Determination of plasma amino acids by gas chromatography

A complete methodology, incorporating a novel clean-up technique, for quantitative determination of amino acids in plasma by gas chromatography is described. Glucose, a component causing major analytical interference, is removed by an enzymic reaction included in the pre-chromatographic clean-up. The procedure for derivatisation of amino acid standards is shown to be reproducible down to a level of 2.5 micrograms for each amino acid, relative standard deviations for all amino acids except arginine and histidine being 3% or lower. For the entire procedure applied to plasma, relative standard deviations for most amino acids are below 5% with recoveries ranging from 90 to 120%. Normal values, obtained using eighteen plasma samples, are in reasonable agreement with published data. Plasma amino acid values were determined simultaneously by gas chromatographic and ion-exchange chromatographic techniques. Statistical evaluation shows there to be no significant difference between corresponding values for eleven amino acids. Values for tyrosine, histidine and particularly phenylalanine show significant differences (p less than 0.001).     

Polymorphism of N-(2,6-dioxo-3-piperidyl)pthalimide (thalidomide): Structural characterization of a second monoclinic racemic modification

The X-ray crystal structure of the drug N-(2,6-dioxo-3-piperidyl)pthalimide (thalidomide), C₁₃H₁₀N₂O₄, obtained from 11 dimethyformamide-ethanol solution, is reported. This species is monoclinic, space groupC2/c, witha=20.679(5),b=8.042(2),c=14.162(5) , =102.86(3)°,Z=8,R=0.051 for 1674 unique reflections. crystal packing is determined by intermolecular N–H... O hydrogen bonding which is more extensive than that reported in the literature for a racemate of thalidomide crystallizing in space groupP2₁/n. Comparison of the melting behavior and X-ray powder diffractograms of the two racemic polymorphs shows that they are distinctly different, allowing easy identification of these species. By comparing experimental X-ray powder patterns with those calculated from single crystal data, it was concluded that neither of these polymorphs undergoes a phase change on trituration.     

Chalcogen based treatment of InAs with [(NH4)2S/(NH4)2SO4]

A sulphur based chemical, ([(NH₄)₂S/(NH₄)₂SO₄]) to which S has been added not previously reported for the treatment of (111)A InAs surfaces is introduced and benchmarked against the commonly used passivants Na₂S·9H₂O and ((NH₄)₂S + S), using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It has been found that the native oxide layer present on the InAs surface is more effectively removed when treated with ([(NH₄)₂S/(NH₄)₂SO₄] + S) than with ((NH₄)₂S + S) or Na₂S·9H₂O. AES depth profiles of the sulphurized layers revealed the formation of a thin (less than 8.5 nm) In–S surface layer for both ((NH₄)₂SO₄ + S) and ([(NH₄)₂S/(NH₄)₂SO₄] + S) treatments. No evidence for the formation of As―S bonds was found. Treatment with ([(NH₄)₂S/(NH₄)₂SO₄] + S) also affected a significant improvement compared to the more established sulphur treatments in the surface morphology of the otherwise poor as-received n-InAs (111)A surface.     

Dependence of surface distribution of self-assembled InSb nanodots on surface morphology and spacer layer thickness

Self-assembled InSb nanodots (NDs) were grown on a GaSb (1 0 0) substrate using metal-organic vapour phase epitaxy (MOVPE). The effects of etching depth of the substrate and thickness of the GaSb buffer layer on the density and size distribution of single and double layer dots were studied for detector applications. The etch depth of the substrate was varied up to 30 μm. In this particular study, the dots were grown at 450 °C and the GaSb spacer thickness was varied between 50 nm and 200 nm. The optimum substrate etch depth was found to be 30 μm while the best spacer thickness was found to be 200 nm.     

