What’s Inside? Non-Destructive Analysis with XRD and XRF

In archaeology, material characterization is often fundamental to identification and appraisal of artifacts. For example, authenticity assessments of Chinese jade requires confirmation of both mineral composition and microstructure to ensure the correct makeup of tremolite and actinolite (Zhang et al. 2020). However, commonly-used analytical techniques, such as chromatography, require destruction of the sample, which is undesirable for these culturally valuable objects (Musílek et al. 2012). To avoid this, X-ray fluorescence (XRF) and X-ray crystallography can be used; these are non-destructive techniques that use the compound’s “reaction” to X-ray illumination to determine the chemical and structural characteristics of a molecule (Loubser and Verryn 2008).

XRF is an analytical technique used to determine chemical composition (Marguí et al. 2022). X-ray exposure can cause atoms to ionize, assuming the energy of the X-ray is greater than the ionizing energy of the atom. This ionization causes the atom to eject an electron, leaving behind an orbital hole. To fill this space, an electron in a higher orbital will “fall” down into the hole, causing a release of energy (as a photon) that is equal to that energy difference (Figure 1). The energy of this radiation is characteristic for each element, meaning the resultant signals can be interpreted to determine the elemental composition. The intensity of the radiation can further be used to determine the relative amount of each element within the sample. However, this analysis does not give us the full picture; XRF tells us composition, but the compound of interest could be any of several different isomers (Marguí et al. 2022).

To gain a more detailed understanding of the compound, X-ray crystallography is used. This technique uses X-ray diffraction (XRD) and thus requires a crystal sample (Ameh 2019). This time, the X-rays are diffracted by the sample, producing a two-dimensional diffraction pattern. The crystal is slowly rotated, with pattern recordings taken at each orientation. The position and intensities of the spots in these patterns directly correlate to the dimensions of the unit cell, which is the smallest repeating unit within the crystal. The amplitude and phase of the scattered X-rays can further be determined from the diffraction pattern. A Fourier transform can then convert these two-dimensional patterns to a three-dimensional electron density map. This map defines the electron density at every point within the unit cell, with the maxima taken as the atoms in the sample. This creates the preliminary model of the structure, which is then refined using complementary chemical information (including the chemical formula, from XRF) to produce the final crystal structure (Figure 2; Ameh 2019).

Accurate techniques still possess limitations. For example, in practice, elements lighter than sodium often cannot be characterized by XRF (Marguí et al. 2022). Further, XRD requires a single-crystal sample, meaning it has a continuous, unbroken lattice, which is not possible for all compounds (Ameh 2019). Consequently, XRF cannot be used to identify organics, but is key to the composition analysis of ceramics and metals to identify possible trade routes (Musílek et al. 2012). Similarly, XRD cannot be used for non-crystalline materials, but provides a detailed analysis of mineral inclusions or corrosion products in artifacts, which can be used for origin determination and dating, respectively (Garrison 2014). Regardless, both techniques are incredibly valuable in material characterization and continue to be the gold standard in archeological applications.

References

Ameh, E. S. 2019. “A Review of Basic Crystallography and X-Ray Diffraction Applications.” The International Journal of Advanced Manufacturing Technology 105 (7–8): 3289–302. https://doi.org/10.1007/s00170-019-04508-1.

Calvero. 2006. Schematic Representation of X-Ray Fluorescence (XRF). Selfmade with ChemDraw. https://commons.wikimedia.org/wiki/File:X-ray_fluorescence_simple_figure.svg.

Garrison, Ervan. 2014. “X-Ray Diffraction (XRD): Applications in Archaeology.” In Encyclopedia of Global Archaeology. Springer, New York, NY. https://doi.org/10.1007/978-1-4419-0465-2_339.

Hasegawa, Kimiko. 2012. “Introduction to Single Crystal X-Ray Analysis.” The Rigaku Journal 28 (1): 5.

Kuckova, Stepanka, Ivan Nemec, Radovan Hynek, Janka Hradilová, and Tomas Matys Grygar. 2005. “Analysis of Organic Colouring and Binding Components in Colour Layer of Art Works.” Analytical and Bioanalytical Chemistry 382 (June): 275–82. https://doi.org/10.1007/s00216-005-3108-5.

Loubser, Maggi, and Sabine Verryn. 2008. “Combining XRF and XRD Analyses and Sample Preparation to Solve Mineralogical Problems.” South African Journal of Geology – S AFR J GEOL 111 (September): 229–38. https://doi.org/10.2113/gssajg.111.2-3.229.

Marguí, E., I. Queralt, and E. de Almeida. 2022. “X-Ray Fluorescence Spectrometry for Environmental Analysis: Basic Principles, Instrumentation, Applications and Recent Trends.” Chemosphere 303 (September): 135006. https://doi.org/10.1016/j.chemosphere.2022.135006.

Musílek, Ladislav, Tomáš Cechák, and Tomáš Trojek. 2012. “X-Ray Fluorescence in Investigations of Cultural Relics and Archaeological Finds.” Applied Radiation and Isotopes: Including Data, Instrumentation and Methods for Use in Agriculture, Industry and Medicine 70 (7): 1193–202. https://doi.org/10.1016/j.apradiso.2011.10.014.

Zhang, Baoshuai, Xiaotong Wu, Yufeng Sun, et al. 2020. “Complex Raw Materials and the Supply System: Mineralogical and Geochemical Study of the Jade Artefacts of the Longshan Culture (2400–2000 Bce) from Sujiacun Site in Coastal Shandong, China.” Archaeometry 63 (December): 1–18. https://doi.org/10.1111/arcm.12634.