Diameter Determination of Frozen-Hydrated Virions Using Polyoma Virus as a Calibration Standard

1988 ◽  
Vol 46 ◽  
pp. 166-167
Author(s):  
N. H. Olson ◽  
T. S. Baker
Keyword(s):  
Accurate Determination ◽  
Carbon Substrate ◽  
Specimen Preparation ◽  
X Ray Diffraction ◽  
Polystyrene Spheres ◽  
X Ray ◽  
Negative Stain ◽  
Highly Sensitive ◽  
Particle Images

Accurate determination of particle dimensions requires both a reliable measure of the instrumental magnification and reproducible, non-distorting specimen preparation procedures. Typical calibration standards for measuring microscope magnification include replica gratings, polystyrene spheres, and negatively-stained catalase crystals. Polystyrene spheres and catalase crystals may be used as internal standards but both are highly sensitive to beam damage. Calibrations with replica gratings are subject to greater inaccuracies at magnifications exceeding 10,000-20,000 X. Furthermore, for negatively-stained biological specimens, the object of interest as well as the standard (e.g. catalase) are susceptible to significant distortions produced when the stained sample dries on the grid. The stain itself also moves during the initial stages of irradiation.1.2Large discrepancies are often found between diameter measurements from particle images with circular profiles (e.g. spherical viruses) made in the microscope and from those measurements determined by x-ray solution scattering or other x-ray diffraction techniques. Measurements from virions embedded in negative-stain, suspended over holes in a carbon substrate, are typically much lower than the corresponding measurements by x-ray techniques, reflecting a probable shrinkage of virions in the stain.

Layer stacking disorder in Mg-Fe chlorites based on powder X-ray diffraction data

2020 ◽  
Vol 105 (3) ◽  
pp. 353-362
Author(s):  
Katarzyna Luberda-Durnaś ◽  
Marek Szczerba ◽  
Małgorzata Lempart ◽  
Zuzanna Ciesielska ◽  
Arkadiusz Derkowski
Keyword(s):  
Accurate Determination ◽  
Unit Cell ◽  
Powder Xrd ◽  
X Ray Diffraction ◽  
Unit Cell Parameters ◽  
Xrd Pattern ◽  
X Ray ◽  
Cell Parameters ◽  
Layer Stacking

Abstract The primary aim of this study was the accurate determination of unit-cell parameters and description of disorder in chlorites with semi-random stacking using common X-ray diffraction (XRD) data for bulk powder samples. In the case of ordered chlorite structures, comprehensive crystallographic information can be obtained based on powder XRD data. Problems arise for samples with semi-random stacking, where due to strong broadening of hkl peaks with k ≠ 3n, the determination of unit-cell parameters is demanding. In this study a complete set of information about the stacking sequences in chlorite structures was determined based on XRD pattern simulation, which included determining a fraction of layers shifted by ±1/3b, interstratification with different polytypes and 2:1 layer rotations. A carefully selected series of pure Mg-Fe tri-trioctahedral chlorites with iron content in the range from 0.1 to 3.9 atoms per half formula unit cell was used in the study. In addition, powder XRD patterns were carefully investigated for the broadening of the odd-number basal reflections to determine interstratification of 14 and 7 Å layers. These type of interstratifications were finally not found in any of the samples. This result was also confirmed by the XRD pattern simulations, assuming interstratification with R0 ordering. Based on h0l XRD reflections, all the studied chlorites were found to be the IIbb polytype with a monoclinic-shaped unit cell (β ≈ 97°). For three samples, the hkl reflections with k ≠ 3n were partially resolvable; therefore, a conventional indexing procedure was applied. Two of the chlorites were found to have a monoclinic cell (with α, γ = 90°). Nevertheless, among all the samples, the more general triclinic (pseudomonoclinic) crystal system with symmetry C1 was assumed, to calculate unit-cell parameters using Le Bail fitting. A detailed study of semi-random stacking sequences shows that simple consideration of the proportion of IIb-2 and IIb-4/6 polytypes, assuming equal content of IIb-4 and IIb-6, is not sufficient to fully model the stacking structure in chlorites. Several, more general, possible models were therefore considered. In the first approach, a parameter describing a shift into one of the ±1/3b directions (thus, the proportion of IIb-4 and IIb-6 polytypes) was refined. In the second approach, for samples with slightly distinguishable hkl reflections with k ≠ 3n, some kind of segregation of individual polytypes (IIb-2/4/6) was considered. In the third approach, a model with rotations of 2:1 layers about 0°, 120°, 240° was shown to have the lowest number of parameters to be optimized and therefore give the most reliable fits. In all of the studied samples, interstratification of different polytypes was revealed with the fraction of polytypes being different than IIbb ranging from 5 to 19%, as confirmed by fitting of h0l XRD reflections.

