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The heart of any CEM system, whether extractive or in‐situ, consists of the analyzers. An extractive system transports and conditions the flue gas, but the analyzers perform the job of measurement. The selection or evaluation of CEM system analyzers must consider both regulatory specifications and performance characteristics. Although most analyzers are advertised as meeting or exceeding required specifications, care must be exercised in their selection because an analyzer's performance in the field can differ greatly from its performance on a laboratory bench.

CEM system analyzers must measure gases without interference from other gases. They are increasingly being required to measure accurately in low‐concentration ranges, and they must perform well often in hostile environments. Current U.S. EPA, Canadian, and International Organization for Standardization (ISO) standards for CEM system analyzers do not specify analytical techniques that are to be used (except for opacity monitors), but rather provide performance‐based specifications. Therefore, it is left to the CEM system manufacturer or the user to determine the measurement techniques that would be the most appropriate for a given application. This chapter provides a basis for the understanding of several techniques employed in commercially available CEM system analyzers.