Formaldehyde 2,4-Dinitrophenylhydrazone-13C6 is the isotope labeled version of Formaldehyde 2,4-Dinitrophenylhydrazone. Formaldehyde 2,4-Dinitrophenylhydrazone is a dinitrophenylhydrazone (DNPH) derivative of an aliphatic aldehyde found in mainstream cigarette smoke.