Low-temperature opto-galvanic spectroscopy without discharges

Hyperfine Interactions - In the CRYRING storage ring weak beams of electron-cooled highly charged ions have been studied. When the intensity of the ion beam decays below a certain limit the...     

The measurement of the isotopic composition of plutonium in an Irish Sea sediment by mass spectrometry

Samples from sediment cores, collected from a site close to the Sellafield outfall and dated using conventional - and -spectrometry, have been analysed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Thermal Ionisation Mass Spectrometry (TIMS). This has provided information on the isotopic composition of plutonium in the liquid discharges from Sellafield; such information is not available using conventional -spectrometry alone. There was very good agreement between the²⁴⁰Pu/²³⁹Pu ratios obtained by the two MS methods. More recent discharges have been characterised by a progressive decrease in the relative proportion of²³⁹Pu.     

Kinetic studies of grafting of maleic anhydride to hydrocarbon substrates

The kinetics of grafting of maleic anhydride to various hydrocarbon substrates has been investigated. Grafting to eicosane and squalane was effected in the pure hydrocarbons and in 1,2-dichlorobenzene solution, while polyethylene was grafted only in solution. The initiator was 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne which has a half life of about 1 h at a typical reaction temperature of 150°C. At high concentrations of initiator (Ca. 0.02M), the rate of disappearance of maleic anhydride is linear with time. In the pure hydrocarbons the order with respect to initiator is close to 0.5. In squalane, the overall activation energy is 112 kJ mol^?1; the average number of maleic anhydride molecules grafted per molecule of peroxide decomposed varies from 8 at high rates of initiation to 57 at low rates of initiation. The results are interpreted in terms of a chain mechanism, including a slow propagation step in which a succinic anhydride radical abstracts hydrogen from the same or a different chain. The same general mechanism is proposed for grafting of maleic anhydride to polyethylene and the hydrocarbons in 1,2-dichlorobenzene solution.     

Determination of total taurine in pet foods by liquid chromatography of the dansyl derivative: collaborative study

A liquid chromatographic (LC) method for the determination of total taurine in pet foods was evaluated in a collaborative study. Ten laboratories assayed 6 blind duplicate pairs of wet and dry pet foods. The taurine in the 6 sample pairs ranged from low (170 mg/kg) to high (2250 mg/kg) concentrations as is. Collaborators also assayed a sample of known taurine concentration for familiarization purposes. Samples were hydrolyzed to release bound taurine, which was subsequently converted to the dansyl derivative and quantitated by gradient-elution LC with fluorescence detection. Repeatability relative standard deviations, RSDr, ranged from 3.2 to 10.0%; reproducibility relative standard deviations, RSDR, ranged from 6.1 to 16.1%. The method has been adopted Official First Action status by AOAC INTERNATIONAL.     

Direct measurement of the energy thresholds to conformational isomerization in tryptamine: experiment and theory

The methods of stimulated emission pumping-hole filling spectroscopy (SEP-HFS) and stimulated emission pumping population transfer spectroscopy (SEP-PTS) were applied to the conformation-specific study of conformational isomerization in tryptamine [TRA, 3-(2-aminoethyl)indole]. These experimental methods employ stimulated emission pumping to selectively excite a fraction of the population of a single conformation of TRA to well-defined ground-state vibrational levels. This produces single conformations with well-defined internal energy, tunable over a range of energies from near the zero-point level to well above the lowest barriers to conformational isomerization. When the SEP step overcomes a barrier to isomerization, a fraction of the excited population isomerizes to form that product. By carrying out SEP excitation early in a supersonic expansion, these product molecules are subsequently cooled to their zero-point vibrational levels, where they can be detected downstream with a third tunable laser that probes the ground-state population of a particular product conformer via a unique ultraviolet transition using laser-induced fluorescence. The population transfer spectra (recorded by tuning the SEP dump laser while holding the pump and probe lasers fixed) exhibit sharp onsets that directly determine the energy thresholds for conformational isomerization in a given reactant-product conformer pair. In the absence of tunneling effects, the first observed transition in a given X-Y PTS constitutes an upper bound to the energy barrier to conformational isomerization, while the last transition not observed constitutes a lower bound. The bounds for isomerizing conformer A of tryptamine to B(688-748 cm(-1)), C(1)(860-1000 cm(-1)), C(2)(1219-1316 cm(-1)), D(1219-1282 cm(-1)), E(1219-1316 cm(-1)), and F(688-748 cm(-1)) are determined. In addition, thresholds for isomerizing from B to A(<1562 cm(-1)), B to F(562-688 cm(-1)), and out of C(2) to B(<747 cm(-1)) are also determined. The A-->B and B-->A transitions are used to place bounds on the relative energies of minima B relative to A, with B lying at least 126 cm(-1) above A. The corresponding barriers have been computed using both density functional and second-order many-body perturbation theory methods in order to establish the level of theory needed to reproduce experimental results. While most of the computed barriers match experiment well, the barriers for the A-F and B-F transitions are too high by almost a factor of 2. Possible reasons for this discrepancy are discussed.