Studies of transport and collection characteristics of gaseous mercury in natural gases using amalgamation and isotope dilution analysis

Abstract

Transport and collection characteristics were studied for gaseous elemental mercury (Hg⁰(g)) in natural gases using newly developed methodology based on amalgamation, isotope dilution with permeation tubes and inductively coupled plasma mass spectrometry. The study involved different Au–Pt collection tube designs, tubing materials and gaseous matrices, including air, natural and sales gas, as well as methane and sales gas to which hydrogen sulfide (H₂S) had been added. The Hg⁰(g) capacity of the Au–Pt tubes was determined to 3.5 ± 0.1 µg. Blanks and detection limits of gaseous mercury (Hg(g)) were 58 ± 17 pg m⁻³ and 50 pg m⁻³, respectively, for a 60 L sample volume. For the gases tested, added Hg⁰(g) tracers could be collected with 90% or higher efficiency at flow rates and volumes of up to 10 L min⁻¹ and 100 L, respectively. The collection efficiency was found to be independent of the type of gas tested, even in the presence of H₂S. However, for the gases containing H₂S, the apparent transport efficiency of added Hg⁰(g) tracers through stainless steel tubing varied from 50 to 150% upon changing the temperature from 25 to 100 °C. The interaction of stainless steel with Hg⁰(g) leading to either a sink, or source of Hg, was not observed in the absence of H₂S, nor was it observed for PTFE tubing in the presence of H₂S. These observations raise questions about the applicability of currently used sampling procedures for determination of Hg(g) in H₂S rich natural gases, including the 6978-2 ISO standard method, in which stainless steel is a prescribed material for tubing and valves of the sampling apparatus.