Provision of primary NIST traceability to support vapor phase mercury emissions monitoring of combustion sources using isotope dilution inductively coupled plasma mass spectrometry

Primary mercury gas standards, traceable to the SI, have been developed which can be used to underpin a NIST traceability framework for the calibration of vapor phase continuous emission monitoring systems (CEMS). A gas standard, which is based on a commercial gas generator, has been certified for gaseous elemental mercury output at specified generator output set points. The standard, termed the NIST Prime (NP) was certified at selected set points in the range 0.25 μg/m3 to 38 μg/m3 by isotope dilution inductively coupled plasma – mass spectrometry (ID-ICP-MS) using two alternative approaches, based on directly coupled gas sampling, and activated carbon sorbent trapping. Both approaches have been used to provide NIST traceability for electric utility mercury CEMS. The directly coupled method yielded expanded measurement uncertainties ranging from approximately 5.5% relative at 0.5 μg/m3, to approximately 1% relative at 38 μg/m3, and an estimated limit of quantitation (LOQ) of 0.06 μg/m3. The sorbent trapping method in contrast, yielded expanded uncertainties of approximately 1% relative at all sampling points, with an LOQ of 0.001 μg/m3.