Home About us Contact
|
Home About us Contact | ||
|
|
||
Standard Uncertainties (standard + uncertainty)
Selected AbstractsImprovement of scale factors for harmonic vibrational frequency calculations using new polarization functionsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2008Anibal Sierraalta Abstract Density functional calculations were carried to improve the calculated CO vibrational frequencies for transition metal carbonyls. Two types of density functionals were studied, hybrid and generalized-gradient methods. Using the simplex optimization method, new polarization functions for C and O atoms were obtained. With these new optimized functions, new scaling factors were obtained. The results reveal that, with the new polarization functions, the agreement between the calculated and the experimental values improves considerably. In general, the new scaling factors are very close to unit, with standard uncertainties close to ±0.006 cm,1. The use of the new polarization functions allows more precise calculations of the transition metal carbonyl CO vibrational frequencies. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Comparison of gas chromatography and liquid chromatography mass spectrometric measurements for high accuracy analysis of cholesterol in human serum by isotope dilution mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 9 2002Céline S. J. Wolff Briche Cholesterol measurements are of vital clinical importance and reliable reference materials are essential for method validation. Gas chromatography with mass spectrometry (GC/MS) is usually used for the high accuracy analysis of cholesterol by isotope dilution. A certified reference material for cholesterol content in human serum was analysed by isotope dilution utilising GC/MS and liquid chromatography mass spectrometry (LC/MS). The use of LC/MS avoided the need for a derivatisation step. Both LC/MS and GC/MS produced results on the measurement of cholesterol that agreed within 0.5% of the certified value. Moreover, the precision obtained for ratio measurement using both techniques are comparable and lead to relative expanded standard uncertainties (with a coverage factor of 2) varying between 0.2 and 0.5%. Copyright © 2002 John Wiley & Sons, Ltd. [source] Determination of zinc incorporation in the Zn-substituted gallophosphate ZnULM-5 by multiple wavelength anomalous dispersion techniquesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010M. Helliwell The location of isomorphously substituted zinc over eight crystallographically different gallium sites has been determined in a single-crystal study of the gallophosphate ZnULM-5, GaZn(PO4)14(HPO4)2(OH)2F7, [H3N{CH2}6NH3]4, 6H2O, in an 11 wavelength experiment, using data from Station 9.8, SRS Daresbury. The measurement of datasets around the K edges of both Ga and Zn, as well as two reference datasets away from each absorption edge, was utilized to selectively exploit dispersive differences of each metal atom type in turn, which allowed the major sites of Zn incorporation to be identified as the metal 1 and 3 sites, M1 and M3. The preferential substitution of Zn at these sites probably arises because they are located in double four-ring (D4R) building units which can relax to accommodate the incorporation of hetero atoms. As the crystal is non-centrosymmetric, with space group P21212, it was also possible to use anomalous differences to corroborate the results obtained from the dispersive differences. These results were obtained firstly from difference Fourier maps, calculated using a phase set from the refined structure from data measured at the Zr K edge. Also, refined dispersive and anomalous occupancies, on an absolute scale, could be obtained using the program MLPHARE, allowing estimates for the Zn incorporation of approximately 22 and 18 at. % at the M1 and M3 sites to be obtained. In addition, f, and f,, values for Ga and Zn at each wavelength could be estimated both from MLPHARE results, and by refinement in JANA2006. The fully quantitative determinations of the dispersive and anomalous coefficients for Ga and Zn at each wavelength, as well as metal atom occupancies over the eight metal atom sites made use of the CCP4's MLPHARE program as well as SHELXL and JANA2006. The results by these methods agree closely, and JANA2006 allowed the ready determination of standard uncertainties on the occupancy parameters, which were for M1 and M3, 20.6,(3) and 17.2,(3),at %, respectively. [source] How precise are measurements of unit-cell dimensions from single crystals?ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2000Frank H. Herbstein The results of single-site and many-site measurements of cell dimensions from single crystals are compared for Bond and four-circle diffractometers using samples of corundum (essentially pure rhombohedral ,-Al2O3, aluminum oxide) of high diffraction quality, where the effects of small changes in temperature and composition (Cr2O3, chromium oxide, in solid solution) can be taken into account. Similar comparisons are made for four-circle diffractometer measurements on ruby (,-Al2O3, with 0.46 wt % Cr in solid solution). The precisions are some parts in 105. There is partial support for the Taylor,Kennard [Acta Cryst. (1986), B42, 112,120] dictum that standard uncertainties (s.u.s) of cell parameters from routine four-circle diffractometer measurements are less than those for many-site measurements by factors of 5 for cell lengths and 2.5 for cell angles. For organic crystals, independent repetitions of adequate quality for comparison and analysis of routine four-circle diffractometer measurements are available only for ,-oxalic acid dihydrate and anthracene. The experimental standard uncertainties given for these two crystals agree reasonably well with the sample s.u.s at room temperature, but appreciably less well at ,100,K, again giving partial support to the Taylor,Kennard dictum. The relation between specimen characteristics and attainable precision is emphasized; the precisions for routine measurements on good quality organic crystals are some parts in 104. Area-detector measurements of cell dimensions have also been appraised; currently published s.u.s from such measurements appear to