Comparison of primary laser spectroscopy and mass spectrometry methods for measuring mass concentration of gaseous elemental mercury
Abneesh Srivastava, Stephen E. Long, James E. Norris, Colleen E. Bryan Sallee, Jennifer Carney, Joseph T. Hodges
We present a direct comparison between two independent methods for the measurement of gaseous elemental mercury (GEM) mass concentration: isotope dilution cold-vapor inductively coupled plasma mass spectrometry (ID-CV-ICP-MS) and laser absorption spectroscopy (LAS). The former technique combined with passive sorbent tube sampling is currently the primary method at NIST for SI- traceable mercury gas standards. This traceability is achieved via measurements on a mercury- containing reference material. The latter technique has been recently developed at NIST and involves real-time measurements of light attenuation caused by GEM, with SI traceability based in part on the known spontaneous emission lifetime of the probed 6 1S0 → 6 3P1 inter- combination transition of elemental mercury (Hg0). Using a steady-flow Hg0-in-air generator to produce samples measured by both methods, we use LAS to measure the sample gas and in parallel we collect the Hg0 on sorbent tubes to be subsequently analyzed using ID-CV-ICP-MS. Over the examined mass concentration range (41 υg/m3 to 287 υg/m3 Hg0 in air) the relative disagreement between the two approaches ranged from 1.0% to 1.8%, which is (1.0 to 1.9) times the relative combined standard uncertainty for both methods. We discuss the comparison method and results, and we conclude that the LAS technique is an attractive alternative primary method for SI- traceable measurements of GEM.