Species specific isotope dilution calibration for determination of mercury species by gas chromatography coupled to inductively coupled plasma- or furnace atomisation plasma ionisation-mass spectrometry

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Abstract
Standard solutions of (CH3)2Hg, CH3HgCl and HgCl2 were prepared in toluene from isotopically enriched mercury. Methods for synthesising (CH3)2Hg and CH3HgCl, from HgCl2, were developed with reaction yields of 95.2% and 92.7%, respectively, without noticeable loss of isotope. Isotopically enriched species standards were then used for species specific isotope dilution calibration and method assurance tests for mercury species determination. An interface was built to couple a gas chromatograph (GC) to an inductively coupled plasma mass spectrometer (ICP-MS) that resulted in rapid and efficient transfer of organic mercury compounds eluted from natural gas condensates. Oxygen, introduced to the plasma auxiliary gas, reduced matrix interferences and improved sensitivity by a factor of two. The GC-ICP-MS method with species specific isotope dilution calibration gave recovery values of 97.7 ± 2.0, 104.1 ± 0.7 and 106.6 ± 2.1% for (CH3)2Hg, CH3HgCl and HgCl2, respectively, in natural gas condensate compared to a standard solution of the species in toluene. Detection limits of the three species, calculated from 3s of the peak areas derived from 20 pg of the species (as Hg) in solution, were 8, 2 and 3 pg. Furnace atomisation plasma ionisation mass spectrometry (FAPIMS) was also evaluated as a detector with the GC methodology. When the plasma was operated with optimised conditions for the generation of elemental ions, recoveries were 100.0 ± 2.9, 98.3 ± 1.5 and 97.5 ± 1.2% for (CH3)2Hg, CH3HgCl and HgCl2 in a 10% condensate solution, and a detection limit of 33 pg was obtained for methylmercury. A plasma was also generated and sustained with reduced rf power and graphite furnace temperature so that molecular ions and fragments were produced and detected by the same spectrometer, which demonstrated the potential of FAPIMS as a sensitive elemental detector additionally capable of providing molecular mass spectra for the identification of species eluted from a GC column.

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