Optical properties of ZnO nanoparticles synthesized by varying the sodium hydroxide to zinc acetate molar ratios using a Sol-Gel process

Material property dependence on the OH⁻/Zn²⁺ molar ratio of the precursor was investigated by varying the amount of NaOH during synthesis of ZnO. It was necessary to control the water content and temperature of the mixture to ensure the reproducibility. It was observed that the structural properties, particle size, photoluminescence intensity and wavelength of maximum intensity were influenced by the molar ratio of the precursor. The XRD spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO, indicating a single phase. UV measurements show the absorption that comes from the ZnO nanoparticles in visible region. The absorption edge of these ZnO nanoparticles are shifted to higher energies and the determined band gap energies are blue shifted as the OH⁻/Zn² molar ration increases, due to the quantum confinement effects. The photoluminescence characterization of the ZnO nanostructures exhibited a broad emission band centred at green (600 nm) region for all molar ratios except for OH⁻/Zn²⁺ = 1.7 where a second blue emission around 468 nm was also observed. The photoluminescence properties of ZnO nanoparticles were largely determined by the size and surface properties of the nanoparticles.     

Metalorganic chemical vapor deposition of ZnO:N using NO as dopant

Highly c-axis orientated ZnO was grown by metal organic chemical vapor deposition (MOCVD) using NO as both oxidant and nitrogen dopant source. The properties of the deposited material are investigated by X-ray diffraction to study the crystalline quality of the thin films. Photoluminescence measurements are used to determine the optical properties of the material as a function of VI/II ratio and post growth-annealing temperature. Two transitions appear at 3.228 and 3.156 eV and are interpreted as involving active nitrogen acceptors. An increase in the NO flow increases the concentration of nitrogen in the films, which are activated by subsequent annealing at 600 °C in an oxygen ambient.     

dc-Hydrogen plasma induced defects in bulk n-Ge

Bulk antimony doped germanium (n-Ge) has been exposed to a dc–hydrogen plasma. Capacitance–voltage depth profiles revealed extensive near surface passivation of the shallow donors as evidenced by ∼a 1.5 orders of magnitude reduction in the free carrier concentration up to depth of ∼3.2 μm. DLTS and Laplace-DLTS revealed a prominent electron trap 0.30 eV below the conduction (E_C –0.30 eV). The concentration of this trap increased with plasma exposure time. The depth profile for this defect suggested a uniform distribution up to 1.2 μm. Annealing studies show that this trap, attributed to a hydrogen-related complex, is stable up to 200 °C. Hole traps, or vacancy-antimony centers, common in this material after high energy particle irradiation, were not observed after plasma exposure, an indication that this process does not create Frenkel (V–I) pairs.     

Quantitative mineralogical properties (morphology-chemistry-structure) of pharmaceutical grade kaolinites and recommendations to regulatory agencies

The physical and chemical characteristics of kaolinite (kaolin) may be variable, and minor amounts of other clay minerals, nonclay minerals, and other impurities may affect the properties of kaolinites. Thus specific technical properties of pharmaceutical grade kaolinites become very important because these clays are used in medical applications, e.g., as pharmaceutical excipients, and will be consumed by humans. Seven pharmaceutical grade kaolinite specimens were used in this study: K1004, KA105, 2242-01, K2-500, Acros, Acros-mono, and KX0007-1. In addition, two kaolinites from the Clay Minerals Society Source Clays, KGa-1b and KGa-2, were used for comparison purposes. The Acros-mono and 2242-01 kaolinites contained minor amounts of illite, which was demonstrated both compositionally and structurally by using inductively coupled plasma spectroscopy and powder X-ray diffraction. The KX0007-1 kaolinite powder was found to be heavily contaminated with quartz, cristobalite, and alunite. Crystal structure computations also showed excess Si in its tetrahedral site, and the mineral no longer has the typical kaolinite crystal structure. These widely-used industrial standards should be quantitatively characterized morphologically, compositionally, and structurally. Results of the mineralogical characteristics should be clearly labeled on the pharmaceutical grade kaolinites and reported to the relevant regulatory agencies.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

The global element method and its extension to parabolic problems

This article describes the derivation of the global element method, originally proposed by Delves and Hall, using the classical Euler–Lagrange theory. The method is then extended so as to be applicable to parabolic partial differential equations. The main advantages of the method, the ease of adjusting its accuracy and applying it to problems with discontinuous coefficients and point singularities, are also discussed.