16 - Comparison of Primary Laser Spectroscopy and Mass Spectrometry Method for Gaseous Elemental Mercury Concentration

"Metrological traceability is an important element of regulatory compliance. For gaseous elemental mercury, Hg0 (GEM), Environmental Protection Agency (EPA) 2012 MATS (mercury and air toxic standards) promulgation sets the regulatory guideline in USA. NIST maintains NIST Prime Hg generators as part of its mercury traceability program for GEM. The low and high NIST prime generators, (0.2 to 39 and 40-300) ug/m3, provide an International Systems of Units (SI) traceable method to certify vendor Hg generators. Internally, NIST attains metrological traceability of the NIST prime generators against a MS (mass spectrometry) based method, traceable to the SI via the Hg NIST SRM (standard reference material, NIST SRM 3133).

Recently, Srivastava and Hodges (2018) developed a frequency resolved laser absorption spectroscopy (LAS) primary method to determine the absolute concentration of GEM in air. By applying well-established light-matter interaction equations, spectral line shape parameters, and the Hg probe 253.7 nm transition line intensity, the measured spectra is converted to a GEM SI concentration value. This method provides several advantages over the MS based method: 1) calibration-material free, 2) high throughput, 3) low uncertainty, 4) active continuous measurement and 4) isotope resolution to list a few.

In this study we compare the two primary methods. Over the examined mass concentration range (41 ug/m3 to 287 ug/m3 Hg0 in air ) the methods are shown to agree within 1.0% to 1.8%. The relative combined standard uncertainty on average is 0.4% and 0.9% for the LAS and MS methods, respectively. Our comparison studies help validate the accuracy of the ID-CV-ICP-MS (isotope dilution-cold vapor-inductively coupled plasma-mass spectrometry) primary method as well as establish the LAS technique as a rapid calibration-free alternative primary method for SI-traceable GEM concentration measurements. Key results covering various aspects of the above description will be presented."

Authors: Abneesh Srivastava, Stephen Long, James Norris, Colleen Bryan, Jennifer Carney, Joseph Hodges