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Crystal Form Quantitative Analysis
Crystal Form Quantitative Analysis
Single-peak, multi-peak method
Traditional quantitative X-ray powder diffraction methods rely more on calibration curves established with internal standards, the method is based on internal standards for quantification. The single-peak method essentially establishes a linear relationship between the ratio of crystalline content and the ratio of diffraction peak intensity. The single-peak method is simple and sensitive, and is particularly suitable for long-term crystal form quantification that only requires little information about the sample. However, this method is heavily dependent on the purity, particle size, and sample preparation method of the standard sample, and the sample selection orientation effect can seriously affect the application of the single-peak method.
Rietveld Full Spectrum Fitting (self-developed)
The principle of Rietveld fitting method is to use the whole diffraction spectrum data for analysis. By using peak parameters, crystal structure parameters (cell parameters, atomic positions, phase ratio, etc.), the simulated and experimental intensities y_(obs,i) and y_(cal,i) can be calculated for each 2θ position, and then the least squares method is used to compare with the measured spectrum, and the parameters are continuously adjusted and refined to minimize the simulated-experimental difference. The advantage of the Rietveld full-spectrum fitting method over the conventional X-ray quantification method is that it does not require standards or pre-developed standard curves, but only the structural model of each phase in the mixture (lattice parameters, space group symmetry, atomic positions, position occupancy) to simulate the powder diffraction pattern, so that quantitative analysis can be performed. More importantly, the full-spectrum fitting method has the ability to correct systematic errors and overlapping peak splitting caused by merit orientation, instrument construction, and other factors by mathematical models, so that intensity values can be extracted more accurately.
