Accuracy Of Quantitative Phase Analysis Research Articles

Accurate and time-saving quantification of a component present in a very small amount in a mixture by the direct derivation method

In quantitative phase analysis (QPA) using the direct derivation (DD) method, total sums of diffracted/scattered intensities for individual components are used as observed quantities. Fluctuation in their relative intensity ratios induces errors in derived weight fractions, and it ought to be suppressed for improving the accuracy in QPA, in particular, of a component that is present in a small amount. The fluctuation is primarily caused by the termination in summing/integrating diffracted/scattered intensities on the high-angle side. It is usually associated with changing the 2θ range in whole-powder-pattern fitting (WPPF) used to decompose the mixture pattern into individual component patterns. In this study, calculated patterns for individual components, fitted in WPPF, are normalized so as to give the unit area when they are separately integrated over their definition ranges in 2θ. The termination effect could effectively be reduced by extending the definition range to a certain high-angle limit. Scale parameters for adjusting the calculated patterns become non-fluctuating against the change of the 2θ range in WPPF. Thus, the time spent for intensity data collection of mixture patterns can be reduced by shortening the scan range. The present procedure has been tested with binary mixtures containing small amounts of crystalline phases of 0.02–0.4 wt%. QPA could be conducted with errors of 0.01–0.03 wt% for both inorganic materials chosen as ideal samples and pharmaceutical materials as practical ones. QPA of an amorphous component present in a small amount is also discussed.

Study of the phase composition of fine-grained tungsten carbide based ceramic materials by x-ray phase analysis

The results of X-ray diffraction analysis of qualitative and quantitative phase composition of powder and solid polycrysral samples of the system a-WC/W 2 C are presented. Powder (nanopowder) samples were obtained by plasmochemical synthesis from tungsten oxide and hydrocarbon. High-density fine-grained structure of ceramics was formed by electropulse plasma sintering (spark plasma sintering) of initial industrial a-WC powders (AlfaAesar). Experimental data were obtained on a Shimadzu XRD-7000 X-ray diffractometer (CuKa, λ = 1.54 A). The optimal modes of X-ray shooting of the ceramic samples based on tungsten carbide are presented. In the absence of reference standards for initial crystalline phases (WC and W2C), quantitative phase analysis was carried out using the reference intensity ratio (RIR) or corundum number (instead of the method of calibration curve). The required quantitative ratios were calculated using the structural parameters of corundum a-Al 2 O 3 and phases a-WC, W 2 C. The results of determining the repeatability of the intensity values were used to select the optimal exposure time and width of the receiving slit of the detector of diffracted X-rays. Study of the effect of the quality of sample preparation on the sensitivity of qualitative X-ray phase analysis showed the feasibility of grinding of the surface of solid ceramic samples and their pre-polishing with a diamond paste (grain size not less than 5/3 pm). The accuracy of quantitative phase analysis of powder and sintered ceramic samples was evaluated. The layer-by-layer phase analysis of the ceramic preform revealed a preferential crystallite orientation (texture) in the surface layer and the absence of the impurity phase W 2 C. The sensitivity of X-ray phase analysis to the content of a-W 2 C phase in nanopowders of tungsten monocarbide has been estimated.

EVOLUTIONARY QUANTITATIVE FULL-PROFILE X-RAY PHASE ANALYSIS BASED ON THE RIETVELD METHOD, A SELF-CONFIGURABLE MULTIPOPULATION GENETIC ALGORITHM AND ELEMENTAL ANALYSIS DATA

We developed a self configuring genetic algorithm to quantify phase concentrations in a crystalline sample from powder X-ray diffraction data. The algorithm does not require the fine-tuning of parameters, which is inherent to most evolutionary algorithms. The software executing the algorithm uses parallel computing and allows performing reference-free quantitative phase analysis on a personal computer, a computing cluster or with the help of a computer network. The suggested method was tested on a set of trial samples with known composition. It was demonstrated that one may use data on the chemical composition of a sample to increase the accuracy of quantitative phase analysis.

