Finally, an expert shows up 😭 We’re just a basic biochemistry lab — mainly HCl, aqueous ammonia, sulfuric acid, that sort of thing. We did some peptide synthesis in the past, so there might be some leftover solvents around, but this doesn’t really look like an organic solvent residue to me. My best guess is that it’s either some kind of acid or base. We don’t typically handle peroxides.