Explain This If You Can!

[Probeman]

No speculations on what could be causing these different TDI trends on 3 simultaneous spectrometers...?

https://probesoftware.com/smf/index.php?topic=144.msg12205#msg12205

speculations... there is a bit too little information on other stuff to exclude the hardware effects. In case there is no hardware effect, I would speculate that maybe, maybe, this is crystallographic orientation thing. I cant remember now where I had read this, but as far I could remember it was somewhere shown that analyzing apatite-F at different orientations shows sever F loss where at other orientations shows nearly no F loss. I could get from that experiment the mind-shortcut that it is e-beam--crystallographic orientation controlled F loss. But Your experiment makes me start to think that maybe there is no loss at all (it was always for me illogical that negative (-1 valence) F ion would move toward negatively charged sample surface! There is probably only crystallographic repositioning of F, where for some spectrometer it gets hidden behind other atoms in lattice and thus absorption significantly goes up (decreasing intensity), while for different angle spectrometer they stay completely unhindered.

I think it would be easy to throw this hypothesis out the window If same behavior would be observed after redoing experiment with rotated sample.

This is a reasonable explanation I think.

The only thing that bothers me with this idea is that this would assume that the orientations of both the BaF2 and the CaF2 would be similar, yes?  Also I note that these are both cubic compounds, so not sure how orientation would affect ion migration...  but it's an interesting idea.

Here is a drawing of the std mount with the positions of CaF2 and BaF2 outlined and the locations of the Cameca spectometers 1, 2 and 4 shown:



The thing that makes a little more sense to me would be that perhaps the carbon coat connection to the mount might be only in one spot (towards spec 1?), so the sub surface negative charge would flow towards that point, and therefore "push" the F- ions away from spec 1, causing the large intensity drop over time, whereas spec 4 might see the least reduction in F- ions because it was getting more F- ions "pushed" towards it over time?

How does that sound as an explanation?

[sem-geologist]

easy to check (both hypothesis), as it is 1 inch round mount. I suggest to rotate it about 45 degrees and retry the experiment. If result will be the same – then our both hypotheses are failed, and the reason is buried somewhere in hardware.

[Probeman]

Yeah I thought of that, but I would have to ensure that conductive contact is made at specific points relative to the spectrometer orientation.  I think it might be easier to try using a thin insulator on the top of the mount and then bridge that insulator in specific places using a small drop of carbon.  Maybe this weekend...