1) gas-phase elemental mercury and as
2) oxidized/divalent mercury compounds, such as mercuric chloride.
3) particulate or non gas-phase mercury
The EPA PS-12A performance specification standard was promulgated for evaluating the acceptability of total vapor phase mercury continuous emission monitoring systems (CEMS) installed on exit gases from fossil fuel fired boilers. Particle-bound mercury is not included.
Most mercury CEMS are extractive monitors, which remove a continuous or semi-continuous gas sample from a stack and subsequently transfer it to an analyzer. Difficulties arise from the fact that current mercury CEMS only measure elemental mercury directly, while mercury emissions in the flue gas can consist of elemental, oxidized or particle-bound mercury. Most commercially available CEMS filter out particulate matter, including particle-bound mercury, and hence the mercury measured typically refers to gas-phase mercury only.
The most commonly used detection methods used in current mercury CEMS:
* Cold Vapor Atomic Absorption Spectroscopy (CVAAS) (photo)
* Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS)
* Atomic Emission Spectroscopy (AES)
* UV Differential Optical Absorption Spectroscopy (DOAS)
* Zeeman-Modulated Atomic Absorption Spectroscopy (ZAAS)
CVAAS and CVAFS are the most commonly used.
(Pollution Engineering, March 2006, "Inside Mercury CEMS)