Comparison of Primary Laser Spectroscopy and Mass Spectrometry Methods for Measuring Mass Concentration of Gaseous Elemental Mercury

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 mercury gas standards traceability
to the International System of Units (SI). 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 intercombination
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 μg/m3 to 287 μg/m3 Hg0 in air), the
relative disagreement between the two approaches ranged from (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 primary method
as well as establish the LAS technique as an attractive alternative
primary method for SI-traceable measurements of GEM.