Low-Energy Electron Interactions with Methyl-p-benzoquinone: A Study of Negative Ion Formation.

Methyl-p-benzoquinone (MpBQ, CH3C6H3(=O)2) is a prototypical molecule in the study of quinones, which are compounds of relevance in biology and several redox reactions. Understanding the electron attachment properties of MpBQ and its ability to form anions is crucial in elucidating its role in these reactions. In this study, we investigate electron attachment to MpBQ employing a crossed electron-molecular beam experiment in the electron energy range of approximately 0 to 12 eV, as well as theoretical approaches using quantum chemical and electron scattering calculations. Six anionic species were identified: C7H6O2 -, C7H5O2 -, C6H5O-, C4HO-, C2H2 -, and O-. The parent anion is formed most efficiently, with large cross sections, through two resonances at electron energies between 1 and 2 eV. Potential reaction pathways for all negative ions observed are explored, and the experimental appearance energies are compared with calculated thermochemical thresholds. Although exhibiting similar electron attachment properties to pBQ, MpBQ's additional methyl group introduces entirely new dissociative reactions, while quenching others, underscoring its distinctive chemical behavior.

New chloromethane preparation line for the stable chlorine isotope composition analysis.

Stable isotopes of chlorine have great application potential due to the widespread occurrence of the chlorine anion in water, minerals, living organisms and the environment. In most studies, the chloride ion from the samples is converted to chloromethane, which is then analysed isotopically using a mass spectrometer. In the present study, a new design for a chloromethane preparation line is proposed. In particular, the new chloromethane preparation line uses a new system for injecting iodomethane into the preparation system, as well as ampoules with Teflon valves and the u-shaped freezers. These improvements reduced the preparation time to about 1 h, and also achieved a decent measurement uncertainty of 0.05 permil.

Building a Nest by an Automaton.

A robot modeled as a deterministic finite automaton has to build a structure from material available to it. The robot navigates in the infinite oriented grid Z × Z . Some cells of the grid are full (contain a brick) and others are empty. The subgraph of the grid induced by full cells, called the shape, is initially connected. The (Manhattan) distance between the furthest cells of the shape is called its span. The robot starts at a full cell. It can carry at most one brick at a time. At each step it can pick a brick from a full cell, move to an adjacent cell and drop a brick at an empty cell. The aim of the robot is to construct the most compact possible structure composed of all bricks, i.e., a nest. That is, the robot has to move all bricks in such a way that the span of the resulting shape be the smallest. Our main result is the design of a deterministic finite automaton that accomplishes this task and subsequently stops, for every initially connected shape, in time O ( s n ) , where s is the span of the initial shape and n is the number of bricks. We show that this complexity is optimal.

Negative ion formation and fragmentation upon dissociative electron attachment to the nicotinamide molecule.

Nicotinamide (C6H6N2O) is a biologically relevant molecule. This compound has several important roles related to the anabolic and metabolic processes that take place in living organisms. It is also used as a radiosensitizer in tumor therapy. As a result of the interaction of high-energy radiation with matter, low-energy electrons are also released, which can also interact with other molecules, forming several types of ions. In the present investigation, dissociative electron attachment to C6H6N2O has been studied in a crossed electron-molecular beams experiment in the electron energy range of about 0-15 eV. In the experiment, six anionic species were detected: C6H5N2O-, C5H4N-, NCO-, O-/NH2 -, and CN-, with NCO- being the most prominent anion. We also provide detailed computational results regarding the energetic thresholds and pathways of the respective dissociative electron attachment (DEA) channels. The experimental results are compared with the theoretical ones and on this basis, the possible DEA reactions for the formation of anions at a given resonance energy were assigned as well as the generation of neutrals fragments such as pyridine and its several derivatives and radicals are predicted. The pyridine ring seems to stay intact during the DEA process.

Negative Ion Formation by Thermal Surface Ionization of Sulfur Dioxide.

