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November 2016

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JEOL-Focused Probe Users List <[log in to unmask]>
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Tue, 29 Nov 2016 10:04:47 -0800
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Try turning the sample 180 degrees to see if the deflection is in the opposite direction.If so likely magnetism.


Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: "Johnson, Melissa J" <[log in to unmask]> Date: 11/29/16  9:01 AM  (GMT-08:00) To: [log in to unmask] Subject: Re: [PROBEUSERS] Weird beam deflection issue: magnetism? charging? other? 


HI Julien-
 
Could you take the sample in question and take it out of the epoxy mount and remount it in epoxy you have used before or some other mounting material and repolish
 it? Perhaps that would at least tell you if it’s something is weird with the epoxy or if it’s the sample. I analyze material in epoxy all of the time, and while I do have problems with occasional charging (I use a lot of copper tape to cover the epoxy), I
 haven’t encountered the beam deflection issues you are having. I don’t think I’ve analyzed many things in epoxy that’s more than a year or two old though.
 
Anyway, good luck! I look forward to hearing what the issue/solution is.
J
-Melissa
 
 


From: JEOL-Focused Probe Users List [mailto:[log in to unmask]]
On Behalf Of Julien Allaz

Sent: Monday, November 28, 2016 7:59 PM

To: [log in to unmask]

Subject: [PROBEUSERS] Weird beam deflection issue: magnetism? charging? other?


 
Dear probers, 

 


I hope you all had a great Thanksgiving break. Today a colleague brought three epoxy mounts (1’’) containing each one 5-10 mm meteorite chip. Whereas the day started well with acceptable (large) Px analyses (100-101%), troubles arrived
 when I hit a CL-active plagioclase: the beam was actually off-centered by 10-15 um compare to my usual well-centered position. As a result, a series of 20+ points on well-polished and large surface yielded 103-104% total, even when using 20 nA and 10 um beam
 size (2 min analysis time). Problem is that the electron beam is perfectly centered on any other standards, and other CL active minerals in other mounts. So… the problem appears to be sample-related?


 


From there, I started wondering about possible (a) charging (old epoxy => loose connection, plus sample is badly polished and fractured), and (b) presence of magnetic minerals (magnetite especially).


 


I have not seen any “flash”, abnormal SE intensity (except on some cracks as expected), or image displacement, even during prolonged beam exposure. Despite this, I opt to re-coated a SECOND time (so TWO coating layers of C ~20-30 nm at
 least) hoping it would cover the cracks & so. Additional re-coating (after re-polishing) did not help. I also added some C-paint to connect the sample surface directly to the sample holder. None of this improve the behavior. The beam was still showing some
 displacement. The weird thing is that on one single sample, the beam deviation can be variable within one single sample: more or less centered in one zone, 10 um on the right in one other and 10 um to the top-left on the other. And it is variable in each sample,
 too. So… Not a charging issue?


 


Could magnetism be an issue? The users have actually run some QEMSCAN, and beside some Cr-Fe oxide and ilmenite there does not appear to have magnetite, s.str. Could other minerals (in meteorite!) be a problem? BTW, I do NOT have access
 to a demagnetizer, but I have some hope to find some (or buy one) soon?


 


To double check everything, in one coating run, I freshly re-coated all the samples (after brief re polishing with .25 Al2O3 + ultrasonic bath), and added an other similar sample made with a new epoxy mount (~2 month old) of another meteorite,
 not the same composition / mineralogy. This one worked perfectly (beam was centered)… Could the problem come from “old” epoxy (how old is old???). Living in CO, any compounds like epoxy might behave differently than on the west and east coast due to our well-known
 near-absence of H2O, and the high elevation near the mile-high city…)?


 


At this point I would welcome any suggestion / opinion…


 


Best,


 


Julien


 


=============================

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



Lab website: http://geode.colorado.edu/~jallaz/

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