Adsorption equilibria are reported for isopropyl methylphosphonofluoridate (GB) on activated carbon at relative pressures (p(i)/p(s)) approaching 10(-10) and temperatures between 30 and 100 degrees C. A novel desorption method and analytical apparatus employing purge-and-trap chromatography (PTC) were used to overcome the difficulties in quantifying the vapor phase at ultralow concentrations ( < 1 ppb). The use of desorption-PTC analysis has extended the detection threshold range by several decades over that of conventional volumetric methods.