The generation of negative ions from SO2 in the gas phase was studied using the thermal surface ionization method. Six anion types were measured: O- , S- , SO- , and SO2- and anions with m/z=96 and m/z=128. The most abundant anion formed was S- and the formation routes are discussed for each of the six anions. O- , S- , and SO- are formed via dissociative electron attachment to the molecule, whereas the generation of SO2- and anions with m/z=96 and m/z=128 are probably associated with the formation of H2 SO4 in the gas inlet system and the ion source. Using statistical thermodynamics the dissociation temperatures of SO2 and SO in the gas phase are calculated and values of above 1800 °C are obtained for both molecules. We also estimated the optimal filament temperatures for the formation of all anions measured, indicating that for SO2 the optimal temperature is related to the electron affinity of the molecules: the optimal temperature increases with decreasing value of the electron affinity for the molecule corresponding to the respective anion.

Trace elements and stable sulfur isotopes in plants of acid mine drainage area: Implications for revegetation of degraded land.

The abundances of trace elements, a low pH of water and soil in areas impacted by the acid mine drainage (AMD) may cause an excessive uptake of potentially toxic elements and nutritional imbalances in plants. Metal-tolerant, native plants are used for revegetation of degraded mining areas. We established levels of selected trace elements and stable sulfur isotopes in the above-ground plant biomass collected in a mining area in south-central Poland. In 2016, 20 samples of the most common species were collected from sites with a different influence of acid mine drainage and analyzed for trace elements by the inductively coupled plasma mass spectrometry technique. On the basis of the results obtained in 2016, the most contaminated site was selected for a more detailed study, in which sulfur contents and stable sulfur isotope ratios were determined together with trace elements in 17 samples. The results confirmed that the plants native to the AMD area efficiently accumulated trace elements, especially As and rare earth elements. Mosses showed the highest content of trace elements, but exhibited the lowest concentrations of sulfur accompanied by the highest δ34S values. It has been shown for the first time that stable sulfur isotope composition of AMD plants in south-central Poland is significantly depleted in the 34S isotope showing an average δ34S value of -10.5‰ in comparison with positive δ34S values in local vegetation growing outside the AMD area and in local precipitation.

Improved three-filament thermal ionization mass spectrometry ion source for isotope ratio determination of two samples.

RATIONALE: In order to make a single-filament thermal ionization mass spectrometry (TIMS) instrument more versatile and efficient in element isotope analysis, a multifilament ion source has to be employed. In the currently used three-filament ion sources, the same sample must be loaded on all the evaporator filaments, because of the possibility of cross-contamination. METHODS: The elimination of cross-contamination in a TIMS three-filament ion source was achieved by installation of a wide ionizer filament (ribbon shape) perpendicular to the extracting slit plane between two parallel evaporators. In such a configuration, the ionizer filament serves also as a screen separating two evaporator filaments on which two different samples can be loaded. RESULTS: The lack of cross-contamination (on a measurement uncertainty 1σ level of 0.3‰) has been demonstrated by measurements of the isotope ratios of a potassium chloride sample (39 K/41 K = 13.7801 ± 0.0023) versus a spike (39 K/41 K = 1.5670 ± 0.0003). The reproducibility of the isotope ratios for potassium 41 K/39 K and lithium 7 Li/6 Li is about 600 ppm (1σ). CONCLUSIONS: The small change of the ionizer filament configuration significantly improves the functionality of the ion source. The proposed modification enables us to perform alternating isotope analyses under the same working conditions for two different samples (e.g. studied sample and standard).

Formation of Negative Ions upon Dissociative Electron Attachment to the Astrochemically Relevant Molecule Aminoacetonitrile.