Stresses in Passivated Films

1990 ◽  
Vol 188 ◽  
Author(s):  
Paul A. Flinn
Keyword(s):  
Tensile Stress ◽  
Accurate Determination ◽  
Silicon Oxynitride ◽  
X Ray Diffraction ◽  
X Ray ◽  
Curvature Measurement ◽  
Formation Of Cracks ◽  
Patterned Films ◽  
Strain Tensors

ABSTRACTAlthough wafer curvature measurement provides a rapid and accurate determination of stress in a uniform thin film, the technique is not applicable to patterned films. To study the stress in metal lines, and the effect of passivation on that stress, it is necessary to use X-ray diffraction. To obtain the sensitivity and precision required, a generalized focusing diffractometer (GFD), that had been developed especially for work on thin films, was used in this study.The elastic strain tensors for aluminum and aluminum-silicon films and patterned lines were determined by X-ray diffraction. The corresponding stress tensors were calculated with the use of the known elastic constants of aluminum. The effect of various oxide and oxynitride passivations was investigated. Passivation over uniform metal films has very little effect, while passivation over patterned metal results in substantial triaxial tensile stress in the metal. Contrary to the conventional wisdom, high compressive stress in the passivation does not result in additional tensile stress in the metal. A possible explanation for the frequently observed deleterious effect (increased tendency for formation of cracks and voids) of highly compressive silicon nitride and silicon oxynitride passivations will be discussed.

Theoretical and experimental developments for accurate determination of crystallinity of cellulose I materials

2010 ◽  
Vol 44 (1) ◽  
pp. 184-192 ◽  
Author(s):  
Carlos Driemeier ◽  
Guilherme A. Calligaris
Keyword(s):  
Accurate Determination ◽  
Dry Weight ◽  
Orientation Distribution ◽  
X Ray Diffraction ◽  
Cellulose I ◽  
Experimental Development ◽  
X Ray ◽  
Pure Cellulose ◽  
Better Than

This work defines the crystallinity of cellulose I materials on a dry-weight basis. Theoretical and experimental developments in X-ray diffraction lead to a crystallinity determination method that is estimated to reach 1σ accuracies of better than 0.05 (crystallinity defined between 0 and 1). The method is based on Rietveld modelling, to resolve cellulose I Bragg peaks, and a standard truncated invariant integral. Corrections are derived to account for incoherent scattering, moisture content and other compositional deviations from pure cellulose. The experimental development uses X-ray diffraction in transmission fibre geometry with two-dimensional pattern Rietveld modelling, including a crystal-orientation distribution function. The crystallinities of a few commercial cellulose I materials were determined with the aim of illustrating the applicability of the method.

Accurate determination of lattice mismatch in the epitaxial AlAs/GaAs system by high-resolution X-ray diffraction

1993 ◽  
Vol 132 (3-4) ◽  
pp. 427-434 ◽  
Author(s):  
C. Bocchi ◽  
C. Ferrari ◽  
P. Franzosi ◽  
A. Bosacchi ◽  
S. Franchi
Keyword(s):  
High Resolution ◽  
Lattice Mismatch ◽  
Accurate Determination ◽  
X Ray Diffraction ◽  
X Ray

scholarly journals Extracting Extremely Localized Molecular Orbitals from X-ray Diffraction Data

2014 ◽  
Vol 70 (a1) ◽  
pp. C284-C284 ◽  
Author(s):  
Alessandro Genoni
Keyword(s):  
Wave Function ◽  
Diffraction Data ◽  
Accurate Determination ◽  
Molecular Orbitals ◽  
X Ray Diffraction ◽  
Electron Densities ◽  
X Ray ◽  
Localized Molecular Orbitals ◽  
Electron Distributions

The accurate determination of electron densities in crystals from high-resolution X-ray diffraction data has become more and more important over the years. The existing techniques to accomplish this task can be subdivided into two great families: the multipole models and the wave function-based strategies. The former, which are the most widely used, are essentially linear scaling and allow an easy chemical interpretation of the obtained molecular charge densities, but they are also characterized by some drawbacks, such as the possible presence of unphysical negative regions in the resulting electron distributions. On the contrary, the latter always provide quantum mechanically rigorous electron densities, but they are more computationally expensive and, above all, the ease of chemical interpretation is almost completely lost. In this context, in order to combine the easy chemical interpretability of the multipole models with the quantum mechanical rigor of the wave-function based methods, we have recently extended the X-ray constrained wave function approach proposed by Jayatilaka in the framework of a quantum chemistry technique for the a priori determination of Extremely Localized Molecular Orbitals (ELMOs), namely we have developed a new strategy that allows to extract from X-ray diffraction data a single Slater determinant built up wit Molecular Orbitals strictly localized on small molecular fragments (e.g., atoms, bonds or functional groups). Preliminary tests have shown that the determination of X-ray constrained ELMOs is really straightforward. Furthermore, given the reliable transferability of the obtained Molecular Orbitals, we are constructing new ELMOs databases that can be used as alternative to the existing pseudo-atoms libraries for refining crystallographic structures and electron distributions of large systems. A detailed comparison between the new technique and the multipole models is also currently under investigation.

Oxford HEXameter: Laboratory High Energy X-Ray Diffractometer for Bulk Residual Stress Analysis

2006 ◽  
Vol 524-525 ◽  
pp. 743-748 ◽  
Author(s):  
Alexander M. Korsunsky ◽  
Shu Yan Zhang ◽  
Daniele Dini ◽  
Willem J.J. Vorster ◽  
Jian Liu
Keyword(s):  
Accurate Determination ◽  
High Energy ◽  
Energy Dispersive ◽  
Detector Response ◽  
X Rays ◽  
X Ray Diffraction ◽  
X Ray ◽  
New Instrument ◽  
Synchrotron Beamlines

Diffraction of penetrating radiation such as neutrons or high energy X-rays provides a powerful non-destructive method for the evaluation of residual stresses in engineering components. In particular, strain scanning using synchrotron energy-dispersive X-ray diffraction has been shown to offer a fast and highly spatially resolving measurement technique. Synchrotron beamlines provide best available instruments in terms of flux and low beam divergence, and hence spatial and measurement resolution and data collection rate. However, despite the rapidly growing number of facilities becoming available in Europe and across the world, access to synchrotron beamlines for routine industrial and research use remains regulated, comparatively slow and expensive. A laboratory high energy X-ray diffractometer for bulk residual strain evaluation (HEXameter) has been developed and built at Oxford University. It uses a twin-detector setup first proposed by one of the authors in the energy dispersive X-ray diffraction mode and allows simultaneous determination of macroscopic and microscopic strains in two mutually orthogonal directions that lie approximately within the plane normal to the incident beam. A careful procedure for detector response calibration is used in order to facilitate accurate determination of lattice parameters by pattern refinement. The results of HEXameter measurements are compared with synchrotron X-ray data for several samples e.g. made from a titanium alloy and a particulate composite with an aluminium alloy matrix. Experimental results are found to be consistent with synchrotron measurements and strain resolution close to 2×10-4 is routinely achieved by the new instrument.

scholarly journals A simple pressure-assisted method for MicroED specimen preparation

