Volatile Back Titration

[Noble]

Hi, I'm glad I found this forum and community. I have a rather peculiar question, does any body know of a case that a common substance contained harmful impurites? I hope to write a story and chemistry seems to be one theme I can articulate on.

The plot structure will have a student who discovers the substance is ??% pure by doing a back titration. And the rest of it is volatile (possibly by some form of contamination) and reactive with common substances (i.e. water, skin, etc) that other students will have accidents with. No way im going to leave all the work up to you guys, I am very willing to do research and get the correct numbers and substances, but I need a place to start. Your help is appreciated.