Graphing Data

The light that the synchrotron produces is used by scientists to gather information about the structural and chemical properties of various materials at the molecular level. The Education programs that utilize the IDEAS beamline are able to use two techniques to examine a sample: X-ray Fluorescence (XRF) and X-ray Absorption Near Edge Structure (XANES). Each technique will tell a different story when it comes to what is happening in a sample. Check out our How Does it Work PowerPoint for more information on the synchrotron (to the right) and how these techniques work. Be sure to also check out our Videos that shows step-by-step how to make data graphs.

Here are a few extra resources that may help:


CLS How Does It All Work.pdf

X-ray Fluorescence (XRF)

With XRF, students are able to examine what elements are present and absent in their sample. XRF starts with X-rays being directed at a sample. These incident X-rays are energetic enough to excite electrons in the atoms and cause the excited electrons to either get promoted to a higher energy shell or get emitted out of the atom. Another electron, from a higher energy shell within the atom, releases energy and drops down to fill the hole of the emitted electron. The energy released, is referred to as a fluoresced X-ray or photon and is recognized by the IDEAS detector and counted.

The fluoresced X-rays, or photons, from each element are unique to that element, which is why we can determine what elements are present in a sample. For example, iron fluoresces photons at 6400 eV while copper fluoresces photons at 8030 eV. An XRF scan lasts from 3-5 minutes to complete and in this time, the detector is counting how many photons are fluoresced from, or coming off, the sample. This data is then plotted on a graph where we can see what peaks or elements are present in a sample. An XRF scan will only provide relative concentrations of elements. These instructions tell you how to convert the raw XRF data you receive into graphs. There are also supplementary videos on our YouTube channel (check our videos page) to help visually show the graphing process.

X-ray Absorption Near Edge Structure (XANES)

With XANES, students are able to look at the speciation of an element they are interested in. Similar to XRF, X-rays are directed at a sample when the scan starts. However, the beamline has tuned the incident X-rays to a specific energy level (as it relates to a chosen element) and the X-rays gradually increase in energy as the scan progresses. The electrons, within the atoms in the sample, will not get excited until the X-rays reach the specific energy level the element absorbs at, which is different than the level the element fluoresces at. For example, calcium absorbs at 4038.5 eV and so with a calcium XANES scan, the incident X-rays are set around 3880 eV at the start of the scan and increase over time. As the energy closes in on 4038.5 eV, the inner-shell electrons start to become excited and we see a distinct edge or sharp increase in the spectra, followed by more subtle variations.

To analyze XANES graphs, we focus on these distinct variations, or features, of the spectra as we can make suggestions as to what the oxidation and chemical state (also referred to as speciation) of the element of interest. We focus in on the pre-edge features, the position of the edge, the incline of the edge, and the post-edge features. Each speciation of an element has a distinct spectrum and by comparing your graphs to literature reviews of similar elemental XANES graphs, one can speculate what species is present. Do note though, that there may be multiple species of an element in your sample, especially if it is organic! A sample may have a mixture of various species but the XANES spectra will only show one plot which would combine features of XANES spectras of that element. A XANES scan can vary in length due to the amount of the chosen element within the sample, which can depend on the type of sample being scanned (ex: water sample vs. soil sample). If there is a sufficient amount of chosen element present, a scan can take 15-20 minutes per element per sample. These instructions tell you how to convert the raw XANES data you receive into graphs. There are also supplementary videos on our YouTube channel (check our videos page) to help visually show this graphing process.