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Electron Microprobe Lab, University of Minnesota

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Probe folks,

A researcher in our lab (an experimental petrologist) is trying to  
analyze a few of his samples (experimental andesitic melts/glasses  
containing between 3% and 6% water), and he is trying to (1) get good  
totals and (2) correlate measurements of the water content using FTIR  
and oxygen analysis in the microprobe.  He is getting a linear fit  
between water contents from the FTIR and microprobe, but his totals  
are low on the probe when we analyzes his samples as both oxides and  
metals.  He is an experienced user, and we've gone through all of the  
usual procedures for testing the causes of low totals -- nothing.

The FTIR measured total water contents ranging of 3.37, 4.12, 4.54,  
5.25, 5.45, and 5.88 to 5.9 wt%.  When we analyze the glasses with the  
electron microprobe, assuming a deficit from a 100% total is  
attributed to H2O, the calculated water contents are 5.29, 6.45, 7.12,  
7.75, 7.56, and 9.21 wt. %, respectively.  So his totals are about 5%  
low, occasionally more, causing overestimation of water.

Here is what he has tried (based on the notes he's given me):

1)  Periodically aligned beam and checked for astigmatism on willemite.
2)  Beam conditions: 10 nA current, 30 micron spot size.
3)  Always analyzed Na first (monitored counts rates, found Na counts  
did not drop for at least 30 seconds).
4)  Analyzed for oxygen using three different standards: enstatite,  
pyrope, and almandine -- all yielded similar results.
5)  Counted the area under the peak for oxygen.
6)  Adjusted the detector settings for oxygen: PHA Gain: 32, High V:  
1708, Base L. (V) 0.70, Window: 2.5, Mode: Diff.  Set up this way to  
filter out high-order Al and Na peaks.  WDS scans indicated these  
peaks were eliminated by this filtering method but had little effect  
on improving the water contents of the glasses when compared to an  
“unfiltered” method.
7)  Set oxygen backgrounds based on WDS scans and software overlap  
database: Back +: 12.000 mm, Back -: 6.500 mm.
8)  Total counting time on oxygen: ~44 seconds.  Other elements: 10  
seconds for peak, 5 seconds for background.
9)  Elements present in samples (and analyzed for): Si, Ti, Al, Cr,  
Fe, Mn, Mg, Ca, Na, K, O, H (known from FTIR)
10)  Full WDS scans (including light elements) revealed no missed  
elements that could have led to low totals.
11)  Used basaltic glass to standardize MgO, CaO, FeO.  Rhyolitic  
Glass (basaltic glass) for Al2O3.  SiO2 Glass (Quartz, basaltic glass,  
rhyolitic glass) for SiO2.  Benitoite for TiO2.  Chromite for Cr2O3.   
Mn-Hortonolite for MnO. Omphacite (albite) for Na2O.  K-Spar for K2O.   
Standards in brackets were also tries, and produced worse results.   
Standardized as oxides, and then measured the unknowns as a metal.   
Oxygen was standardized as a metal on the enstatite standard, and  
analyzed as a metal on the unknowns.
12)  Sample thicknesses were 100 microns or greater.
13)  Two carbon coat thicknesses tried: about 50 and 200 angstroms --  
produced similar results, so charging isn't a problem.
14)  Glasses were free of quench crystals, so the interaction volume  
should be homogeneous.
15)  Moving elements to different spectrometers had no effect on  
improving the results.
16)  Using these conditions, analyses on two different basaltic glass  
standards yielded matching elemental concentrations.  These glasses  
were relatively dry (a few tenths of a wt.% H2O) compared to the  
specimens in question.
17)  This technique was also performed on hydrous rhyolitic glasses  
with 1.3, 3.3, 4, 5, and 6 wt. % H2O (determined by FTIR) and produced  
the similar water contents (1.06, 3.11, 4.74, 5.65, and 7.24,  
respectively) and overall chemistry.

The only real issue I see above is the benitoite as a Ti standard  
because it has CL, but the TiO2 content of these glass samples isn't  
high enough to account for the difference.  Because these glasses are  
amorphous (presumably), even most of my usual "exotic" explanations  
for low totals are ruled out, like X-ray polarization between the  
crystals in the sample and the dispersing crystal.  I'd like to have a  
better explanation for these low totals other than "well, something is  
causing the electrons and/or X-rays to be more strongly absorbed or  
behave in an unexpected way" -- that's not very satisfying.

So I need to get some fresh ideas or find out if others have had  
similar problems with hydrous andesitic melts.  Anyone have an ideas  
about what to try next or what the problem might be (even if we can't  
fix it)?

Thanks,
Ellery

--------------------
Ellery E. Frahm
Research Fellow & Manager
Electron Microprobe Laboratory
University of Minnesota - Twin Cities
Department of Geology & Geophysics
Lab Website: http://probelab.geo.umn.edu
Personal Website: http://umn.edu/~frah0010