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Moderator: Ellery Frahm, [log in to unmask],
Electron Microprobe Lab, University of Minnesota
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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 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).
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.
At 08:02 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
>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
>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
>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