Thanks for all the response! They are all very helpful!
Just want to make sure that I get the information right and a few follow-up questions:
1. I'd better contact the beamline engineer to ask for the sensitivity for Tl.
2. Fe fluorescence interference could be decreased by adding a few layer of Al foil. I knew that I did that when I measured As fluorescence. So this is ok.
3. Pb fluorescence interference is very hard to eliminate. Therefore, EXAFS for Tl is very hard to get. Actually, I didn't see any papers on Tl EXAFS and maybe that is why. The research question I want to answer is: what structure Tl is bond to in the fly ash? Tl(I) and Tl(III) could theoretically bind to sulfate, nitrate, bicarbonate, etc.. I saw some papers that are using nitrate, sulfate, bicarbonate Tl salt as references for micro-XANES measurement. And then they compared the XANES in the end. Is that a better approach than EXAFS to answer the question?
4. To increase the intensity, we can condense the ash by pressing it into pellet. The fly ash from coal combustion power plants are already pretty compacted. They are mainly gypsum in most cases. I dont' know if that helps or not.
5. This is the first time I heard about non-energy dispersive detector, e.g. PIPS diode. We are planning to use 13-element detector. What is the advantage of using one way or the other? If I do want to concentrate on XANES, would be helpful to ask the beamline engineer for the possibility of changing detectors?
Thank you all again,
Mengling
2011/9/29 "Dr. Dariusz A. Zaj±c"
<kicaj@ifj.edu.pl>
Dear Matt,
this is very good introduction for fluorescence measurements on diluted samples... Like a tutorial...
I just want to add small comments: as I understood the sample is an ash, so maybe will be possible to condense it, e.g. press it into the pellet. If there will be no possible to measure exafs (due to heavier elements), and is needed to focus on xanes, than I suggest to use non-energy dispersive detector, e.g. PIPS diode. It is very often more sensitive than regular fluorescence detectors, and can take higher count-rate.
please correct me if I am wrong...
kicaj
W dniu 11-09-29 17:02, Matt Newville pisze:
Dear Mengling,
The basic issues are: will there be enough Tl fluorescence counts in
the detector, and will you be able to see a decent edge. From you
spreadsheet of concentrations, the elements that stand out are: Fe
and to a lesser extent Zn, and Pb.
The Fe and Zn will dominate the fluorescence getting to your detector.
This hurts sensitivity to Tl because the solid-state detectors can
handle a limited flux themselves (typically on the order of 100K Hz).
The high Fe concentration means the Fe Kalpha will dominate the
emission when you shine 12.7keV x-rays on the sample. The
simple-but-effective remedy is to put aluminum foil on the detector,
as aluminum will absorb Fe fluorescence (at 6.4keV) about 4x more than
the Tl fluorescence (at 10.3keV). You might need a few hundred
microns of aluminum foil, and may need to try a few different amounts
to cut down the Fe counts sufficiently.
The other issue is Pb. The Tl L3 edge is at 12.658keV. once you hit
the Pb L3 edge, at 13.1 keV, the Pb Lalpha fluorescence will dominate
the Tl Lalpha fluorescence -- these lines (10.3 and 10.5 keV,
respectively) are too close together to effectively filter
electronically. Even well below the Pb L3 edge, you'll start to see
some Pb Lalpha fluorescence. Since the Pb concentration might be 100x
the Tl concentration, that would be something to look for, but there's
not a lot that can be done about it. Anyway, that really only limits
the EXAFS, not so much the ability to get XANES. The Tl
concentrations are low, but I think that at a good APS BM line like
5-BM, getting Tl L3 XANES is just a matter of time.
Hope that helps,
--Matt
On Thu, Sep 29, 2011 at 9:39 AM, Mengling Yi Stuckman
<stuckman.12@osu.edu> wrote:
Dear Ifeffit Community,
I am currently working on project "Evaluating Speciation of Thallium in Coal
Fly Ash". The project was accepted by APS and we will conduct our
measurement on beamline 5-BM-D.
My fly ash sample has Tl concentration ranging from 0.5ppm to 13ppm. How can
I know whether the sample can generate enough photon counts if using
fluorescence mode? what would be the appropriate sample preparation
procedure I should follow? Is there any interference I should pay attention
to?
I understand that I should use Hephaestus, but I don't know how to do that
if my sample contains lots of different oxides. I attached the elemental
composition in this email. Most metals should be in form of oxides, or
sulfates.
Any input would be highly appreciated,
Mengling
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Mengling Yi Stuckman
Graduate Research Assistant
Civil and Environmental Engineering and Geodetic Science
Ohio State University
2070 Neil Ave., Columbus, OH, 43210