Beamline 101 - Basics of running an experiment

Preparing Samples

How samples must be prepared depends on the type of measurement being performed, with transmission XAS, fluorescence XAS / HERFD XAS, and XES all having different requirements. What follows here is a general outline of what each technique requires which certainly does not capture the details of every system which may be measured; for questions about a specific system, it is generally best to discuss with a beamline scientist well before beamtime is scheduled to begin.

Transmission XAS

Transmission XAS measures the intensity of x-rays that are passed through a sample as a function of energy, analogous to how a typical UV/vis experiment works. The transmission signal is calculated as the ln(incident / transmitted) or, in standard ID2A parlance, ln(I2 / I3). This expression communicates that the sample must be sufficiently absorbing in order to generate a difference between the incident and transmitted beams while also still allowing enough beam to penetrate the sample so that it may be detected. In other words, transmission XAS has both an upper and a lower limit in term of sample concentration: if the concentration is too low you will get no signal and if it is too high your data will be distorted and unusable. While the specifics of these limits depend on the element and edge being measured, in order to be suitable for transmission a sample must generally have a few weight % of the element of interest. Few samples are going to natively possess the perfect concentration of the absorbing element for transmission XAS, and so specific preparation of the sample is generally going to be required. For materials that are more concentrated than they need to be, it is standard practice to dilute the material of interest with an unreactive, low atomic number diluent such as boron nitride (BN) in order to lower the effective concentration of the absorbing element to be within the acceptable range. The amount required can be calculated manually, though it is often more convenient to use calculators that are designed explicitly for this purpose such as the one provided by SSRL here. The step-by-step process for how diluting samples works is detailed in the Sample Preparation Guide. Importantly, the BN+sample mixture must be highly uniform and homogeneous and there cannot be chunks of sample that have not been broken up. Similarly, when packed into the sample plate, the packing must also be uniform with no pinholes or regions that are significantly thicker than others, otherwise the data will be distorted.

For samples much less concentrated than they need to be, there is little that can be done to make them suitable for transmission and fluorescence XAS may be a better option.

Fluorescence XAS / HERFD XAS

Fluorescence XAS is similar to transmission in that it records the amount of x-ray absorption as a function of energy, though in this detection scheme the signal from a solid state detector is used rather than an ion chamber after the sample. The general idea behind fluorescence detection is that, when an atom absorbs an x-ray photon to promote a core electron, this leaves behind a hole in a core orbital that is very quickly (< fs) filled by higher-lying electrons. This decay process can produce secondary fluorescent x-ray photons, and, for the edges measured at ID2A, the amount produced is proportional to the amount of absorption that took place. Thus, by measuring these fluorescent photons the amount of absorption may be recorded. Fluorescence detection allows for much lower concentrations of absorbing element to be probed, down to ~10 mM when using the four-element Vortex detector. The downside to fluorescence detection is that the data can suffer from an effect called "self-absorption" or "over-absorption", wherein samples that are too concentrated become distorted.

Unfortunately, unlike in transmission there is no easy calculator available to determine when a sample is too concentrated.

Pinholes are better tolerated in fluorescence than in transmission.

XES

Distinct from XAS, in XES the incident beam energy does not vary and instead is fixed at a value (well) above the absorption edge while fluorescent photon from the sample are recorded. This greatly simplifies sample preparation as the sample volume of sample is probed regardless of what emission energy is being detected. In terms of sample concentration XES is the most forgiving of the methods employed as it has no upper limit to the amount of the element being measured; even pure metals can be measured without issues. The lower limit for concentration depends on the element and emission line being measured and generally increases according to Kα < Kβ << VtC. In practice a lower limit of 1 mM or a few 10s of ppm can be accommodated with anything lower needing explicit approval of staff. Since high concentrations do not distort XES data it is generally best practice to try and make samples as concentrated as possible and dilution with BN should only be done in cases such as when not enough sample would otherwise be available to fill a sample cell. Whether diluted or not, the Sample Preparation Guide outlines how to make samples in standard sample holders.

Mounting Samples

Aligning Samples

Once a sample is mounted on the beamline, it must be aligned before data can be collected.

-- ChrisPollock - 07 Oct 2024