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141.
P. L. Reeder T. W. Bowyer R. W. Perkins 《Journal of Radioanalytical and Nuclear Chemistry》1998,235(1-2):89-94
A beta-gamma coincidence counting system has been developed for automated analysis of Xe gas samples separated from air. The
Xe gas samples are contained in a cylindrical plastic scintillator cell located between two NaI(T1) scintillation detectors.
The X-ray and gamma spectra gated by coincident events in the plastic scintillator cell are recorded for each NaI(T1) crystal.
The characteristic signatures of the131mXe,133gXe,133mXe, and135gXe isotopes of interest for nuclear test-ban verification as well as the procedures and results of absolute efficiency measurements
are described. A NaI(T1) crystal with provision for 4 sample cells has been implemented for the system to be deployed in the
field. Examples of data on ambient air samples in New York City obtained with the field prototype are presented. 相似文献
142.
143.
McCreery R Dieringer J Solak AO Snyder B Nowak AM McGovern WR DuVall S 《Journal of the American Chemical Society》2003,125(35):10748-10758
Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3 V was 0.71 A/cm(2), or about 10(5) e(-)/s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1 e(-)/molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude. 相似文献
144.
Researchers continue to emphasize the importance of covariational reasoning in the context of students’ function concept, particularly when graphing in the Cartesian coordinate system (CCS). In this article, we extend the body of literature on function by characterizing two pre-service teachers’ thinking during a teaching experiment focused on graphing in the polar coordinate system (PCS). We illustrate how the participants engaged in covariational reasoning to make sense of graphing in the PCS and make connections with graphing in the CCS. By foregrounding covariational relationships, the students came to understand graphs in different coordinate systems as representative of the same relationship despite differences in the perceptual shapes of these graphs. In synthesizing the students’ activity, we provide remarks on instructional approaches to graphing and how the PCS forms a potential context for promoting covariational reasoning. 相似文献
145.
Stacy L. Beun 《代数通讯》2013,41(4):1334-1352
Symmetric k-varieties are a generalization of symmetric spaces to general fields. Orbits of a minimal parabolic k-subgroup acting on a symmetric k-variety are essential in the study of symmetric k-varieties and their representations. In this article, we present the classification of these orbits for the group SL(2,k) for a number of base fields k, including finite fields and the 𝔭-adic numbers. We use the characterization in Helminck and Wang (1993), which requires one to first classify the orbits of the θ-stable maximal k-split tori under the action of the k-points of the fixed point group. 相似文献
146.
Electrophoretic exclusion, a novel separations technique that differentiates species in bulk solution using the opposing forces of electrophoretic velocity and hydrodynamic flow, has been adapted to a microscale device. Proof-of-principle experiments indicate that the device was able to exclude small particles (1 μm polystyrene microspheres) and fluorescent dye molecules (rhodamine 123) from the entrance of a channel. Additionally, differentiation of the rhodamine 123 and polystyrene spheres was demonstrated. The current studies focus on the direct observation of the electrophoretic exclusion behavior on a microchip. 相似文献
147.
Cara L. Copeland Stacy D. Brown Brooks B. Pond 《Biomedical chromatography : BMC》2013,27(12):1587-1589
Methylphenidate, a psychostimulant used for the treatment of attention deficit hyperactivity disorder and narcolepsy, is administered as a 50:50 racemic mixture, despite the fact that d‐methylphenidate has been shown to have greater pharmacologic activity. This paper presents a validated LC‐MS/MS approach to separation and quantification of methylphenidate enantiomers using a vancomycin column and triethylammonium acetate to enhance the chiral separation. The method is applicable to the monitoring of these enantiomers in mouse brain, with a limit of detection of 0.5 ng/mL and a lower limit of quantification of 7.5 ng/mL. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
148.
Mauro Claudino Inge van der Meulen Stacy Trey Mats Jonsson Andreas Heise Mats Johansson 《Journal of polymer science. Part A, Polymer chemistry》2012,50(1):16-24
The increasing demand for bioderived polymers led us to investigate the potential use of the macrolactone globalide in thermoset synthesis via the photoinduced thiol–ene reaction. A series of six lipase‐catalyzed poly(globalide‐caprolactone) copolyesters bearing internal main‐chain unsaturations ranging from 10 to 50 and 100 mol % were successfully crosslinked in the melt with equal amounts of thiol groups from trimethylolpropane‐trimercapto propionate affording fully transparent amorphous elastomeric materials with different thermal and viscoelastic properties. Three major conclusions can be drawn from this study: (i) high thiol–ene conversions (>80%) were easily attained for all cases, while maintaining the cure behavior, and irrespective of functionality at reasonable reaction rates; (ii) parallel chain‐growth homopropagation of the ene monomer is insignificant when compared with the main thiol–ene coupling route; and (iii) high ene‐density copolymers result in much lower extracted sol fractions and high Tg values as a result of a more dense and homogeneous crosslinked network. The thiol–ene system evaluated in this contribution serve as model example for the sustainable use of naturally occurring 1,2‐disubstituted alkenes in making semisynthetic polymeric materials in high conversions with a range of properties. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012. 相似文献
149.
Michele M. Schantz Danielle Cleveland N. Alan Heckert John R. Kucklick Stefan D. Leigh Stephen E. Long Jennifer M. Lynch Karen E. Murphy Rabia Olfaz Adam L. Pintar Barbara J. Porter Savelas A. Rabb Stacy S. Vander Pol Stephen A. Wise Rolf Zeisler 《Analytical and bioanalytical chemistry》2016,408(16):4257-4266
150.
A series of 1,3‐bis(2′‐hydroxyethyl)imidazolium ionic liquids is reported where 1H NMR chemical shift values and thermal stabilities (Td), as determined by thermogravimetric analysis, are correlated with the hydrogen bonding capability of various anions ([Cl?], [Br?], [CF3CO2?], [NO2?], [MsO?], [NO3?], [TfO?], [BF4?], [NTf2?], and [PF6?]). Use of anions with the strongest hydrogen bonding capability, such as chloride [Cl?], bromide [Br?], and trifluoroacetate [CF3CO2?], led to the furthest observed downfield chemical shift values in DMSO‐d6 and the poorest thermal stabilities ([CF3CO2?] < 200 °C). Thermal stabilities in excess of 350 °C and upfield chemical shift values were observed for ionic liquids, which employed the weakly coordinating triflate [OTf?], tetrafluoroborate [BF4?], or bis(trifluoromethylsulfonyl)imide [NTf2?] anion. Optimized structures of selected ionic liquids, as determined by density functional theory calculations at the B3LYP/6‐31G + (d,p) level, indicated that the anion preferred to be located above the imidazolium ring and in close proximity to the hydroxyl groups. Calculated dissociation energies (ΔE) and a comparison of key bonding distances (C2―H, (C2)H···X, O―H, and (O)H···X) also confirmed this structural preference. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献