Low totals in oxides

[theo_nt]

We have a problem in analyzing titanomagnetites. We always getting low totals, even if we assume that 2/3 of the Fe is Fe3+.

Our microprobe is a Cameca SXFive FE and the analytical conditions are 15 kV, 20 nA and the beam beam size 1 micron.

In the attachement you will find the detailed analytical conditions (.doc file) and some analyses (.xlsx file) to demonstrate the problem.

Also we observed a negative correlation between Ti and totals: The higher the TiO2 contain the lower the totals. We tried using different standards to calibrate Ti and Fe. Also we are using as a secondary standard a synthetic ilmenite. Calibrating Fe in FeO or even on almandine and Ti on rutile we always get a total of 100 wt% on this imenite.

Also we thought about peak shift. Calibrating Fe on EDS we are getting the same results. Low totals for titanomagnetite and good totals for ilmenite.

The only thing that we have not yet checked is the ABS coefficients. Here is the question:

Is it possible that the Cameca ABS coeficients (in SXFive FE) have a specific problem in the system Fe-Ti for the specific range of concentration applying to titanomagnetite?

I would appreciate any suggestion/comment

Theo Ntaflos

[jon_wade]

From memory alone, it looks like your backgrounds are too tight, especially for Ti (-550, +600)

[Probeman]

Hi Theo,
Would it be possible to plot up and post here some WDS scans showing the background positions of the elements?
john

[Mike Jercinovic]

Hi Theo!
A couple of comments... First, just looking over the data table you attached.  First, seems like the TiO2 contents of points 10 and 11 are high even for end member ulvospinel, so must be some exsolution with ilmenite or rutile (?) if this is supposed to be titanomagnetite.  Boundary fluorescence of Ti in fine Fe-Ti oxide mixtures can be problematic too. 

Second point, maybe a little out there, but magnetite-titanomagnetite can be magnetized, having picked up permanent magnetization especially if there is maghemite or other impurities that might increase the magnetic retentivity.  If they have been polished on a system using magnetic disks, that could do it too.  We don't do magnetite very often, but we have noted beam deflection from time to time compromising x-ray focus, and we wonder if the current flow from the beam could actually induce some of this effect under the right circumstances.  We have analyzed tetrataenite which is even more problematic, with some very dramatic magnetic field effects, but we could compensate by storing a beam deflection (in the setup) to get the beam back into correspondence with the position during calibration.  Very tedious because the fields would vary grain to grain.  So, anytime we think about probing ferromagnetic minerals, we consider beam shift as a possible variable.  As you note, your ilmenites seem fine, so appealing to MAC tables is unlikely.  Anyway, maybe beam deflection is a possibility?

[theo_nt]

I think the Ti backgrounds are not too narrow. Maybe the lower Fe background is little bit narrow.

John I attached the WDS scans for Fe and Ti.

Mike, you are write about the points 10 and 11.

As to the beam deflection seems not to be the issue. I attached the optical and the BSE images where the beam contamination is visible on the standard ilmenite and on unknown titanomagnetite. Both kind of images show the same point.

If there was a background problem why the EDS is giving as you see in the excle file I sent identical results?

ciao
Theo

[Mike Jercinovic]

Theo, it would be interesting to compare the beam position between your Fe standard and some of the points with lowest totals, like 8, 9, or 42.  Not sure how to evaluate point 10 other than it is getting closer to an ilmenite composition anyway.

[Probeman]

The Fe backgrounds shown will likely cause low totals on unknowns assuming that the backgrounds on your Fe standard are wider.  Could that be the main problem?

Here's something else to check: what is the database composition of your ilmenite standard?  Have you entered the excess oxygen properly? What do you see if you analyze your ilmenite standard as an unknown?  Can we see the entered composition for your ilmenite standard and an analysis of the ilmenite standard as an unknown?

Or maybe even better, since you see an issue with increasing Ti content, can you try analyzing a TiO2 standard as an unknown using the ilmenite standard as the primary standard or perhaps the other way around?

[jon_wade]

Our LPETs is definitely 'flabbier' than the small PET (your set up has Ti on LPET?)
Fe backgrounds are too narrow for my taste.

