X-ray determination of the mean amplitudes of vibration and debye temperatures of some rare earth monochalcogenides

The x-ray diffraction intensities of Bragg reflections have been measured at room temperature for thulium selenide, samarium sulphide, samarium selenide and samarium telluride. On the basis of a common amplitude approximation, the Debye-Waller factor, the mean amplitude of vibration and the Debye temperature have been evaluated. The values of the Debye temperatures and mean amplitudes of vibration are 176±16°K, 0·185 ± 0·017 Å (TmSe), 155 ± 7°K, 0·244 ± 0·012 Å (SmS), 153 ± 14°K, 0·221 ± 0·020 Å (SmSe) and 151 ± 20°K, 0·204 ± 0·027 Å (SmTe).     

Analysis of coefficient identification problems associated to the inverse Euler-Bernoulli beam theory

A rigorous investigation of the identification of a heterogeneousflexural rigidity coefficient in the Euler-Bernoulli steady-statebeam theory in the presence of a prescribed load is presented.Mathematically, this study is an extension to higher-order differentialequations of the coefficient identification problem analysedby Marcellini (1982) for the one-dimensional Poisson equation.In addition, various types of boundary conditions are discussed.Conditions for the well-posedness of these inverse problemsare established and, furthermore, numerical results obtainedusing a regularization algorithm are presented.     

A critical investigation of the effects of the radio frequency potential on the trapping of externally injected ions in ion trap mass spectrometry

The sensitivity of all ion trap mass spectrometry (ITMS) methods is dependent on the trapping efficiency of the instrument. For ITMS instruments utilizing external ion sources, such as laser desorption, trapping efficiency is known to depend on the phase and amplitude of the radio frequency (RF) potential applied to the ring electrode at the time of ion introduction. It is remarkable that, in a considerable body of literature, no consensus exists regarding the effects of these parameters on the efficacy of trapping externally generated ions. In this paper, a summary of the literature is presented in order to highlight significant discrepancies. New laser desorption ion trap mass spectrometry (LD-ITMS) data are also presented, from which conclusions are drawn in our effort to clarify some of the confusion. Copyright 1999 John Wiley & Sons, Ltd.     

Colloidal stability and drug incorporation aspects of micellar-like PLA–PEG nanoparticles

The drug delivery properties of a series of poly(lactic acid)–poly(ethylene glycol) (PLA–PEG) micellar-like nanoparticles have been assessed in terms of their colloidal stability and their ability to incorporate a water soluble drug. These studies have focused on a range of PLA–PEG copolymers with a fixed PEG block (5 kDa) and a varying PLA segment (3–110 kDa). In aqueous media, these copolymers formed micellar-like assemblies following precipitation from water miscible solvents. There was a controlled increase in the particle size as the molecular weight of the PLA block was increased. The characteristics of the PEG corona were also highly dependent on the PLA moiety. Copolymers with a low molecular weight PLA block (3–15 kDa) formed highly colloidally stable dispersions, with a complete PEG surface coverage. However, increasing the molecular weight of the PLA block resulted in significantly less colloidally stable nanoparticle dispersions, which flocculated in solvents that were significantly better than θ-solvents for the stabilising PEG chains. This can be attributed to a reduced PEG surface coverage and the probable presence of naked PLA ‘patches’ on the particle surface. These larger PLA–PEG nanoparticles (30:5–110:5) were found to be stabilised in the presence of serum components, which are thought to adsorb into the gaps on the particle surface and prevent flocculation. All of the dispersions were found to be stable under physiological conditions and therefore suitable for in vivo administration. A reasonable loading (3.1% w/w) of the micellar-like PLA–PEG 30:5 nanoparticles with the water soluble drug procaine hydrochloride was achieved. The incorporated drug was found to have no effect on the nanoparticle structure or recovery, which can be attributed to the micellar character of these assemblies and the presence of the stabilising PEG chains.     

Metal electrodes for continuous amperometric measurement of free hydrofluoric acid in acidic solutions containing complexing ions

Titanium-based electrodes have been demonstrated to be useful for measuring free HF concentration in the range 0.01-0.07M at temperatures between 19 and 61 degrees C in acidic, complexed HF solutions, by measurement of the current density resulting when the electrode potential is approximately +0.6 V with respect to an inert electrode. The current density, i, is linearly related to the HF concentration when the solution is adequately stirred to remove diffusion effects at the electrode, and is independent of HNO(3) concentration up to 1.75M. The dependence on the absolute temperature is ln i = - A/T + B + C ln T, and the activation energy at an absolute temperature T(r) is R(A + T(r)C), where A,B and C are constants and R is the gas constant. For a Ti-2% Pd electrode, the activation energy for anodic dissolution is found to be 43.7 +/- 0.2 kJ/mole at 312 K. The equations relating the current density to HF concentration at different temperatures can be combined to give a single equation for [HF] as a function of both i and T.