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June 2008

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Subject:
From:
"John J. Donovan" <[log in to unmask]>
Reply To:
JEOL-Focused Probe Users List <[log in to unmask]>
Date:
Wed, 11 Jun 2008 09:17:01 -0700
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JEOL Probe Users Listserver

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Electron Microprobe Lab, University of Minnesota

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*

Hi Ellery,
I'm sure you are correct about the lack of Na loss (especially since 
you were only counting 10 seconds) but I will just mention that on 
the Withers glasses I measured I saw an over 100% correction in Na 
intensity over time (about half the intensity was lost over a 
admittedly longer period of 60 secs) with column conditions of 15 
keV, 10 nA and a 20 um beam. (So a somewhat more focused beam than 
you are using.)

Just to be sure, do you see an increase in the Si concentration when 
the H2O is included in the matrix correction? Note in my analysis 
below the SiO2 is 70.57 with H2O in the matrix and only 70.22 without 
H2O. Also you can see the 107% positive correction for Na loss (the 
line labeled VOL%) and in addition, a -0.7% correction for Si GAIN 
and also a 2.9% gain for oxygen loss (over 120 seconds). Maybe this 
last issue is related to your low totals?

Finally, did you perform an area peak factor correction for the 
oxygen measurement relative to your oxygen standard? In my case I 
used MgO as the oxygen standard which yields an APF correction of 
1.031 (3.1%) relative to my unknown composition.

I guess the other question is what oxygen mass absorption coefficient 
did you use? I used

       O       ka      Si  8.7900e+03    Bastin  (1992)

This could make a difference too since the matrix is mostly silicon 
(besides oxygen). I'm guessing that you will find the low totals 
being due to a number of small effects rather than a single large effect.
john

Emacs!



At 08:39 AM 6/11/2008, you wrote:
>JEOL Probe Users Listserver
>
>Moderator: Ellery Frahm, [log in to unmask],
>Electron Microprobe Lab, University of Minnesota
>
>Post a message: send your message to [log in to unmask]
>
>Unsubscribe: send "SIGNOFF PROBEUSERS" to [log in to unmask]
>
>On-line help and FAQ: http://probelab.geo.umn.edu/listserver.html
>
>
>
>*
>
>Hi John,
>
>>The first question is: are you performing the intensity at zero time
>>correction for Na loss? This glass (even in 10 seconds) will lose
>>some Na relative to "omphacite (albite)". Obviously if you have lost
>>Na then the H20 from 100% calculation will be high.
>
>The only treatment of Na so far has been charting the counts on the
>peak over time using his conditions to show that there is no loss of
>counts over a relevant period of time, at least 30 seconds.  I'll have
>him double-check his measured vs expected Na totals based on the
>experiment.  I will also check if there is enough Na in the glasses to
>possibly explain totals at least 5% low.
>
>>The other consideration is that the H2O needs to be added to the
>>matrix correction to adjust the SiO2 concentration (all the elements
>>are affected by the addition of H2O to the matrix, but SiO2 being
>>the largest concentration requires this the most). This is because
>>Si ka is more absorbed by oxygen than by itself. In these
>>composition glasses one will typically see an increase in the total
>>(or decrease in the H20 by difference) of about 1% absolute.  These
>>corrections should be performed in the matrix correction loop for
>>best accuracy (as opposed to an Excel spreadsheet).
>
>Yes, we are definitely adding H2O to the matrix correction, not just
>adding it later in Excel by difference.
>
>>I'll send you a short note I wrote on these water by difference
>>correction issues off-line that also includes a discussion on a new
>>"blank" correction for accuracy I've developed for traces that
>>amazingly can also be used for calibration of major concentrations
>>oxygen.
>
>Thanks -- I'll definitely check that out as well as pass it along to
>our researcher.
>
>Best,
>Ellery


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