X-ray diffraction (XRD) is much more than a phase identification tool. In many sectors (pharmaceutical, cement, or energy), the question is not only to know the chemical composition of the sample but also the quantity of components present.
However, when faced with a diffractogram, several quantification methods are available to the FILAB laboratory. Rietveld method, RIR, amorphous phase analysis: how to choose? Here are the keys to understanding their differences and applications.
The RIR (Reference Intensity Ratio) method: a rapid quantitative analysis
For routine analysis requiring a rapid response, the RIR method stands out as a pragmatic solution by comparing the peak intensity of the sample to that of a universal reference standard. This approach allows for the estimation of crystalline phase proportions without requiring complex structural calculations, making it ideal for simplified quality control. However, its accuracy remains limited by sensitivity to crystal orientation effects and signal overlap, which can introduce bias in the interpretation of the densest mixtures.
For example, in a case study at the FILAB laboratory on titanium dioxide (TiO2), the RIR method estimated the Anatase content at 97.1%, illustrating a tendency to overestimate certain phases compared to more comprehensive methods.
The Rietveld method or WPPF method: maximum precision
Unlike single-peak methods, Rietveld analysis, or Whole Powder Pattern Fitting, relies on a mathematical model of the entire diffractogram. By fitting a complete theoretical profile to the measured data, this technique takes into account lattice parameters and atom positions, thus offering unparalleled accuracy for complex mixtures. It is now the gold standard for research laboratories because it eliminates potential external experimental phenomena that can distort the results.
In the TiO2 analysis mentioned above, this refinement corrected the measurement to 91.2% of Anatase, demonstrating the method's ability to overcome measurement biases and handle complex mixtures where peaks are nested.
Quantifying the amorphous phase: how to see the invisible?
Amorphous phase analysis addresses the need to quantify non-crystalline compounds that, instead of producing sharp peaks, generate characteristic scattering halos on the background signal. To isolate and measure this "invisible" part of the material, FILAB implements specific strategies such as adding an internal standard of known concentration or using calibration charts based on standard mixtures. This expertise is essential to ensure the conformity of products whose properties depend on the degree of crystallinity, such as certain polymers or active pharmaceutical ingredients whose stability can be altered by a residual amorphous phase.
FILAB laboratory's expertise in X-ray diffraction (XRD) analysis
Thanks to state-of-the-art equipment and excellent mastery of operating software, the technicians of the Materials Characterization unit of the FILAB laboratory support manufacturers in choosing the method best suited to their problems.
To conclude
In conclusion, while XRD is a powerful non-destructive method, the reliability of the results depends primarily on the choice of quantification strategy. A methodological error can lead to erroneous conclusions about the true composition of your products.