Wednesday, April 3, 2013

Sulfur X-Ray Microprobe and XAS at SSRL: A First Look Into My Beamline Science

I'm working this week at the Stanford Synchrotron Radiation Lightsource (SSRL)!  

SSRL is a synchrotron particle accelerator.  When electrons are sped up to relativistic speeds and forced to bend radially along their path they emit electromagnetic radiation in the microwave to high-energy (hard) x-ray range.  This emitted radiation provides for a wide range of potential instrumental applications, which benefit from the high intensity, high brilliance, and high stability of the source radiation.  

SSRL, a part of the SLAC (Stanford Linear Accelerator) National Accelerator Laboratory, is operated by Stanford University on behalf of the U.S. Department of Energy (DOE) and provides scientists with the ability to access a wide range of instrumentation which utilizes synchrotron radiation.

SSRL from the side (for more info on SSRL click here)

Sam Webb, one of our longtime collaborators and lab-friends, is commissioning his new beamline at SSRL, BL 14-3, which will give researchers and users at SSRL the ability to do low-energy x-ray microprobe mapping and spot-XAS (X-ray absorption spectroscopy).  The low energy region allows us to target the K-alpha absorption/reflection of elements like Cl, P, Si, Al, Mg, and, most importantly for me, sulfur.

This new beamline will allow me to create x-ray microprobe maps over regions of interest in thin sections of my samples.  Once I target good regions, I can come back at those regions and create microprobe maps at various energies which target various sulfur oxidation states.  I can use these maps to determine where the primary sulfur compounds of interest may be located.  Once I target desired spots which likely show the variation in a sample, I'll come in with a focused X-ray beam and conduct XAS on each spot.  So far in the commissioning time I have mapped 6 samples and run a slew of absorption scans.  Things are just now starting to get interesting as I get used to using the instrumentation (and now that the instrumentation appears to be less glitchy than it was earlier in the run).

Right now I'm running a sample of material we're calling paleopipe, which was collected in the arctic by our collaborator Bob Pappalardo in 2011.  A paleopipe is a sedimentary structure that is likely the remains of sulfide rich springs which once flowed onto our glacier at Borup Fiord Pass.  The paleopipe sample I'm running now was prepared by taking a small chunk of sediment, embedding it in epoxy, and grinding it down to expose some surfaces of the solid material (these latter steps were done by Paul Boni, our rock-shop guru in the Geological Sciences Department at the University of Colorado Boulder).  Here's an image of the initial x-ray microprobe map conducted at coarse resolution (30x30 micron^2 step-sizes).  

The coloring here signifies the relative intensity at which x-rays are absorbed by the sample, with red being more and blue being less. The map shown here signified where sulfide is most likely present in the sample (where there's very strong absorption - i.e. red areas are sulfides).

The higher-resolution maps at various energies are showing some interesting potential variations in sulfur compounds in the sample.  I am pretty hopeful for the data I'm getting from this sample right now.

Beamline science is exciting and fun, although tedious at times.  I've watched a lot of episodes of Red Dwarf while working here at BL 14-3.  As I finish up this blog post, I have x-ray spectra coming off of this paleopipe sample which will help me to determine the composition of the material.  This is an important first step for x-ray microprobe mapping of my samples from the arctic.  I hope in the not-too-distant future to be able to prepare samples for microprobe without removing them from their natural location in the arctic, as that may be the best way to preserve any potential remaining signatures of past/present biological activity.