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Jingjing Zhao ◽  
Hongyi Xu ◽  
Hugo Lebrette ◽  
Marta Carroni ◽  
Helena Taberman ◽  
...  
Keyword(s):  
High Viscosity ◽  
Specimen Preparation ◽  
Universal Method ◽  
X Ray Diffraction ◽  
Simple Method ◽  
X Ray ◽  
Wide Range ◽  
Major Bottleneck ◽  
Crystal Electron

AbstractMicro-crystal electron diffraction (MicroED) has shown great potential for structure determination of macromolecular crystals too small for X-ray diffraction. However, specimen preparation remains a major bottleneck. Here, we report a simple method for preparing MicroED specimens, named Preassis, in which excess liquid is removed through an EM grid with the assistance of pressure. We show the ice thicknesses can be controlled by tuning the pressure in combination with EM grids with appropriate carbon hole sizes. Importantly, Preassis can handle a wide range of protein crystals grown in various buffer conditions including those with high viscosity, as well as samples with low crystal concentrations. Preassis is a simple and universal method for MicroED specimen preparation, and will significantly broaden the applications of MicroED.

An accurate determination of the crystal structure of triclinic potassium dichromate, K2Cr2O7

1968 ◽  
Vol 46 (6) ◽  
pp. 933-941 ◽  
Author(s):  
J. K. Brandon ◽  
I. D. Brown
Keyword(s):  
Crystal Structure ◽  
Potassium Dichromate ◽  
Accurate Determination ◽  
X Ray Diffraction ◽  
X Ray ◽  
Space Groups ◽  
Cell Refinement ◽  
Final Agreement ◽  
Dichromate Ions

The crystal structure of triclinic potassium dichromate has been determined by single crystal X-ray diffraction. The cell constants are a = 13.367, b = 7.376, c = 7.445 Å, α = 90.75°, β = 96.21°, γ = 97.96° with four K2Cr2O7 units per cell. Refinement of 2600 reflections in both the space groups P1 and [Formula: see text] leads to the same structure. This is in agreement with the results of anomalous dispersion measurements, confirming that [Formula: see text] is the correct space group. The final agreement index, R, is 0.054. The two crystallographically independent dichromate ions are similar, deviating only slightly from C2v) symmetry. The Cr—O (terminal) distance is 1.63 Å, the Cr—O (bridging) distance is 1.79 Å and all angles at the chromium atoms are tetrahedral except for one of the O(bridging)—Cr—O(terminal) angles in each ion which is 106°. The angles at the bridging oxygen atoms are 124° and 128°. The geometry of the anion is compared with that found in a number of similar groups.

The Determination of Quartz in Perlite by X-ray Diffraction

1989 ◽  
Vol 33 ◽  
pp. 493-497 ◽  
Author(s):  
R. D. Hamilton ◽  
N. G. Peletis
Keyword(s):  
Detection Limit ◽  
Sample Preparation ◽  
Accurate Determination ◽  
Analytical Techniques ◽  
Crystalline Silica ◽  
X Ray Diffraction ◽  
X Ray ◽  
Precision And Accuracy ◽  
Improved Technique

AbstractIARC's designation of crystalline silica as a “probable carcinogen” triggered the requirement to label products containing greater than 0.1 % crystalline silica. For perlite and other materials which may contain crystalline silica in levels close to 0.1% an accurate determination is critical from both legal and marketing considerations.Existing analytical techniques for the determination of crystalline silica at levels of less than 1.0% were found to be inadequate to meet the new requirements. An improved technique based on x-ray diffraction has been developed specifically to analyze perlite for crystalline silica, which occurs largely in the form of quartz, at the 0.1%. level. The technique employs long counting times and improved sample preparation and mounting to increase both precision and accuracy, and to lower the detection limit to less than 0.1%.The technique was tested on a large number of samples from a variety of sources and proven to give excellent results for all types of expanded perlites and perlite ores. The procedures developed are applicable to a wide variety of materials in addition to perlite.

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