Accuracy in quantitative phase analysis: the impact of instrument geometry

Accuracy in quantitative phase analysis: the impact of instrument geometry

Field-based accounting of CO2sequestration in ultramafic mine wastes using portable X-ray diffraction

Carbon mineralization, the sequestration of carbon within minerals, presents one method through which we could control rising levels of anthropogenic carbon dioxide (CO) emissions. The mineral wastes produced by some ultramafic-hosted mines have the ability to sequester atmospheric CO via passive carbonation reactions. Carbon accounting in mine tailings is typically performed using laboratory-based quantitative X-ray diffraction (XRD) or thermogravimetric methods, which are used to measure the abundances of carbonate-bearing minerals such as hydromagnesite [Mg(CO)(OH)·4HO] and pyroaurite [Mg Fe23+ $\text{Fe}^{3+}-{2}$ (CO)(OH)·4HO]. The recent development of portable XRD instruments now allows for the characterization and quantification of minerals in the field. Here we assess the feasibility of using a portable XRD instrument for field-based carbon accounting in tailings from the Woodsreef Chrysotile Mine, New South Wales, Australia. Modal mineralogy was obtained by Rietveld refinements of data collected with an inXitu Terra portable XRD. The Partial Or No Known Crystal Structures (PONKCS) method was used to account for turbostratic stacking disorder in serpentine minerals, which are the dominant phases in tailings from Woodsreef. Weighed mixtures of synthetic tailings were made to evaluate the precision and accuracy of quantitative phase analysis using the portable instrument. An average absolute deviation (bias) of 8.2 wt% from the actual composition of the synthetic tailings was found using the portable instrument. This is comparable to the bias obtained using a laboratory-based diffractometer (9.6 wt% absolute) and to the results from previous quantitative XRD studies involving serpentine minerals. The methodology developed using the synthetic tailings was then applied to natural tailings samples from Woodsreef. Surface crusts forming on the tailings pile were found to contain hydromagnesite (∼5.8 wt%) and pyroaurite (∼2.1 wt%). Comparable results were obtained using the laboratory-based instrument and these results are expected to have similar biases to the analyses of the synthetic tailings. These findings demonstrate that portable XRD instruments may be used for field-based measurement of carbon sequestration in minerals in engineered and natural environments.

Quantitative phase analysis of challenging samples using neutron powder diffraction. Sample #4 from the CPD QPA round robin revisited

Quantitative phase analysis (QPA) using neutron powder diffraction more often than not involves non-ambient studies where no sample preparation is possible. The larger samples and penetration of neutrons versus X-rays makes neutron diffraction less susceptible to inhomogeneity and large grain sizes, but most well-characterized QPA standard samples do not have these characteristics. Sample #4 from the International Union of Crystallography Commission on Powder Diffraction QPA round robin was one such sample. Data were collected using the POWGEN time-of-flight (TOF) neutron powder diffractometer and analysed together with historical data from the C2 diffractometer at Chalk River. The presence of magnetic reflections from Fe3O4 (magnetite) in the sample was an additional consideration, and given the frequency at which iron-containing and other magnetic compounds are present during in-operando studies their possible impact on the accuracy of QPA is of interest. Additionally, scattering from thermal diffuse scattering in the high-Q region (<0.6 Å) accessible with TOF data could impact QPA results during least-squares because of the extreme peak overlaps present in this region. Refinement of POWGEN data was largely insensitive to the modification of longer d-spacing reflections by magnetic contributions, but the constant-wavelength data were adversely impacted if the magnetic structure was not included. A robust refinement weighting was found to be effective in reducing quantification errors using the constant-wavelength neutron data both where intensities from magnetic reflections were ignored and included. Results from the TOF data were very sensitive to inadequate modelling of the high-Q (low d-spacing) background using simple polynomials.

Accuracy in quantitative phase analysis of mixtures with large amorphous contents. The case of stoneware ceramics and bricks

For the first time, this work inspects the accuracy of quantitative phase analysis of both crystalline and amorphous components of stoneware tiles and bricks. A number of variables were considered: the nature of the internal standard, experimental conditions and counting statistics. The so-calledG-factor method has also been applied. The results of the X-ray powder diffraction analysis have been compared with the results obtained with optical microscopy and image analysis. Only the mixtures spiked with corundum and silicon yielded accurate weight estimates of the amorphous fraction, whereas the use of highly X-ray absorbing internal standards (such as fluorite, rutile and zincite) resulted in gross underestimations. In fact, microabsorption effects are found to drastically reduce the accuracy of the results when standards with linear X-ray absorption coefficients higher than 100 cm−1are employed. It was found that very low counting statistics reduced the calculated amorphous fractions in both bricks and stoneware tiles owing to partial masking of the major peak of the internal standard, namely corundum. The application of theG-factor method to the systems investigated was also evaluated. The results are poorer than those obtained using the internal standard.

