International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 3.9, pp. 346-347

Section 3.9.3.2.1. Selection of an internal standard

I. C. Madsen,a* N. V. Y. Scarlett,a R. Kleebergb and K. Knorrc

aCSIRO Mineral Resources, Private Bag 10, Clayton South 3169, Victoria, Australia,bTU Bergakademie Freiberg, Institut für Mineralogie, Brennhausgasse 14, Freiberg, D-09596, Germany, and cBruker AXS GmbH, Oestliche Rheinbrückenstr. 49, 76187 Karlsruhe, Germany
Correspondence e-mail:  ian.madsen@csiro.au

3.9.3.2.1. Selection of an internal standard

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The selection of an appropriate material for use as an internal standard for QPA is not always straightforward. Ideally, the material selected should:

  • (1) Have a simple diffraction pattern resulting in minimal overlap with peaks of interest in the sample.

  • (2) Have a mass absorption coefficient similar to that of the sample to avoid introducing microabsorption effects and thus reducing accuracy.

  • (3) Have minimal sample-related aberrations that may affect observed intensities. For example, it should be fine-grained to ensure minimal grain-size effects on the observed intensities and not be subject to preferred orientation. Importantly, it should have 100% (or known) crystallinity.

  • (4) Be stable over an extended time and be unreactive, especially for in situ studies where it may be subjected to extreme conditions.

Some possibilities for use as internal standard include α-Al2O3 (corundum), TiO2 (rutile), ZnO (zincite), Cr2O3 (eskolaite), α-Fe2O3 (haematite), CeO2 (cerianite), CaF2 (fluorite) and C (diamond). Cline et al. (2011[link]) have described the certification of the standard reference material SRM 676a with accurately known amorphous content for use as an internal standard for QPA (see Chapter 3.1[link] ). Alternatively, it is possible to use an independent measure (e.g. chemical analysis) to derive the concentration of a phase already present in the sample and then to designate it as the internal standard.

Selection of the amount of internal standard to add is often based on folklore or local practices with reported additions ranging from 5 to 50 wt%. Westphal et al. (2009[link]) have described the mathematical basis for selecting the optimal internal standard addition in the context of amorphous phase determination. The amount of internal standard added has a strong influence on the precision of the determination of amorphous content and `a poor choice can make determination impossible, while a clever choice can enhance the precision'.

With the exception of diamond, all of the phases listed above tend to have absorption coefficients that are too high for use with organic materials. The development and verification of a suitable low-absorption-coefficient standard material that meets the criteria given above remains an important area of research.

References

Cline, J. P., Von Dreele, R. B., Winburn, R., Stephens, P. W. & Filliben, J. J. (2011). Addressing the amorphous content issue in quantitative phase analysis: the certification of NIST standard reference material 676a. Acta Cryst. A67, 357–367.Google Scholar
Westphal, T., Füllmann, T. & Pöllmann, H. (2009). Rietveld quantification of amorphous portions with an internal standard – mathematical consequences of the experimental approach. Powder Diffr. 24, 239–243.Google Scholar








































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