I realize that this is a EPMA forum, but here is a quick question from the imaging side of things.
I have always been told to never try CL imaging of uncoated samples (we have a Hitachi S-3400 with VP mode), because the air bled into the chamber to keep the charging down would kill your CL signal and maybe break your CL detector. This is generally what I tell people when they come to me for CL imaging, and I turn them away if their samples are uncoated. Today however, I had a particularly insistent user who insisted that we try CL imaging with uncoated samples because they will be measuring trace C with SIMS in this sample.
We devised an experiment were we took one of his less precious samples and coated 1/3 of it with the standard 20nm carbon, 1/3 we left uncoated, and 1/3 we coated with 0.6nm iridium. What we found is that carbon was actually produced by far the worst images out of the three (both in terms of overall signal and ability to see zonations), and best images were taken on the uncoated sample in VP mode (with the iridium coated samples being almost as good as the uncoated).
This goes against everything I have been told about CL. Has anyone else ever been told to not CL image uncoated samples, or I did I just get bad advice from someone? Now that I think about it again, it makes sense to me that uncoated would yield the best images since there will be no attenuation of the CL signal by the coating. Can anyone else come up with a better reason why this work? We have tried this is a few different types geologic samples now, and they all seem to show the same results.
I have attached three images taken under the exact same brightness/contrast conditions with the only difference being the coating and using VP mode (the fourth image is just a repeat of the third image with the brightness turned out as the original was too bright).
Cheers,
phil