As the U.S. and world population grows, energy use increases. Increased energy usage requires an increase in electric power generation, which constitutes one of the major components of the total energy needs. Increased power generation from fossil-fired power plants also results in increased emissions of pollutants, such as NOx, SOx, Hg, heavy metals and particulate matter (PM). Pollution emissions are regulated by Government rules and regulations, which typically require continuous measurement of emissions and periodic calibration of the continuous emission monitoring system (CEMs) using approved reference methods. Instrumentation, techniques, and reference methods for the measurement of NOx and SOx emissions are mature technologies, with well established procedures, accuracy and reliability. Of the 189 substances designated hazardous air pollutants by the U.S. Environmental Protection Agency (EPA), mercury (Hg) has attracted significant attention in the world due to its increased levels in the environment and well-documented food chain transport and bio-accumulation.