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With 2014 being the International Year of Crystallography, we focus this month on the X-ray diffractometer in our equipment section, celebrating modern technology that helps geologists, inorganic chemists and biochemists alike to probe the structure of often unknown compounds. A staple component in the crystallographer’s toolkit, X-ray diffractometry (XRD) remains a powerful and often definitive technique for determining the structure of a crystal or powder.

We interview Jonathan Britton, a nanotechnology specialist at the Nanotechnology Innovation Centre at Rhodes University who supervises the operation of the XRD at the NIC facility.

What information does it provide to scientists?

For those scientists who are able to use the XRD on their samples it provides valuable structural information. This is very important to help determine the structures of newly created compounds (usually in combination with other techniques). In fact, if a scientist can get an XRD spectrum of a single crystal of their compound, as opposed to a powder, it is considered one of the most definitive ways to prove its structure. Aside from this, knowing the structure accurately can allow a person to predict certain chemical properties of the compound.

So what is XRD and how does it work?

The X-ray diffractometer is the actual instrument for analyzing the structure of a material based on the scattering pattern produced when a beam of X-rays interacts with it. XRD is also a technique that can be used to determine the structure of proteins.

The atoms of the crystalline sample cause a beam of incident X-rays to diffract when they encounter the atom’s electrons and nucleus, and the nature of the scattering depends on the number of electrons that are present in each atom and the organization of the atoms in space. When I say diffract I mean that if an X-ray encounters an electron or the nucleus it has its velocity changed due to bumping into either of them. If enough of these diffractions are moving in the same direction, they generate an interpretable pattern of spots. The generated diffraction patterns are used to build a three dimensional image of the electron clouds of the molecule known as an electron density map. The structural model of the protein is built within this electron density map.

How does XRD compare to other similar techniques?

XRD can be considered an old technique, having come into development in the late 1800s and early 1900s. Nuclear magnetic resonance (NMR) and electron microscopy could be considered as alternative techniques to XRD. However, electron microscopy offers only molecular resolution, while XRD and NMR produce atomic level resolution. NMR is more difficult to use and much more expensive. To summarise, an XRD is simpler to run and still gives comparable information to that generated from the alternatives. While XRD itself isn’t making any major breakthroughs for science, it is a solid, basic structural analysis technique which still needs to be used for analysis of newer ground-breaking compounds.

What do you use the XRD for at Rhodes?

As a nanotechnology specialist working with Distinguished Professor Tebello Nyokong, the XRD is widely used. It helps us determine nanoparticle size as well as compound structural analysis. The geologists use it to identify the mineral composition of various rocks and sedimentary layers.

How easy is it to run the XRD?

The actual operation of the machine is rather simple. It’s the interpretation of the XRD pattern you obtain which is trickier. Essentially the XRD has a removable platform where samples can be loaded and then placed into the machine. Then a parameter file is selected to suit the needs of the sample. A parameter file essentially contains all the needed data that tells the XRD where to start and stop running, as well as how quickly it takes these measurements. If a parameter file cannot be found which is satisfactory, then one can be created.

For information on how to access the XRD at Rhodes University’s Nanotechnology Innovation Centre, email Ms Gail Cobus: gail.cobus@ru.ac.za

Writer: Jonathan Britton