typically 6-7 x 10(-17) cm(2) between 11 and 19 eV. Its energy dependence is characterized by a small feature around 3 eV, a strong rise from 6 eV up to a maximum at 12.5 eV, followed by two structures centered around 15 and 18 eV. The aldehyde production is discussed in terms of the formation of electron resonances or transient anion states, which may lead to the fragmentation of the molecule and explain the structures seen in the energy dependence of the measured cross section.     

Rapid determination of decabromodiphenyl ether in polystyrene by thermal desorption-GC/MS.

A rapid determination of decabromodiphenyl ether (DeBDE) in polystyrene (PS) by thermal desorption (TD)-GC/MS was studied. The TD-GC/MS method using a pyrolysis-GC/MS system allowed the quick quantification of DeBDE in a waste TV back plate composing of a PS flame-retarded with polybrominated diphenyl ethers on the basis of the resulting chromatogram with a ca. 4% relative standard deviation without using any tedious sample pretreatment, such as solvent extraction.     

Kinetics of interaction of gas phase oxygen with cerium-gadolinium oxide

The method of isotopic exchange with gas phase analysis was used to study the kinetics of oxygen interaction with the Ce_0.80Gd_0.20O_{1.90 ? ??} oxide at the oxygen pressure of 0.27?C1.33 kPa in the temperature range of 700?C800°C. The values of oxygen interphase exchange rate and diffusion coefficient, and also effective activation energies of the processes of oxygen exchange and diffusion were determined. The contributions of three exchange types and amounts of equivalent exchangeable oxygen were calculated. It was shown that the limiting exchange stage is the process of dissociative oxygen adsorption/desorption on the surface of the Ce_0.80Gd_0.20O_{1.90 - ??} oxide.     

Photodegradation study of decabromodiphenyl ether by UV spectrophotometry and a hybrid hard- and soft-modelling approach

This work presents an exploratory study of the photochemical degradation process of decabromodiphenyl ether (decaBDE) and gives an interpretation of the kinetic pathway, species and effects of the key factors involved in the degradation process. Use of lowly brominated diphenyl ethers (PBDE) has been banned by the EU and there seems to be evidence of the photolytic degradation of highly brominated PBDEs into lowly brominated congeners. Hence, the importance of knowing the photodegradation process of decaBDE.The photodegradation was investigated under UV light by UV-spectrophotometric monitoring. A novel hybrid data analysis approach, based on the combination of hard- and soft-spectrophotometric multivariate curve resolution, was applied to elucidate the mechanism of the degradation process, to resolve kinetic profiles and pure spectra of the photodegradation products and to evaluate the rate constants. The photodegradation process could be described with a kinetic model based on three consecutive first-order reactions and a decrease of the degradation process was observed as solvent polarity increased. Complementary identification of photodegradation products by gas chromatography coupled to mass spectrometry using negative chemical ionization (GC-NCI-MS) is attempted.This work presents a novel attempt of describing in a comprehensive way the photochemical degradation process of decaBDE, with all successive steps and related rate constants. This study proves also the potential of the proposed hybrid data analysis methodology as a general strategy to interpret the evolution of these photochemical reactions.     

Formation of negative ions by interaction of electrons with heterocyclic compounds

Curves are derived for the effective yield of negative ions from the interaction of electrons with thiophene, 2-methylthiophene, 2-propylthiophene, 3-methylthiophene, 3-propylthiophene, furan, and selenophene. It is found that there are two quasi-stationary states of the molecular negative ions having lifetimes of about 10^–14 and 4 · 10^–15 sec. The cross sections for formation of these states are estimated, and also the cross sections for resonant elastic scattering of electrons. The states are shown to be related to excited states of the molecule. It is found that alkyl substitution has the following effects on resonant electron capture: a) reduces the probability of ring breakage and b) reduces the energy levels of the quasi-stationary state. Some aspects of dissociative ionization are discussed.     

Interaction of low-energy electrons with the pyrimidine bases and nucleosides of DNA

We report computed cross sections for the elastic scattering of slow electrons by the pyrimidine bases of DNA, thymine and cytosine, and by the associated nucleosides, deoxythymidine and deoxycytidine. For the isolated bases, we carried out calculations both with and without the inclusion of polarization effects. For the nucleosides, we neglect polarization effects but estimate their influence on resonance positions by comparison with the results for the corresponding bases. Where possible, we compare our results with experiment and previous calculations.