MEX-2
- AS Spectroscopy Wiki
MEX-2 Sample Preparation
See below for your sample preparation kit!
Types of measurements
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.
Sample Holders
MEX2 sample holders are different to MEX1.
Fluorescence and drain current measurements: MEX2 has adopted the magnetic mounting ruler type sample holder (“sticks”) used on SXR fast NEXAFS chamber (Photos a and b). SXR sticks are interchangeable on MEX2, with the exception that on MEX2, samples may be mounted on both sides of the stick. The most convenient method of mounting most samples for fluorescence and drain current measurements is to stick them to the ruler with double sided Carbon tape (black strips on ruler b). To visualize the beam, the first ~ 7 mm of the ruler will be left blank for the adhesion of a white film (Photos b and d) that glows when struck with X-rays. See Figure e) below for more detail.
Carbon tape and fluorescent paper are available at the beamline.
Transmission measurements: the mounting ruler has windows (Photo c) allowing various transmissible samples to be mounted (Photo d). Transmission measurements and sample preparation is difficult at the MEX2 energy ranges. Please contact the MEX beamline scientists to discuss feasibility if transmission is your only option.
Users are able to purchase the rulers (Toledo 150MSE) and mount samples prior to arriving at the beamline. This can also be done at the beamline (in the case of non-hazardous samples) or in the Synchrotron’s Chemistry lab.
Sample preparation kit
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: