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!!!
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
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/