Author Topic: Smithsonian Standard Sample Preparation  (Read 5891 times)

Anette von der Handt

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Smithsonian Standard Sample Preparation
« on: November 20, 2013, 04:30:03 pm »
Ok, I have a question about mounting standards, specifically the Smithsonian mineral standards.

I have to replace one of our old standard blocks and use this as a chance to change to a more flexible system. For this I have a new holder that houses the standards in individual small brass tubes that I can switch in and out and polish individually. So far so good when it comes to the more generous sized standard grains in our collection. However, the mineral grains I got from the Smithsonian are barely visible to the eye.

I don't want to go through too many trial and error cycles and waste time and material. Therefore I would love to hear from others how they handle these tiny grains, specifically:

1) How do I get them best out of their plastic container? I have a problem with some that they are so tiny and cling to the sides. Are vacuum tweezers the answer?

2) What epoxy did you use? I am worried about the viscosity of the resin and the grains "swimming" up or away (we have PetroThin here in the lab). Pete McSwiggen once mentioned that he actually used a mixture of three different epoxies to deal with that which started me to ponder on it.

3) I usually use Scotch tape to keep my samples in place when I pour epoxy mounts. I am worried that the adherence will be too great for this grain size and that it will introduce too much of a surface relief. I have seen recommendations for Kapton double-sided tape as well as simple Post-it notes. Any preference?

So, any comments especially to the second question would be greatly appreciated.
« Last Edit: February 07, 2014, 02:36:00 pm by John Donovan »
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Gareth D Hatton

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Re: Smithsonian Standard Sample Preparation
« Reply #1 on: November 26, 2013, 12:45:30 am »
All of my Smithsonian samples came in gelatine capsules inside the plastic tubes.  I just used some very small tweezers to remove them.  I had a well lit bench with a small sheet of glass which I put strips of double sided sticky tape on.  I wrote on which standards were which and then placed them gently onto the tape.  After this I used some silicon tubing of about 3mm diameter and placed this over the sample (a short length of only 5mm).  This I had pre-labelled and then filled with Epofix using a small pipette.  It was very time consuming and I had to use a needle to move the bubbles from the sample.  When set I removed the tubes and had cylinders of resin containing my standards.  I pre-polished using a fine SiC paper and placed all of the samples, again labelled, into a mould and back filled with resin.

Obviously this took a long time but I hope they give me many years of service.  I think if you used some silicon tube you could make the cylinders and then swap them out.  Keep the tube short though as it will not be true and this is likely to be worse as it gets longer.

Good luck.

Zack Gainsforth

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Re: Smithsonian Standard Sample Preparation
« Reply #2 on: June 27, 2014, 01:45:14 pm »
We used a similar protocol:

1) Pull the grain out using tweezers, or in the case of standards being prepared for TEM we used the electrostatic charge on a glass needle... (probably you don't need this for an SEM sample but if you want more detail ask).

2) Place it on the basin of a 1" silicone mold.

3) Mix some epofix and and put a drop or two on the grain.

This is how I handle grains swimming away.  One drop or two of epoxy from a pipette directly on the particle means that it stays put within a few tens of microns.  Then, if you add the first mm or two of epoxy to the mold slowly, just a few drops at a time from a pipette and circle around the grain then there is no shear at the grain and it doesn't move.  Once the epoxy is more than a mm or two thick, you can just pour (carefully).

4) Slowly pour the epofix in the rest of the way and then bake it.

We typically didn't have much trouble with bubbles.  But in cases where they are a problem, we solve it by cycling a few times in a vacuum chamber before fixing  (or remix the epoxy).  Too much vacuum though and epofix hardens inhomogenously!  I usually just use a few minutes.

It is better to avoid bubbles in the first place.  Pour the two components together using a glass rod so that one doesn't splash into the other.  Then mix the components carefully using a magnetic stir rod for much of the mixing.  To mix up the liquid at the top of the cup, use a glass rod, not a wooden stick.  You're basically trying to keep the surface unbroken so that there is no chance to incorporate air bubbles in the first place.

5) Polish.

6) Carbon coat.

Hope that's helpful!

Anette von der Handt

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Re: Smithsonian Standard Sample Preparation
« Reply #3 on: June 27, 2014, 02:00:48 pm »
Thank you both very much. That is incredibly helpful. I tackled various other tiny standards with a top-down mounting method* recommended by John Fournelle but have been gun shy for the Smithsonians. I will give this a try next.

