Sample conditioning systems and methods especially useful for converting non-elemental forms of mercury to elemental form. The systems involve a reactor having a heater, with the reactor configured such that a heated sample is flowed through a calcareous sorbent during heating and then cooled in the presence of a solid metal that is more chemically active or less noble than mercury. The methods may be used as a part of an emissions monitoring process to insure compliance with emission standards and to precisely gage the amount of abatement needed.