A purge box is typically used so that absorption of THz radiation by gaseous water molecules is minimized. A dry air source is often used for this purpose, however, a nitrogen gas source may also be used.
Water is known to have many discrete absorptions in the THz region that are rotational modes of water molecules. Alternatively, nitrogen, as a diatomic molecule, has no electric dipole moment, and does not (for the purposes of typical THz-TDS) absorb THz radiation. Thus, a purge box may be filled with nitrogen gas so no unintended discrete absorptions in the THz frequency range occur.