Hello Joy and others,
In my experience using both low temp and high temp. systems with glassy carbon, a reaction rim forms around the upper most glassy carbon chips (the ones that have direct contact with a sample). After a tube is “done” as in full (when a crucible is full on the TC/EA, we do use these, and I think it’s a good idea too). I simply place all the chips our on a piece of clean paper and look at the ones that look “brown” or dull, because they have this rim. I remove these chips and add new ones to replace them as needed. I also have a feeling, that these zones of reaction are also more porous, and might affect chromatography. Additionally use the smallest silver capsules you can, that will help. You can also try to compress the material in a pellet without a capsule (I believe Farquhar has this in a paper).
All the best,