Mercury speciation in natural gas condensate by gas chromatography-inductively coupled plasma mass spectrometry

Abstract

A simple and rapid GC-ICP-MS speciation method for mercury in natural gas condensate is described. The use of a polar DB-1701 column, pre-treated with HBr, permitted sharp peaks for monoalkylmercury species and also other species without derivatization. Six organomercury species, DMeHg, MeEtHg, DEtHg, MeHgCl, DBuHg and EtHgCl (D=di-), were baseline separated from Hg⁰ and HgCl₂ within 6 min when the pulsed splitless injection mode was used. Separation of Hg⁰ from HgCl₂ was performed using the on-column injection mode. The detection limits were 150 fg for Hg⁰, 340 fg for HgCl₂, 200 fg for DMeHg, 19 fg for MeEtHg, 35 fg for DEtHg, 74 fg for MeHgCl, 50 fg for DBuHg and 36 fg for EtHgCl as Hg for the pulsed splitless injection mode. The detection limits for Hg⁰ and DMeHg were improved to 34 and 130 fg, respectively, with the on-column injection mode. The relative standard deviations of seven replicates of a mixed standard containing approximately 30 µg l^–1 Hg of each species ranged from 1.6 to 3.8% and from 1.6 to 2.7% with the pulsed splitless and on-column injection modes, respectively. Recoveries for DEtHg, MeHgCl and EtHgCl from natural gas condensates ranged between 90 and 103%, but those for Hg⁰, HgCl₂ and DMeHg deviated considerably from 100% owing to overloading by coexisting hydrocarbons. Transalkylation during GC separation did not take place with either injection mode, but decomposition was observed with the pulsed splitless injection mode. The method was applied to natural gas condensates, naphtha fractions and a crude oil sample. Five species, Hg⁰, HgCl₂, DMeHg, MeEtHg and DEtHg, were identified in the real samples, ranging from the sub-ppb to several tens of ppb levels. The existence of MeEtHg in natural gas condensates was identified for the first time.