* I got much better control for small samples and bubble avoidance by first filling the brass tube with epoxy, have it harden and then drill a new small hole. Then I fill the new hole with epoxy and the samples. This way they cannot sink lower than the new hole bottom and I have no issues with surface roughness from the tape etc. First results can be seen here: https://www.facebook.com/probelab
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Jeremy Wykes

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Re: Smithsonian Standard Sample Preparation
« Reply #4 on: June 29, 2014, 04:05:49 pm »
Another technique for removing bubbles after mixing the epoxy and hardener but before casting the mould is to centrifuge the epoxy for a little while.
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Anette von der Handt

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Re: Smithsonian Standard Sample Preparation
« Reply #5 on: June 30, 2014, 03:27:46 pm »
Another technique for removing bubbles after mixing the epoxy and hardener but before casting the mould is to centrifuge the epoxy for a little while.

Good to know. Actually, my bubble problem comes not from the mixing stage of the epoxy but rather me trying to fill a tiny, tall tube with a (too) viscous liquid. I always fill the mixed epoxy into hypodermic needles which seemed to work fine to remove any bubbles and gives me more control over the placement. But even when I have it slowly moving down along the tube walls, it sometimes creates a bubble at the bottom. I now use Epothin which to my knowledge is at the lower end of epoxy viscosities but it is still too viscous for my narrow brass tubes. But now I have some more things to try. I will report back when I attacked the Smithsonian standards again and had success.
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John Donovan

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Re: Smithsonian Standard Sample Preparation
« Reply #6 on: June 30, 2014, 10:30:19 pm »
Another technique for removing bubbles after mixing the epoxy and hardener but before casting the mould is to centrifuge the epoxy for a little while.

Good to know. Actually, my bubble problem comes not from the mixing stage of the epoxy but rather me trying to fill a tiny, tall tube with a (too) viscous liquid. I always fill the mixed epoxy into hypodermic needles which seemed to work fine to remove any bubbles and gives me more control over the placement. But even when I have it slowly moving down along the tube walls, it sometimes creates a bubble at the bottom. I now use Epothin which to my knowledge is at the lower end of epoxy viscosities but it is still too viscous for my narrow brass tubes. But now I have some more things to try. I will report back when I attacked the Smithsonian standards again and had success.

My expert technical petrology info comes from Tim Teague at UC Berkeley.

I know that for tall thin tubes, he pre-fills the tubes (e.g., drilled into solid acrylic), with epoxy using a hypodermic and then adds the grains one at a time. When a grain won't sink, he uses a wire to dislodge any bubbles adhering to the grains. Which is to say, the grains won't sink to the bottom, until they are bubble free!. This technique obviously works well only for materials that are higher density than epoxy (which is true for most materials).
« Last Edit: July 01, 2014, 09:09:40 am by John Donovan »
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Re: Smithsonian Standard Sample Preparation
« Reply #7 on: January 07, 2020, 03:37:12 pm »
I wanted to update this topic with some actual photos of the mount because the best aspect of these acrylic mounts (see above post and login to see the attachment!) is that they are super easy to re-polish.

I'm sure most of us have had to deal with standard mounts where each standard material is individually mounted in a brass sleeve and then inserted in a multi-hole brass mount with a set screw for each standard. But this method requires that the brass sleeves and mount be completely disassembled and then each standard "tube" has to be polished separately. Then ultra-sonically cleaned because the epoxy tends to pull away from the inside of the brass sleeve when it originally was cured, resulting in a tiny gap that can collect polishing compounds and oils/solvents.

With these acrylic mounts seen here:



the epoxy completely adheres to the acrylic hole and when the mount is cured, the *entire* mount shrinks by a small amount. But no gaps or cracks!

So to clean these acrylic mounts one merely uses a little 0.05 um colloidial alumina (or silica) on a soft lap and polishes off the carbon (and any oxide buildup and/or beam damage), then just a quick rinse in ethanol (only use pure ethanol to avoid damaging the epoxy!), and then wipe clean with a KimWipe, and dry in a warming oven for a few minutes immediately prior to carbon coating.

This type of acrylic block makes it so easy to re-polish ones standards that you won't hesitate to do it when it needs to be done!  And if you look closely at the above photo you can see the carefully scribed vertices of a triangle which contain our three "fiducial" marks for quickly re-calibrating our digitized standard positions when the mount is replaced within the sample holder.

Another standard mounting method which we don't use, but which I have an example of, is similar to the acrylic mounting method, but the epoxy is filled with copper shavings and it has a metal outer sleeve. This mount was labeled the "MAC" mount (when I first arrived in Oregon) and I suspect it came with a very old microprobe many decades ago:



However, for whatever reason, our lab manager Julie Chouinard, had to vacuum re-impregnate it with epoxy because originally it did have some tiny cracks in a few places.  Now however, it works great for a quick re-polish.
« Last Edit: January 08, 2020, 08:48:20 am by John Donovan »
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