Amorphous surface and quantitative X-ray powder diffractometry

Abstract

Experimental quantitation constants for direct quantitative X-ray powder diffractometry, k_iJ, are compared with values calculated assuming a model of an ideal crystal. Differences between experimental and calculated k_iJ, values demonstrate the existence of significant amounts of amorphous surface in the samples of quartz and corundum investigated.

Using respirable dust specimens from “pure” quartz of diverse origins, k_iJ, for the 101 reflection varied from 40.5 to 54.5 net counts cm²µg^–1 for the diffractometer conditions used. The maximum value expected for quartz (calculated using Linde A corundum to calibrate the diffractometer for intensity) was only 47 net counts cm²µg^–1. After considering all possible influences, the variations in k_iJ were attributed to amorphous surface in the quartz, while the higher values than the theoretical maximum found for quartz were attributed to amorphous surface in the corundum. k_iJ for both corundum and quartz increased after a hot orthophosphoric acid treatment, while k_iJ for quartz decreased with grinding. These results support the amorphous surface concept.

The significance of amorphous surface to quantitative X-ray powder diffractometry is discussed. The likely presence of amorphous surface in powders necessitates that care be taken in the interpretation of quantitative results. It is suggested that theoretically calculated X-ray powder diffraction data may prove useful as a source of quantitation standards that are free of amorphous surface effects.