be highly unreliable, and this is supported by a recent analysis of the operation of such diffractometers [Paciorek et al. (1999). Acta Cryst. A55, 543,557]. Formulation of a standard protocol for such measurements is badly needed. The dangers inherent in high degrees of replication are illustrated by recounting Kapteyn's Parable of the Chinese Emperor. Attention is drawn to the fact that there has been little improvement in claimed precisions over the past 40,60 years. [source] An unusual methylene aziridine refined in P21/c and the nonstandard setting P21/nACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2009George C. Feast The unusual methylene aziridine 6- tert -butyl-3-oxa-2-thia-1-azabicyclo[5.1.0]oct-6-ene 2,2-dioxide, C9H15NO3S, was found to crystallize with two molecules in the asymmetric unit. The structure was solved in both the approximately orthogonal and the oblique settings of space group No. 14, viz. P21/n and P21/c, respectively. A comparison of these results clearly displayed an increase in the correlation between coordinates in the ac plane for the oblique cell. The increase in the corresponding covariances makes a significant contribution to the standard uncertainties of derived parameters, e.g. bond lengths. Since there is yet no CIF definition for the full variance,covariance matrix, there are clear advantages to reporting the structure in the nonstandard space-group setting. [source] Refined structure of bovine carboxypeptidase A at 1.25,Ĺ resolutionACTA CRYSTALLOGRAPHICA SECTION D, Issue 2 2003Alexandra Kilshtain-Vardi The crystal structure of the bovine zinc metalloproteinase carboxypeptidase A (CPA) has been refined to 1.25,Ĺ resolution based on room-temperature X-ray synchrotron data. The significantly improved structure of CPA at this resolution (anisotropic temperature factors, R factor = 10.4%, Rfree = 14.5%) allowed the modelling of conformational disorders of side chains, improved the description of the protein solvent network (375 water molecules) and provided a more accurate picture of the interactions between the active-site zinc and its ligands. The calculation of standard uncertainties in individual atom positions of the refined model of CPA allowed the deduction of the protonation state of some key residues in the active site and confirmed that Glu72 and Glu270 are negatively charged in the resting state of the enzyme at pH 7.5. These results were further validated by theoretical calculations that showed significant reduction of the pKa of these side chains relative to solution values. The distance between the zinc-bound solvent molecule and the metal ion is strongly suggestive of a neutral water molecule and not a hydroxide ion in the resting state of the enzyme. These findings could support both the general acid/general base mechanism, as well as the anhydride mechanism suggested for CPA. [source] Interlaboratory validation of oxidation-index measurement methods for UHMWPE after long-term shelf agingJOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 1 2002S. M. Kurtz Abstract An international oxidation index standard would greatly benefit the orthopedic community by providing a universal scale for reporting oxidation data of ultra-high molecular weight polyethylene (UHMWPE). We investigated whether severe oxidation associated with long-term shelf aging affects the repeatability and reproducibility of area-based oxidation index measurement techniques based on normalization with the use of 1370- or 2022-cm,1 infrared (IR) absorption reference peaks. Because an oxidation index is expected to be independent of sample thickness, subsurface oxidation was examined with the use of both 100- and 200-,m-thick sections from tibial components (compression-molded GUR 1120, gamma irradiated in air) that were shelf aged for up to 11.5 years. Eight institutions in the United States and Europe participated in the present study, which was administered in accordance with ASTM E691. On average, the 100-,m-thick samples were associated with significantly greater interlaboratory relative standard uncertainty (40.3%) when compared with the 200-,m samples (21.8%, p = 0.002). In contrast, the intralaboratory relative standard uncertainty was not significantly affected by the sample thickness (p = 0.21). The oxidation index method did not significantly influence either the interlaboratory or intralaboratory relative standard uncertainty (p = 0.32 or 0.75, respectively). Our interlaboratory data suggest that with the suitable choice of specimen thickness (e.g., 200 ,m) and either of the two optimal oxidation index methods, interlaboratory reproducibility of the most heavily oxidized regions in long-term shelf-aged components can be quantified with a relative standard uncertainty of 21% or less. Therefore, both the 1370-cm,1 and the 2022-cm,1 reference peaks appear equally suitable for use in defining a standard method for calculating an oxidation index for UHMWPE. © 2001 Wiley Periodicals, Inc. J Biomed Mater Res (Appl Biomater) 63: 15,23, 2002 [source] Centrosymmetric and pseudo-centrosymmetric structures refined as non-centrosymmetricACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2006H. D. Flack The behaviour of the Flack parameter for centrosymmetric and pseudo-centrosymmetric crystal structures based on crystal structures published as being non-centrosymmetric is presented. It is confirmed for centrosymmetric structures that the value obtained for the Flack parameter is critically dependent on the Friedel coverage of the intensity data, approaching 0.5 for a coverage of 100% and sticking near the starting value for a coverage of 0%. For pseudo-centrosymmetric structures, even those very close to being centrosymmetric, it is found that it is often possible to obtain significant values of the Flack parameter. A theoretical basis for this surprising result is established. It has also been possible to establish an a priori estimate of the standard uncertainty of the Flack parameter based only on the chemical composition of the compound and the wavelength of the radiation. The paper concludes with preliminary presentations of bias in the Flack parameter and of inconsistent chemical and crystallographic data. [source] | ||||||||||