We have measured the electrical response to NO2, N2, NH3, and CO for epitaxial graphene and quasi freestanding epitaxial graphene on 6H-SiC substrates. Quasi freestanding epitaxial graphene shows a 6 fold increase in NO2 sensitivity compared to epitaxial graphene. Both samples show a sensitivity better than the experimentally limited 1¿ppb. The strong increase in sensitivity of quasi freestanding epitaxial graphene can be explained by a Fermi-energy close to the Dirac point, leading to a strongly surface doping dependent sample resistance. Both sensors show a negligible sensitivity to N2, NH3, and CO.