MEX-1 Sample Preparation

Well-considered sample preparation is an important pre-beamtime task. It has the potential to critically influence the experimental outcome and success. In summary, the small vertical beam size coupled with small but noticeable beam motion demands highly uniform samples, as sample homogeneity is directly related to obtained data quality.

Concentration

Optimising sample concentration for either transmission of fluorescence measurements will help you maximise data quality and consistency within your sample set.

For transmission, aim for an edge step of approximately 1. An edge step below 0.3 will not be suitable, especially if you want EXAFS information. We use XAFSmass to estimate the edge step of samples based on their composition.

• A guide to using XAFSmass is available here

• A more modern version of XAFSMass is available for users familiar with python. It is available from github, pypi and anaconda. This is the recommended version of XAFSmass

• Old version of XAFSMass for Windows can be downloaded from the link provided on this page. It is NOT recommended to use the old version of XAFSmass.

• A guide to sample preparation using XAFSMass from the XAS beamline is available here.

For fluorescence, aim to achieve a concentration of approximately 1000 ppm. Exceeding 2000 ppm will increase self-absorption effects. We commonly use cellulose to dilute samples to an appropriate concentration.

We appreciate that guidance on the limits of detection and quantitation at MEX1 would be very useful. Robustly providing this information is a complex task that the beamline team is working on. However, to provide some sense of what is achievable, the plots below show the Zn and Ti XANES measured from ZnTiO₃ at various concentrations.

Please note these samples were very homogenous (vide infra).

Homogeneity

If movement of the sample occurs (e.g. due to vibrations), or when the beam moves over the course of a scan (it moves vertically), you will be measuring different parts of the sample. If the sample is not homogenous, spectra can be detrimentally impacted. Below are spectra from iron oxide in cellulose samples, one prepared simply by mixing, one by grinding in a mortar and pestle for 5 revolutions, and one by grinding in a mortar and pestle for 15 revolution. Clearly the longer grinding produced a more homogeneous sample and optimal spectra.

Please do not take the data in the plot below to suggest that 15 revolutions is a sufficient amount of grinding to prepare a sample for transmission. When using a mortar and pestle 15-30 minutes grinding time is recommended. A superior and less tedious solution is to use a ball mill.

Sample Holders

MEX1 uses sample holders described in the technical drawings below.

The round sample holders can hold samples with 13mm (one hole) or 7mm (two holes) diameters or smaller.

The rectangular sample holders (3 holes) are 6mm x 3mm.