(1) A neutral material such as diatomaceous earth used to support a catalyst in a large-scale reaction system. (2) A gas used in chromatography to convey the volatilized mixture to be analyzed over the bed of packing that separates the components. (3) An atomic tracer carrier: a stable isotope or a natural element to which radioactive atoms of the same element have been added for purposes of chemical or biological research.See Tracer.