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.

MEX1 Sample Preparation

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.

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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.

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MEX2 Sample Preparation

MEX2 can make measurements in:

• Drain current (electron yield) mode

• Fluorescence mode

It is best suited to probing edge features (that are mostly rich in this energy range) to study the speciation via XANES. EXAFS in this energy range is currently very difficult.

MEX2 has adopted the magnetic mounting ruler type sample holder (“sticks”) used on the Soft X-ray fast NEXAFS chamber. See Sample Holder section for more information.

Drain Current (electron yield)

This is essentially the easiest measurement to make, providing your sample is relatively electrically conductive and concentrated. The data will have good signal to noise, but depending on the concentration of the element of interest the signal to background might suffer. Unlike fluorescence and transmission, highly concentrated samples work well in drain current.

As in a soft X-ray experiment, drain current measurements must be made under vacuum, so the sample must not outgas. Drain current measurements are also highly surface sensitive (~ several nanometres). A sparing smear of power is best, as the powder film needs to be as thin as possible and forced into the Carbon tape to aid electrical conductivity.

Drain current is a great alternative to transmission for higher concentration samples, keeping in mind that it is very surface sensitive if that’s important for your materials.

Fluorescence

This is analogous to a hard X-ray XAFS measurements, and works on most types of samples. We have a 4 element Silicon drift detector, and measurements can be made either in vacuum (will get slightly better data) or Helium ambient. It is best not to cover the surface of the sample with any material (eg avoid Kapton tape encapsulation unless absolutely necessary).

As for drain current, a thin smearing on Carbon tape works well for powders. In the MEX2 energy range, the measurements are sensitive to the ~ few microns from the surface due to the X-ray attenuation. Ideally, the sample is thin (less than ~ micron) and dilute of the element of interest (~1000-2000 ppm), because self absorption (the absorption of the fluorescent X-rays by the sample itself) becomes a major experimental issue that will suppress the intensity of peaks in the XAS spectrum.

Drain current and fluorescence spectra can be collected at the same time.

How to prepare your samples?

MEX2 uses a ruler mounting system that allows many samples to be stacked one above the other. The document below will guide you through the sample preparation for MEX2. We've designed a sample mount kit specifically for MEX2 users, as shown in the picture below.

If you’d like to receive the kit before starting your beamtime, please contact the beamline scientists well in advance, and they will assist you with the process.

Using the Sample Mounting System for solid or powder MEX2 samples:

All instructions regarding sample exchange procedures under vacuum and Helium at MEX2 can be found in the link below:

Sample Exchange Procedure in MEX2