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.

Powders

Powdered samples are commonly prepared as pellets and sealed in kapton tape.

Optimising powdered sample concentration for either transmission of fluorescence-mode measurements will maximise your data quality and consistency within your sample set.

Transmission mode: for highly concentrated samples that can be diluted

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.

Read how to make a pelleted transmission sample using XAFSmass for sample preparation. Available here.
Mix your sample and inert matrix, such as cellulose, and grind together as homogeneously as possible using a mortar and pestle or ball mill.
Use a hand pellet press to transform your mixture into a pellet (7 mm pellet is 40 mg of sample; for a 13 mm pellet, 90 mg).
Mount the pressed pellet into a sample holder.

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.

Fluorescence mode: for minimally concentrated samples i.e. <2000 ppm

How to make a pelleted fluorescence sample: Skip XAFSmass. Instead, dilute your sample to ~1000 ppm in an inert matrix, then follow the same grinding and pelleting procedure for preparing a transmission sample (steps 2-4 above).

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 Transmission VS Fluorescence spectra below show the Ti (top) and Zn (bottom) XANES measured from ZnTiO₃ at various concentrations.

Fluorescence VS Transmission

Equipment required to make a pellet sample

To make sample pellets, you will need the following equipment available in the Chemistry Lab.

For a more sophisticated grinding and pellet pressing method, the automatic grinder and hydraulic press below can be used, but will require training from the Chemistry Lab technicians outside of beam time.

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.