Aminoacetonitrile (NH2CH2CN, AAN) is a molecule relevant for interstellar chemistry and the chemical evolution of life. It is a very important molecule in the Strecker diagram explaining the formation of amino acids. In the present investigation, dissociative electron attachment to NH2CN was studied in a crossed electron-molecular beams experiment in the electron energy range from about 0 to 17 eV. In this electron energy range, the following six anionic species were detected: C2H3N2(-), C2H2N2(-), C2H2N(-), C2HN(-), CN(-), and NH2(-). Possible reaction channels for all the measured negative ions are discussed, and the experimental results are compared with calculated thermochemical thresholds of the observed anions. Similar to other nitrile and aminonitrile compounds, the main anions detected were the negatively charged nitrile group, the dehydrogenated parent molecule, and the amino group. No parent anion was observed. Low anion yields were observed indicating that AAN is less prone to electron capture. Therefore, AAN can be considered to exhibit a relatively long lifetime under typical conditions in outer space.

Low energy electron attachment to cyanamide (NH2CN).

Cyanamide (NH2CN) is a molecule relevant for interstellar chemistry and the chemical evolution of life. In the present investigation, dissociative electron attachment to NH2CN has been studied in a crossed electron-molecular beams experiment in the electron energy range from about 0 eV to 14 eV. The following anionic species were detected: NHCN(-), NCN(-), CN(-), NH2(-), NH(-), and CH2(-). The anion formation proceeds within two broad electron energy regions, one between about 0.5 and 4.5 eV and a second between 4.5 and 12 eV. A discussion of possible reaction channels for all measured negative ions is provided. The experimental results are compared with calculations of the thermochemical thresholds of the anions observed. For the dehydrogenated parent anion, we explain the deviation between the experimental appearance energy of the anion with the calculated corresponding reaction threshold by electron attachment to the isomeric form of NH2CN--carbodiimide.

Generation of negative ions from SF6 gas by means of hot surface ionization.

RATIONALE: Sulfur hexafluoride (SF(6)) is a man-made compound with many industrial applications. This compound is also one of the most powerful greenhouse gases with a relatively long atmospheric lifetime. Therefore, it is important to investigate processes leading to SF(6) decomposition. METHODS: A magnetic sector mass spectrometer with a thermoemission gaseous ion source was used in this study. The filament temperature was changed and monitored pyrometrically during the course of the studies. In the hot surface ionization process, negative ions may be generated both by free electron attachment to a molecule and by thermal dissociation followed by electron capture to the one of the fragments formed. RESULTS: Eight ion species: SF(5)(-), F(-), SF(6)(-), SF(4)(-), SF(3)(-), SF(2)(-), SF(-) and F(2)(-), with ion current intensities ratios of 1000:200:100:10:5:0.5:0.5:0.05, respectively, were detected. The filament temperature dependencies of the SF(5)(-), F(-), SF(6)(-), SF(4)(-) ion current intensities were measured. The optimal temperatures at which the maximum of the ion current intensity is observed were estimated in the 1830-2000 ± 10 °C range. The formation of F(2)(-) ions is probably disturbed by a dissociation process at high temperatures. CONCLUSIONS: Negative surface ionization on the hot filament is a relatively simple and effective method for carrying out negative ion formation studies. Eight SF(6) decomposition channels leading to the formation of negative ions have been detected and analyzed using this technique.

Inter-laboratory calibration of new silver orthophosphate comparison materials for the stable oxygen isotope analysis of phosphates.

Stable oxygen isotope compositions (δ(18)O values) of two commercial and one synthesized silver orthophosphate reagents have been determined on the VSMOW scale. The analyses were carried out in three different laboratories: lab (1) applying off-line oxygen extraction in the form of CO(2) which was analyzed on a dual inlet and triple collector isotope ratio mass spectrometer, while labs (2) and (3) employed an isotope ratio mass spectrometer coupled to a high-temperature conversion/elemental analyzer (TC/EA) where Ag(3)PO(4) samples were analyzed as CO in continuous flow mode. The δ(18)O values for the proposed new comparison materials were linked to the generally accepted δ(18)O values for Vennemann's TU-1 and TU-2 standards as well as for Ag(3)PO(4) extracted from NBS120c. The weighted average δ(18)O(VSMOW) values for the new comparison materials UMCS-1, UMCS-2 and AGPO-SCRI were determined to be + 32.60 (± 0.12), + 19.40 (± 0.12) and + 14.58 (± 0.13)‰, respectively.

