Chemical identity sensors were a part of the sensor system found aboard Constitution-class starships, used in identifying chemicals.
While scanning an unidentified cosmic cloud in the Pallas 14 system, Spock began getting unusual readings on it from the chemical identity sensors. These sensors reported elements within the cloud that were unknown in Federation periodic tables, helping him determine that the cloud was of extragalactic origin. (TAS: "One of Our Planets Is Missing")