[Ifeffit] Preparation help with fly ash sample with diluted Tl

[Matt Newville]

Hi Dariusz,

Thanks, those are good suggestions.   I was assuming that they'd use a
solid state (say 10+ element Ge) detector for concentrations at and
below 10 ppm.    A large diode detector might work OK -- they can
often take higher total count rate at the expense of energy
discrimination.  I'd be happy to be shown a comparison, but my guess
is that electronically removing the Fe and Compton scattering would be
worth the effort for these concentrations.

--Matt


2011/9/29 "Dr. Dariusz A. Zając" <kicaj at 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 at 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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