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 a small, but noticeable, beam motion demands highly uniform samples as their 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 for transmission, especially if you want EXAFS information. We use XAFSMass to estimate the edge step of samples based on their composition.

XAFSMass executables for Windows can be downloaded from the link provided on this page.

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

A guide to sample preparation using XAFSMass 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.

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.

Open

MEX2 Sample Preparation

MEX2 can make measurements in:

• Drain current (electron yield) mode

• Fluorescence mode

• Transmission 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.

Transmission

Akin to a hard X-ray transmission experiment, however the sample needs to be exceptionally thin due to the low transmission at these X-ray energies. Also, the sample cannot be contained in Kapton or pressed in a BN or cellulose binder. An appropriate sample would be a film deposited on a thin window (eg SiN window, several micron Kapton or mylar film). If a powder is to be used, the grain size must be significantly smaller than the attenuation length of the material at the edge energy (usually means sub micron) and can be, for example, evenly dispersed on the sticky side of thin Kapton or mylar tape. Measurements can be performed in vacuum or Helium; again, vacuum will offer better quality data. Transmission measurements are currently difficult to perform reliably at MEX2.

If you think you will need transmission specifically, please contact us for a more detailed discussion about sample preparation.