The pictures though....the one on the left looks like the analysis crosses a boundary and the one one the right has cracks(?) or  something else (?)  under the spot. 

[theo_nt]

Sorry for not responding. The probe is for the whole month booked. So I will be able to do some tests according to your suggestions in May.
Theo

[John Donovan]

We have a problem in analyzing titanomagnetites. We always getting low totals, even if we assume that 2/3 of the Fe is Fe3+.

Our microprobe is a Cameca SXFive FE and the analytical conditions are 15 kV, 20 nA and the beam beam size 1 micron.

In the attachement you will find the detailed analytical conditions (.doc file) and some analyses (.xlsx file) to demonstrate the problem.

Also we observed a negative correlation between Ti and totals: The higher the TiO2 contain the lower the totals. We tried using different standards to calibrate Ti and Fe. Also we are using as a secondary standard a synthetic ilmenite. Calibrating Fe in FeO or even on almandine and Ti on rutile we always get a total of 100 wt% on this imenite.

Also we thought about peak shift. Calibrating Fe on EDS we are getting the same results. Low totals for titanomagnetite and good totals for ilmenite.

The only thing that we have not yet checked is the ABS coefficients. Here is the question:

Is it possible that the Cameca ABS coeficients (in SXFive FE) have a specific problem in the system Fe-Ti for the specific range of concentration applying to titanomagnetite?

I would appreciate any suggestion/comment

Theo Ntaflos

Hi Theo,
Did you ever figure this out?

Note that you can now calculate the excess oxygen from Fe2O3 using the method of Droop (1987) in Probe for EPMA:

https://probesoftware.com/smf/index.php?topic=92.msg8593#msg8593

This allows the excess oxygen to be included into the matrix correction, which surprisingly affects the Fe concentration significantly.

[theo_nt]

Hi John,

Actually no, I haven't figure out why do I have the low totals. I am attaching a pdf showing an analysis of a single titanomagnetite using three different Fe standards without and with calculation of excess Oxygen. The results using three different Fe standards are close to each other indicating that the used standards are OK. Doing the formula calculation  I am getting ca 98 wt% Sum. I checked the Ti as well using Rutile as a standard. No difference in it.

Another question: is there any progress in using combined wds-eds for quant mapping?
Theo 

[John Donovan]

Actually no, I haven't figure out why do I have the low totals. I am attaching a pdf showing an analysis of a single titanomagnetite using three different Fe standards without and with calculation of excess Oxygen. The results using three different Fe standards are close to each other indicating that the used standards are OK. Doing the formula calculation  I am getting ca 98 wt% Sum. I checked the Ti as well using Rutile as a standard. No difference in it.

Hi Theo,
Did you update PFE recently?  We found a small bug in the calculation of excess oxygen for the sesquioxides (Cr2O3, V2O3, etc.).  That is now fixed.

Another question: is there any progress in using combined wds-eds for quant mapping?

The code for the Thermo EDS-WDS synchronized mapping is done on our end, but we are waiting for Thermo to finish their code. They got distracted porting their new Pathfinder code to 64 bit and integrating their EBSD software.  Hopefully sometime this year they will get around to it...

[theo_nt]

John,
what about Bruker  eds-wds  quant mapping?

[John Donovan]

John,
what about Bruker  eds-wds  quant mapping?

Bruker has told me several times "no problem" but then we never get any information.   Part of it is that I think Cameca wants to implement this feature for PeakSight before they release it to us.  But I think that if a customer bugged them about it they would do it faster. 

[wonachlas]

Hello Theo

I was analyzing Fe-Ti-oxides a few months ago with a very similar routine as you -- see attached spreadsheet with analysis conditions. I was also getting low totals when calculating O by stoichiometry, then I decided to measure O directly and it seemed to fix the problem. O Ka was analyzed on PC0.

I agree with Probeman above that your chosen Fe backgrounds are too narrow.

what about Bruker  eds-wds  quant mapping?

Bruker EDS mapping is well-integrated with PeakSight, but from what I can tell EDS quant accuracy is not particularly good because it uses on-peak calibration only.

Cheers
Will