Sensitive determination of methylmercury δ13C compound specific stable isotopic analysis by purge and trap gas chromatography combustion isotope ratio mass spectrometry

Abstract

Despite several decades of mercury research, answering fundamental questions on where and how methylmercury (CH₃Hg) toxin is naturally produced in aquatic ecosystems, is still highly challenging. Investigating complex and/or coupled processes in the context of global changes requires new high-resolution analytical tools. The purpose of the compound specific carbon stable isotopic analysis (δ¹³C-CSIA) of the methyl group of methylmercury (CH₃Hg), is to explore how the carbon cycle contributes to CH₃Hg sources and formation pathways. The main problem associated with recent CH₃Hg δ¹³C-CSIA methods is the limited sensitivity when using Liquid Injection (LI)-GC-C-IRMS techniques, requiring several micrograms of CH₃Hg (as Hg). In this work, we present the development and application of an original Purge-&-Trap system (PT) coupled to a GC-C-IRMS with the purpose of transferring and analyzing the total amount of CH₃Hg available in a sample vial in the low nanogram range. The new PT-GC-C-IRMS system enhance the sensitivity by a factor better than 200, relative to LI-GC-C-IRMS, by minimizing the sample mass requirements. The δ¹³C_CH3Hg values obtained, following the same sample derivatization approach coupled to PT-GC-C-IRMS (-53.5 ± 1.9 ‰), were in good agreement with the ones obtained in a previous study (-53.8 ± 1.1 ‰). The standard solution was prepared from the same salt, requesting only 25-200 ng of CH₃Hg (as Hg). This new methodology represents a milestone towards the analysis of large array of biological samples displaying CH₃Hg concentrations in the low-mid ng g^-1 range, in order to explore the meaning of the carbon stable isotopic signature of CH₃Hg in the environment.

Keywords: Carbon isotopes; Isotope ratio mass spectrometry; Methylmercury; Purge and trap.