Brain tissue is not as simple as cellulose, but at 600 C with the molecules that make up the molecular soup, there would definitely be some lysing that would take place and form thing like free radicals, these have the potential to react forming longer chains of carbon, which could possibly create a false positive.

At 600 degrees, there is probably some reaction happening there that may be similar to plastics. Basically, creating brain plastic and cooking it off to measure plastics. Im a bit skeptic.