Which of the the following substance in blood is demonstrated by Kunkel's test?

Kunkel's test or tannic acid test: If tannic acid is added to blood it remains cherry-red in CO poisoning, while oxyhemoglobin turns deep brown. Hoppe-Seyler's test and spectroscopic examination are also used to determine presence of CO in blood. Ref: The Essentials of Forensic Medicine and Toxicology by K S Narayan Reddy, 27th edition, Page 555.