I have acquired several WDS scan in the PtTi alloys, and determine the most appropriate exponential factor to constrain appropriately the curvature. The background subtraction and interference correction is done through a third party software, and the whole is integrated within the ZAF iteration. Most of my data in the PtTi alloys are yielding an average within the error close to 0 wt-% or are slightly positive (versus constantly negative without the exponential regression), suggesting that the background subtraction and the peak interference correction are both OK (or at least better than just a linear regression). Again, most of these phases are expected to have low N content (probably 100-1000 ppm, with a few minor phases with detectable content above 0.5-0.8%, which is currently the calculated detection limit depending on the current / analysis time used).

You raise a good point about the Ti Ka and Ti La ratio; however, I’m not sure how bad this can be. I do analyze the Ti Ll line in the pure Ti standard, and this correction procedure is part of the ZAF iterations. I also know that for light element you usually use area-peak-factor for correcting for these effect... Do you expect a large change in peak shape and peak position (hence intensity) between  the pure Ti metal and the Ti-Pt alloy? I know this is a big problem when analyzing a standard that is totally different than the unknown, and here I attempt to minimize this effect by using pure Ti. Using a TiO2 (rutile / Ti-oxide) will certainly be problematic to analyze an alloy… Note that I have NOT observed a peak shift between the scan in Ti and in the Ti-Pt alloy, but of course this is tricky to compare 1:1 as there is a small N Ka peak in the TiPt alloy (although in the test I performed, I selected the sample yielding the lowest intensity / k-ratio for N Ka, hence the sample the most representative as N-poor).

Anyway, please, don’t worry about N, the customer probably will ditch the data anyway and keep these as “informative / qualitative”. Key to them would be to understand the limitations on oxygen analysis. Any insight on this matter?

J.


On Nov 24, 2015, at 15:39, Joe Geller <[log in to unmask]> wrote:

I don't think you can correct precisely for the Ti - N overlap since the peak ratio for TiKα and TiKLα change due to matrix effects.
What procedure do you use for correcting the exponentially curved backgrounds?

Thanks,
Joe

Joe Geller
Geller MicroAnalytical Laboratory, Inc.
426e Boston St.
Topsfield, Ma 01983 (USA)
978 887-7000, fax 978 887-6671
www.GellerMicro.com

-----Original Message-----
From: JEOL-Focused Probe Users List [mailto:[log in to unmask]] On Behalf Of Julien Allaz
Sent: Tuesday, November 24, 2015 5:12 PM
To: [log in to unmask]
Subject: Re: [PROBEUSERS] Pt-Ti alloy

Thanks Joe,

Replacing the BSE is a good idea (I’ve seen it last year, and effectively it is as bad as you would expect it after 27 years of service [!]…), but since our current instrument will be replace in early 2016, this is not an option…

Regarding the Ti Ll interference on N Ka, I’m well aware of this, and I do correct for this. Still, some phases suggest up to 1.8% N after correction for Ti Ll. Again, N is not really important for this customer (they know the strong limitation / huge error associated to this). What was surprising to them was the oxygen content. It is true that the k-ratio is already indicative, but it is important for the customer to know what are the atomic proportion (hence the “as accurate as possible” weight-% element).

Best,

Julien


On Nov 24, 2015, at 14:49, Joe Geller <[log in to unmask]> wrote:

Julien:
I'd recommend you replace the backscattered detector as they do get contaminated over the years.
We did our recently and it made a huge difference.
As far as N and Ti goes there is a pretty complete overlap with the Ti Lα and N Kα. At low concentration forget about it.
The same K-ratios go into the different ZAF corrections so you will see the same relative differences in concentrations but the absolute levels will change.

-Joe

Joe Geller
Geller MicroAnalytical Laboratory, Inc.
426e Boston St.
Topsfield, Ma 01983 (USA)
978 887-7000, fax 978 887-6671
www.GellerMicro.com


-----Original Message-----
From: JEOL-Focused Probe Users List [mailto:[log in to unmask]]
On Behalf Of Julien Allaz
Sent: Tuesday, November 24, 2015 3:09 PM
To: [log in to unmask]
Subject: [PROBEUSERS] Pt-Ti alloy

Dear microprobers,

A customer recently asked me to analyze several alloys of Pt-Ti (in various mixture), and quantify the different Ti-Pt phases within each. In addition, they asked me if it was possible to analyze also O (and N). First importance for this job is the Ti - Pt ratio, and this light element work is just an “extra” if possible. So… I gave it a try... Beside the “usual” light element problem, I have two specific questions for you.

(1) With my vintage instrument (JEOL-8600), it is very hard if not impossible to work on these samples at low keV (<10 keV), as I loose the necessary BSE contrast despite cranking up the BSE gain and adjusting for the contrast - and some phases in a single sample can be ~1-2 µm in size, they are already just barely distinguishable from the mass. Therefore I opt for 15 keV, a level at which the contrast is sufficient. I know… This is definitely not great for light element. The results can vary quite dramatically, notably for O, depending on the matrix correction procedure used (e.g., in one domain, it can vary from 0.7 to 1.2 wt-%, so not negligible (Calculated DetLim for O is around 320 ppm; 1sigma counting precision at 3% relative). Do you have any recommendation for the appropriate ZAF correction procedure (currently I’m using Armstrong ZAF and FFAST MAC table). Is it possible to have oxygen “contamination” during the analyses at high current (100-200 nA).

(2) Results show high O-content from a few hundred ppm to above 1 wt-%. These measurement are repeatable within a single phase in a specific sample (5 analyses each, each with similar wt-% O), and in the same sample, one phase might have 0.9% O and the other almost nothing (100 to 1000 ppm). The customer has actually prepared these sample in a neutral-atmosphere (argon) furnace ~1 year ago. I have yet to double check if the problem could come from Pt Mz (3rd order! - so probably not so important, but we are speaking here of ~0.1-0.4 cps/nA on O Ka in the unknown [Std O Ka at 118 cps/nA]) lines around O Ka, but I don’t see a correlation between O wt-% and Pt wt-%. The samples were re-polished, but I recently learnt that the customer only did a single 0.02 µm fine polishing, which may not have been enough to remove this oxidized layer?!? If you have ever worked with such alloy, do you think it is possible the samples have been oxidized (i.e., do Pt-Ti alloys oxidize easily)?

About the setting: I’m using an LDE1 crystal (60 Å) for O Ka and apply an exponential background correction (curvature determined using WDS scan in a representative TiPt phase), and high current (100 - 200 nA - this further help my BSE detector to “see” the different phases). I opt for Ti Ka on LiF (lower count rate, but with high current / ~60 sec counting, the precision is enough) and Pt La on PET. If I drop the voltage, I would probably have to opt for Pt Ma (and maybe even Ti La) which raises other problems...

Your help will be greatly appreciated! Thanks, and Happy Thanksgiving!!!

Julien


P.S. Let’s not worry about N, another beast… In most (90%) of the phase, the N-level is probably a few hundreds of ppm and with (a) the low count rate (3 cps/nA on BN at 15 keV… I know… way to high absorption in every component and low efficiency in the P-10 gas), and (b) a strong correction from Ti Ll (up to 100% of the signal received).


=============================
Dr. Julien Allaz
Electron microprobe manager
University of Colorado Boulder
Dept. of Geological Sciences
UCB 399
2200 Colorado Av.
Boulder, CO 80309-0399

Phone: (303) 735 2413
Cell: (413) 210 0917
Lab: (303) 492 5251
Fax: (303) 492 2606

Personal website: http://geoloweb.ch
Lab website: http://geode.colorado.edu/~jallaz/
=============================

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