XAFSMass XAFSmass is a powerful and helpful program that lets you easily optimise the dilution of your sample for transmission XAFS analysis. This guide will demonstrate how to use XAFSMass XAFSmass to estimate the appropriate dilution of a sample for transmission, achieving the desired total absorption and edge step.
Terminology
Absorption unit: One absorption unit represents the thickness of material sufficient to decrease in flux to 1/e (36.79%) of its initial value. e.g. A flux of 1E10 photons/second incident on a sample 1 absorption units thick would result in a flux of 3.68E9 photons after the sample, alternatively expressed, 1 photon in every 2.7 photons makes its way through the sample. Absorption units are a logarithmic scale. Thus a sample two absorption units thick will have a transmission of (1/e)*(1/e) which is 13.53% or 1 photon in every 7.4.
Total absorption (μTd): The total attenuation of a beam of photons of a defined energy. All elements in a sample contribute to the total absorption. Total absorption is expressed in absorption units.
Edge step/absorptance step (Δμ): The magnitude of the change in absorption as a monochromatic beam is scanned across an absorption edge. Only atoms of the element corresponding to the absorption edge contribute to the edge step. The size of the edge step is proportional to the concentration of the element of interest in the sample. Edge step is expressed in absorption units.
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MEX-1 can measure edge steps down to 0.1 without problem. Close to 1 is ideal, 0.01 means the sample is probably better suited to fluorescence-mode. |
Optical thickness: In this guide, optical thickness refers to how absorbing a material is, expressed in absorption units. e.g. "My sample has an optical thickness of 2.3 absorption units". Optical thickness and total absorption can be used interchangeably.
XAFSmass: This guide employs XAFSmassQt, the modern version of XAFSmass. It has a much more powerful chemical formula parser than the older version of XAFSmass. Details can be found here: https://xafsmass.readthedocs.io/# It is beyond the scope of this guide to describe installation of XAFSmass. Using the old version of XAFSmass WILL NOT WORK with these instructions.
The problem
The problem to be solved when choosing an appropriate dilution for a transmission sample is to balance the total absorption, the edge step and an appropriate amount of material to make a pellet. In an ideal world, we want an edge step between 0.5 and 1.5 absorption units, whilst keeping the total absorption within reasonable limits, i.e. ideally below 1.5 absorption units, though perhaps relaxing that constraint to 2.5 absorption units or below for more difficult samples. There are two motivations for keeping total absorption at <2.5 absorption units:
the avoidance of "thickness effects". The optically thicker a sample is, the more the measured energy-dependent absorption will be affected by imperfections in sample homogeneity. At a total absorption of 5 absorption units, 1 photon in 150 will make it through the sample. If regions of the sample illuminated by the beam have varying absorption, because your sample was poorly mixed, or worse, there are areas where the element of interest is absent, the measured absorption signal will be dominated by the contributions from those regions. More information can be found in the following papers:
Stern, E. A. & Kim, K. (1981). Thickness effect on the extended-x-ray-absorption-fine-structure amplitude. Physical Review B23, 3781–3787.
Heald, S. M. (1988). Design of an EXAFS experimentEXAFS with synchrotron radiation. In: Koningsberger, D. C. & Prins, R. (eds) X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES. John Wiley & Sons, 119–16287–118.
optimal signal to noise. See here for further explanation.
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Goulon, J., Goulon-Ginet, C., Cortes, R. & Dubois, J. M. (1982). On experimental attenuation factors of the amplitude of the EXAFS oscillations in absorption, reflectivity and luminescence measurements. Journal de Physique 43, 539–548.
optimal signal to noise. See here for further explanation.
In addition, we need a mass of material that is large enough to reliably press into pellets. For many materials, the optimal mass for a 7 mm pellet will be in the single figure milligram range, a mass too small to reliably press high quality pellets. Thus, we must dilute the material of interest with an inert dilutant, preferably a material that has low atomic number (and thus has low absorption) that also forms good pellets. The material of choice at the MEX beamline is microcrystalline cellulose, C6H10O5. A good mass for a 7 mm pellet is 40 mg of sample; for a 13 mm pellet, 90 mg is a good mass.
Diluting the material of interest with cellulose changes both the total absorption and the edge step. Thus, the problem to be solved is to choose a dilution that gives reasonable values for total absorption, edge step and mass of material to be pressed into a pellet. For many real-world samples, optimum values for all three parameters cannot be achieved. Thus, the task for the user is to use tools like XAFSmass to explore the parameter space to find the least worst compromise.
Example 1 - tungsten metal powder
Using XAFSMassXAFSmass, enter W as the compound, µtd (total absorption) as 1.5, S (area of the pellet) as 0.3848451 (the area of a 7 mm diameter pellet expressed in cm2). Choose the energy from the drop down list to be the W L3 edge (+50 eV):
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The intersection of grey dotted lines marks our target of 40 mg and an edge step of 1.0 absorption units. Each of the dots on the coloured lines represents a different diltution (you can inspect the values for mass and edge step in the table to locate each dilution on the plot). The results suggest that a total absorption of a little less than 2.0, i.e. ~1.85 and a dilution with cellulose to between 10 and 20 wt. % will give the optimum result. However, we do not have to stick to making only 40 mg pellets. Depending on , nor do we have to stick to edge steps of 1.0. Depending on your material and your skill making pellets, lower masses will also work. As an accomplished presser of pellets, I wouldattempt would attempt to make a pellet with only 20 mg of material and a dilution of 20 wt % and a total absorption of 1.5 abs. units. Alternatively, for this sample, I could make a 40 mg pellet with a total absorption of 1.5 abs units, and accept an edge step of ~0.7 which will give excellent data at MEX1.
Think of 40 mg as a good starting point for a mass of a pellet that should almost always work, however if you find you can successfully make lower mass pellets, take advantage of the extra flexibility it offers. Similarly, whilst an edge step of 1.0 is ideal, MEX1 can return good data at edge steps as low as 0.1, (or lower for the more adventurous/less discerning user).