Rock, Metal, and Water: Go Team Molybdenum!

Last night we arrived at Argonne National Laboratory in preparation to use one of the beamlines at the Advanced Photon Source (APS—have you noticed yet that we scientists are fond of acronyms and abbreviations??) to analyze some samples. The APS is a synchrotron facility operated by the U.S. Department of Energy Office of Science. A synchrotron produces very bright x-rays by accelerating electrons. The electrons travel around a ring, and as they change direction, they emit radiation which is then harnessed by sophisticated optics and directed to the user’s sample. Some basics about synchrotron radiation and synchrotron sources around the world are available at lightsources.org. An intense and highly tunable source of x-rays is required for the experiments that we are going to conduct, and that is only available from a synchrotron.

We will be using x-ray absorption spectroscopy, known as XAFS (X-ray Absorption Fine Structure). XAFS is broken into two parts: XANES (X-ray Absorption Near Edge Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure). At this stage I will confess that this is all new to me. Anthony has been teaching me about XAFS, and I am here at the beamline to learn as much as I can. This is actually my second trip to the APS, although the first time I was conducting a completely different experiment. Before my first trip, one of the beamline scientists remarked to me that, “It is OK to not know what you are doing; as scientists we do things that we don’t know how to do all the time.” In other words, we learn. So all of you can watch me learn about XAFS. It is difficult to discuss XAFS in depth without getting technical. For those of you who are interested, our beamline host Matt Newville has some excellent tutorials at http://xafs.org/Tutorials. I’m reading them too. Let me know if you want to discuss them at the next book club meeting.

The XAFS spectrum that is acquired contains information about the oxidation state of an element of interest (in our case, molybdenum) and its local coordination environment. Oxidation state is the formal charge on an atom (4+? 2-? 0?). Local coordination environment refers to the identity of the atoms that surround molybdenum, how many there are, and their arrangement in space. Our objective with molybdenum is to discover the geochemical mechanisms by which it is enriched in sediments. Knowing the compound(s) of molybdenum that exist in different environments would provide vital clues for answering our questions. On this trip we have some experimental samples that were prepared in the lab along with some sediment samples. Combining experimental work with observations of the real world is a great way to test ideas in a controlled setting, and then to see if they hold up in nature. Next Friday I will be in Riverside, California, (summer is travel season) visiting Professor Tim Lyons and his group, and this will be a good time to tell you more about why we are interested in molybdenum. It’s a good story, I promise, and it has to do with the origins of life.

For now I am running the risk of this post getting too dry and boring, so here is what all of you really want: pictures!