The column dead volume comprises the entire space accessible to a small molecule that can fully permeate all pores of a packing material. It includes the interstitial volume and the unoccupied pore volume. It is denoted as V_M. The system dead volume includes the additional volume in the tubing that connects the injector and detector to the column. The system dead volume usually is approximated by injecting a small, essentially unretained species. Uracil, acetone and thiourea are most commonly used species in reversed-phase chromatography. See also adjusted retention volume, holdup volume, and void volume.