If you cannot find the answer to your question on this page, you may also refer to XAS tutorials and other learning resources available at our web site and, e.g., the list of tutorials at the International X-ray Absorption Society web pages.
We offer two types of ways to measure XAS: the transmission mode and fluorescence mode. The type of sample determines which mode is preferred.
Another way to classify XAS experiments for functional materials is whether they take place "ex situ" or "in situ"/"operando" conditions. Ex-situ measurements take place without any specific sample environment and usually in ambient conditions, and samples are typically regular powders. In-situ or operando experiments refer to setups where samples are placed in an environment where they perform a function. Examples can be electrochemical cells (e.g., Li-ion batteries) or studies of catalysis. Ex-situ measurements are easier to perform and do usually not require much effort. In-situ/operando experiments have to always be negotiated well in advance and please be aware that we may not have the required equipment available in our facility -- you should normally be prepared to bring your own reaction cells and other equipment.
For samples that are semi-transparent to the used x-rays, the traditional way of measuring XAS is the transmission mode.
A typical solid with 3d metal elements or heavier has an x-ray absorption thickness in the range of 1-20 micrometers depending on the energy. Silicon and aluminium thicknesses can be in the range of 100 micrometers, and a millimeter for polymers and carbon-based materials (including plexi glass, glassy carbon, graphite, and naturally occuring polymers such as cellulose).
Typical samples for transmission XAS include: powders, free-standing films and foils; or films grown on a x-ray transparent substrate such as glassy carbon.
Many samples for transmission mode are prepared in form of powders with grain size not exceeding a few micrometers preferably, so that each individual powder grain is not too absorbing. Powders that are not transparent enough for x-rays are usually mixed with a x-ray-transparent buffer material such as cellulose or other polymer in a proportion that makes the powder pellet properly dilute mixture of the sample and the buffer material. The powders can be pressed to a shape of a pellet using a hydraulic press available at our facility. Samples of soil or sediments are examples of powders. If your sample is an irregular piece of solid but a part of it can be "destroyed" for the measurement, a part of the sample can be pulverized in a ball mill.
Typical amount of sample material needed depends on the concentration and elements within, but most often we prepare samples from about 10-20 mg of powder, or a few micrometers thick films/foils.
You can estimate the proper transmission sample properties by using software such as the X-ray Transmission Mode Sample Analysis tool at the International X-ray Absorption Society web page.
Standard sample holder for transmission XAS: Our standard sample holder is an M8 sized washer (external diameter 16 mm, internal 8.4 mm, thickness 1.6 mm). Label your sample with a permanent fine tip marker on the washer. Attach a piece of a single-sided Scotch tape on one side of the washer, fill in the sample and seal the gasket with another piece of Scotch.
We have an automated sample exchanger for transmission XAS that allows for 15 samples to be loaded in a cartridge for automated sequantial measurements.
Example of a powder sample preparation of a concentrated transition metal oxide: Take 10 mg of the sample material in a fine powder form, mix it with 90 mg of buffer material (e.g., potato starch or microcrystalline cellulos), adding a couple of drops of ethanol to make a slurry. Let the slurry dry and pack the powder into a suitable sample holder preferably using pressure. We are currently using M8 washers and a hydraulic press to pack the powder firmly to make a solid pellet inside the washer.
If your sample is already dilute so that it is x-ray semitransparent with a small amount of metal elements, e.g., a soil or sediment specimen, it is very likely that the buffer material is not required.
Your sample cannot be prepared to be semi-transparent to x-rays if it is a piece of solid that cannot be pulverized, or it is a very thin film (nanometer range thickness) on a thick x-ray opaque substrate. In this case we normally propose the fluorescence mode. In this mode, an energy dispersive x-ray detector is placed near the sample and the intensity of fluorescence x-rays is observed as a function of incident-photon energy. Under certain circumstances, the fluorescence intensity is proportional to the XAS spectrum. These circumstances are that i) the sample should ideally be dilute, or ii) the sample is a thin film in the nanometer range.
In order for the fluorescence mode to work properly, the sample surface should be relatively smooth and uniform over an area of a few millimeters at least with roughness in the micrometer range. However, the surface does not need to be optically flat or polished.
Please make sure that any substrate of a thin film does not contain the element to be studied from the film, otherwise it will constitute a background that cannot be subtracted.
If the sample is a thick piece of solid with high concentration of the element in question, fluorescence mode XAS may suffer from so-called self-absorption saturation effects which may flatten possible sharp features in the spectra.
Suitable samples for fluoresence mode:
Yes, you can ship your samples to us if they do not contain materials that are not accepted by the courier company. The shipping address for private courier services is
[name of your contact person at our facility]
Department of Physics
Physicum
University of Helsinki
Gustaf Hällströmin katu 2
FI-00560 Helsinki
Finland
If you send samples by regular national mail, the address is
[name of your contact person at our facility]
Department of Physics
P. O. Box 64
FI-00014 University of Helsinki
Finland
Yes, certainly. We welcome visitors from academia and industry. Please coordinate your visit with our user office beforehand and see our instructions on how to reach us.
We have only available on our premises the following functionality (all at HelXAS):
If you wish to support us to obtain more equipment, please contact us!