In vacuo reduction of silver orthophosphate with graphite for high-precision oxygen isotope analysis.

The reduction of silver phosphate with graphite under vacuum conditions was studied at final reaction temperatures varying from 430 to 915°C to determine: (i) the CO(2) extraction yield, and (ii) the oxygen isotopic composition of CO(2). The CO(2) yield and oxygen isotopic composition were determined on a calibrated dual inlet and triple collector isotope ratio mass spectrometer. We observed the following three stages of the reduction process. (1) At temperatures below 590°C only CO(2) is formed, while silver orthophosphate decays to pyrophosphate. (2) At higher temperatures, 590-830°C, predominantly CO is formed from silver pyrophosphate which decays to metaphosphate; this CO was always converted into CO(2) by the glow discharge method. (3) At temperatures above 830°C the noticeable sublimation of silver orthophosphate occurs. This observation was accompanied by the oxygen isotope analysis of the obtained CO(2). The measured δ(18)O value varied from -11.93‰ (at the lowest temperature) to -20.32‰ (at the highest temperature). The optimum reduction temperature range was found to be 780-830°C. In this temperature range the oxygen isotopic composition of CO(2) is nearly constant and the reaction efficiency is relatively high. The determined difference between the δ(18)O value of oxygen in silver phosphate and that in CO(2) extracted from this phosphate is +0.70‰.

New isotope ratio mass spectrometric method of precise delta37Cl determinations.

The most precise method of chlorine isotope analysis described to date is based on the isotope ratio mass spectrometry (IRMS) of chlorine quantitatively converted into chloromethane, CH(3)Cl. This gas can be produced from several chlorine-containing compounds and analyzed by IRMS. However, the mass spectrum of chloromethane is rather complicated and the ratio of the most abundant ions (mass-52/mass-50) differs from the (37)Cl/(35)Cl isotope ratio. This difference becomes significant when the delta37 Cl exceeds 10 per thousand. Moreover, the electron ionization source yields approximately 80% of all the ionic species at the useful masses 50 and 52. To overcome these drawbacks, we have devised a negative ion mass spectrometer which retains all the best features of IRMS, including a dual-inlet system with changeover valve, dual collector assembly and CH(3)Cl gas as analyte. In the modified ion source we have replaced the ionization chamber with an electron beam by a metal tube with a hot metal filament inside it. Within this tube the (35)Cl(-) and (37)Cl(-) ions are produced with an efficiency dependent on the filament material and its temperature. No other ionic species were found in the mass spectrum except of traces at masses 26 and 28 at ppm levels, probably due to the formation of CN(-) and CO(-). The minimal amount of Cl used in our method is of the order of 5 micromol (3 mg AgCl) and the precision is better than 0.005 per thousand (1sigma).

Negative ion source for chlorine isotope ratio measurements.

A negative chlorine ion source has been designed and constructed. The source utilizes direct surface ionization of chloromethane gas on a hot metal filament. Four different alloys for the filament material were tested: W99Th1, W75Re25, Hf97.5Zr2.5 and Mo52.5Re47.5. We conclude that the best filament material is the MoRe alloy, for which the signal-to-noise ratio is optimal. The ion source is used for chlorine isotope ratio measurements with higher precision and sensitivity than the positive ionization source used previously. Inasmuch as only negative ions of the two isotopes of interest are observed, no corrections to the measured isotope ratio are necessary, and less rigously purified samples may be analyzed. The negative ion currents are considerably larger than positive ion currents obtained with an electron ionization source. This implies higher analytical precision (typically 0.005 permil) and sensitivity.