Accuracy in quantitative phase analysis of mixtures with large amorphous contents. The case of zircon-rich sanitary-ware glazes

The accuracy of quantitative phase analysis (QPA) of samples with dominant amorphous content, reproducing zircon-rich sanitary-ware glazes, has been investigated. X-ray powder diffraction (XRPD) methods were applied using both conventional Cu Kα radiation and high-resolution synchrotron data. In this work, a combination of the reference intensity ratio (RIR) and Rietveld methods was applied to an artificial mixture (90 wt% glass, 10 wt% zircon), taking into account some of the most common effects that may affect the accuracy in amorphous quantification, such as the degree of crystallinity of the phases, microabsorption and sample preparation. Certified NIST SRM 676a (α-Al2O3) [Cline, Von Dreele, Winburn, Stephens & Filliben (2011).Acta Cryst.A67, 357–367] was used to quantify the amorphous content in zircon and in the different internal standards commonly used when a certified standard is not available or not applicable: the results show that all of the phases invariably contain amorphous material in the range 2.0–15.0 wt%. If the amorphous content of the standard is taken into account, the accuracy of the QPA of the artificial mixture is improved. It was observed that the Brindley correction for microabsorption does not significantly improve the results. Care must be applied if grinding time is increased, since this may increase the amorphous content in the sample. Finally, the sensitivity of the RIR–Rietveld method to the addition of a small amount of zircon (∼1 wt%) has been considered, showing that accurate results can be achieved if great care is taken in the sample preparation and refinement strategy.

Evaluation of laboratory powder X-ray micro-diffraction for applications in the fields of cultural heritage and forensic science

The uniqueness and limited amounts of forensic samples and samples from objects of cultural heritage together with the complexity of their composition requires the application of a wide range of micro-analytical methods, which are non-destructive to the samples, because these must be preserved for potential late revision. Laboratory powder X-ray micro-diffraction (micro-XRD) is a very effective non-destructive technique for direct phase analysis of samples smaller than 1 mm containing crystal constituents. It compliments optical and electron microscopy with elemental micro-analysis, especially in cases of complicated mixtures containing phases with similar chemical composition. However, modification of X-ray diffraction to the micro-scale together with its application for very heterogeneous real samples leads to deviations from the standard procedure. Knowledge of both the limits and the phenomena which can arise during the analysis is crucial for the meaningful and proper application of the method. We evaluated basic limits of micro-XRD equipped with a mono-capillary with an exit diameter of 0.1 mm, for example the size of irradiated area, appropriate grain size, and detection limits allowing identification of given phases. We tested the reliability and accuracy of quantitative phase analysis based on micro-XRD data in comparison with conventional XRD (reflection and transmission), carrying out experiments with two-phase model mixtures simulating historic colour layers. Furthermore, we demonstrate the wide use of micro-XRD for investigation of various types of micro-samples (contact traces, powder traps, colour layers) and we show how to enhance data quality by proper choice of experiment geometry and conditions.

Complete Quantitative Analysis Using Both X-Ray Fluorescence and X-Ray Diffraction

The advantages of using elemental information to assist in the interpretation of x-ray diffraction (XRD) data are well known and many of the modern search/match algorithms utilize chemical information to make the procedure faster and more reliable (1). Elemental data can also be used to facilitate the quantitation of phases ideittified by x-ray diffraction, in some cases eliminating the need for performing the quantitation based upon the diffraction pattern. There are certain advantages to this approach. The first is that certain difficulties encountered in XRD quantitation such as line overlap, particle statistics, preferred orientation, microabsorption, extinction, and lack of pure Standards are alleviated. The second is that the elemental analysis is, as a rule, easier and more sensitive than the diffraction technique. It therefore should be possible to increase the ease and the accuracy of quantitative phase analysis by utilizing the chemical data available. Since most materials submitted for XRD analysis are in form acceptable for x-ray fluorescence (XRF) analysis, this is the quantitative elemental technique of choice. However, it should be noted that other elemental analysis techniques can be used instead of XRF to furnish the chemical analysis.