I am performing a derivatization in serum. I precipitate protein with HCl (10% concentrated HCl final) , pellet the protein, collect supernatant, and react the supernatant in the HCl at 90 degrees C for 90 minutes. After the 90 minutes a dark brown precipitate forms even though the serum is nearly colorless to start. It can't be hemoglobin. It is somewhat fluffy, but not floating. Just a bit hard to pellet. Any ideas about what it is?