High precision delta(17)O isotope measurements of oxygen from silicates and other oxides: method and applications

The use of infrared laser-assisted fluorination to release oxygen from milligram quantities of silicates or other oxide mineral grains is a well-established technique. However, relatively few studies have reported the optimisation of this procedure for oxygen-17 isotope measurements. We describe here details of an analytical system using infrared (10 μm) laser-assisted fluorination, in conjunction with a dual inlet mass spectrometer of high resolving power ( approximately 250) to provide (17)O and (18)O oxygen isotope measurements from 0.5-2 mg of silicates or other oxide mineral grains. Respective precisions (1) of typically 0.08 and 0.04 per thousand are obtained for the complete analytical procedure. Departures from the mass-dependent oxygen isotope fractionation line are quantified by Delta(17)O; our precision (1) of such measurements on individual samples is shown to be +/-0.024 per thousand. In turn, this permits the offset between parallel, mass-dependent fractionation lines to be characterised to substantially greater precision than has been possible hitherto. Application of this system to investigate the (17)O versus (18)O relationship for numerous terrestrial whole-rock and mineral samples, of diverse geological origins and age, indicates that the complete data set may be described by a single, mass-dependent fractionation line of slope 0.5244+/- 0.00038 (standard error). Copyright 1999 John Wiley & Sons, Ltd.     

Test of bell’s inequality using the one-atom micromaser

We examine a local realist bound in the case of a one-atom micromaser. It is shown that such a bound is violated using a simplified treatment of the micromaser. We consider the effect of dissipation in a proposed experiment with the real micromaser. It is seen that the magnitude of violation of a Bell-type inequality depends significantly on the cavity parameters.     

Optical spectroscopy of Cr3+ and Cr3+-Tm3+ in alkaline silicate glasses

Conclusions Alkaline silicate glasses seem to be good candidates as host materials for codoping with Cr³⁺ and Tm³⁺. Cr³⁺ ions occupy mainly low-field sites in them, and their broadband emission overlaps Tm³⁺ excitation. As a consequence very high efficiency for the energy transfer Cr³⁺→Tm³⁺ is achieved with moderate concentrations of these ions. Moreover, the average lifetime of the Cr³⁺ ions in codoped glasses is long enough to allow significant energy storage by flashlamp pumping. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 115–120, September–October, 1995.     

Implications of recording strategy for estimates of neocortical dynamics with electroencephalography

Neocortical dynamics evidently involves very complex, nonlinear processes including top-down and bottom-up interactions across spatial scales. The dynamics may also be strongly influenced by global (periodic) boundary conditions. The primary experimental measure of human neocortical dynamics at short time scales ( approximately few ms) is the scalp electroencephalogram (EEG). It is shown that different recording and data analysis strategies are sensitive to different parts of the spatial spectrum. Thus experimental measures of system dynamics (e.g., correlation dimension estimates) can generally be expected to depend on experimental method. These ideas are illustrated in two ways: a large scale, quasilinear theory of neocortical dynamics in which standing wave phenomenon occur with predicted frequencies in the general range of EEG, and a relatively simple nonlinear physical system consisting of a linear string with attached nonlinear springs. The string/springs system is integrated numerically to illustrate transitions from periodic to chaotic behavior as mesoscopic nonlinear influences dominate macroscopic linear effects. The implications of these results for new theories of neocortical dynamics, experimental estimates of dynamic properties, and cognitive EEG studies are considered.     

Dopant distribution in a Tm(3+)-Yb(3+) codoped silica based glass ceramic: an infrared-laser induced upconversion study

The optically active dopant distribution in a Tm(3+)-Yb(3+) doped silica based glass ceramic sample has been investigated. A systematic analysis of the upconversion fluorescence of the Tm(3+)-Yb(3+) codoped glass and glass ceramic has been performed at room temperature. Tm(3+) and Yb(3+) single doped glass and glass ceramics have also been included in the study. Upon infrared excitation at 790 nm into the (3)H(4) level of the Tm(3+) ions a blue upconversion emission is observed, which is drastically increased in the Yb(3+) codoped samples. A rate equation model confirmed the energy transfer upconversion mechanism. Based on these results, the temporal dynamic curves of the levels involved in the upconversion process, (3)H(4), (2)F(5/2), and (1)G(4) were interpreted in the glass ceramic samples. The contribution of the optically active Tm(3+) and Yb(3+) ions in the crystalline and in the vitreous phase of the glass ceramic was distinguished and the ratio of Tm(3+) ions in the crystalline phase could be quantified for the 1 mol % Tm(3+)-2.5 mol % Yb(3+) glass ceramic. A surprising result was obtained for that concentration: the main contribution to the upconversion emission of the glass ceramic is due to Tm(3+)-Yb(3+) ions in the vitreous phase.     

Carbon-13 as a tool for the study of carbohydrate structures, conformations and interactions

The application of 13C-NMR spectroscopy to problems involving the structures and interactions of carbohydrates is described. Both 13C-enriched and natural abundance compounds were used and some advantages of the use of the stable isotope are described. Carbon-carbon and carbon-proton coupling constants obtained from 1-13C enriched carbohydrates were employed in the assignment of their chemical shifts and to establish solution conformation. In all cases studied thus far, C-3 couples to C-1 only in the beta-anomers while C-5 couples to C-1 only in the alpha-anomers. C-6 and C-2 always couple to C-1 in both anomeric species. The alkaline degradation of glucose [1-13C] to saccharinic acids was followed by 13C-NMR. The conversion of glucose [1-13C] to fructose-1, 6-bisphosphate [1, 6-13C] by enzymes of the glycolytic pathway was shown as an example of the use of 13C-enriched carbohydrates to elucidate biochemical pathways. In a large number of glycosyl phosphates the 31P to H-1 and 31P to C-2 coupling constants demonstrate that in the preferred conformation and phosphate group lies between the O-5 and the H-1 of the pyranose ring. The influence of paramagnetic Mn2 + ions on the proton decoupled 13C-NMR spectra of uridine diphosphate N-acetylglucosamine indicates that the Mn2 + interacts strongly with the pyrophosphate moiety and with the carbonyl groups of the uracil and N-acetyl groups.