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Ultratrace determination of methylmercury in seawater using automated GC-AFS


Methylmercury (MHg) is the most toxic form of mercury present in the environment, is bioaccumulated in the aquatic food web and thereby provides in general a major exposure route for humans through consumption of seafood. Effective marine monitoring is necessary for human risk assessment which is based on reliable analytical methodology. Since MHg in seawater is the starting point of the process of bioaccumulation and biomagnification over several orders of magnitude, its determination at ultratrace levels calls for a highly sensitive and selective procedure.

Most of the applied techniques used for such analysis are based on gas chromatography (GC) coupled with either Atomic Fluorescence Spectrometry (AFS) or Inductively coupled Plasma-Mass Spectrometry (ICP-MS). While these techniques are highly sensitive, the ultratrace levels of methylmercury still call for preconcentration often using large sample volumes that are not easy to handle by on-line mode or to combine them with automated instrumental systems. However, such automated systems would greatly support higher sample throughput required for large scale monitoring studies.

Photo: Location of the IAEA Marine Environment Laboratories at Port Hercule, Monaco (outlined in red) [© IAEA]

The new study:
In view of these challenges, researchers from the IAEA Environment Laboratories in Monaco have developed, optimized and validated a method for the determination of femtomolar MeHg levels in seawater. The proposed method provides significant reduction of sample preparation time, reduction of manual handling and therefore is very suitable for the analysis of numerous samples.  The first step of the sample preparation consists of liquid-liquid extraction of MeHg from 200 g acidified seawater with dichloromethane (DCM). MeHg was then backextracted into aqueous Na2S2O3 solution. For the derivatisation of Hg species 50 μL of freshly thawed sodium tetra-ethyl borate was added to the extract, the vial filled up with the Milli-Q water. Vials were quickly closed, vigorously shacked by hand, and placed in the autosampler for analysis.

The method is using a dedicated MeHg analyser performing automatically the following steps: in line loading and preconcentration of mercury species on Tenax® traps, followed by GC separation of derivatized volatile mercury species on a packed column, pyrolysis and detection with AFS. Quantification of the method is based on external calibration. The calibration curve was built up with 5 standards over the concentration range from 0.2 to 7 pg absolute amount of MMHg.

The performance of the optimised procedure was evaluated  with respect to selectivity; trueness (recovery), repeatability and within-laboratory reproducibility, determination limits (LOD) and quantification limits (LoQs), working range, uncertainty and traceability of measurement results. Also possible sources of measurement uncertainty were carefully identified and combined according to the recommendations of the ISO GUM guidelines.

The use of the optimised extraction procedure together with the automated analyser provided improved sample throughput up to 72 samples per day. The limits of quantification were 0.17 pg or 1.4 pg kg -1. The relative expanded uncertainties obtained were in the range from 16% to 25%, (k = 2). The potential of this analytical procedure was tested and additionally validated via inter laboratory comparison exercise organised under the Geotraces programme. Obtained results were in excellent agreement with the assigned values.

The original publication

Sabine Azemard, Emilia Vassileva, Rapid determination of femtomolar methylmercury in seawater using automated GC-AFS method: Optimisation of the extraction step and method validation, Talanta, 232 (2021) 122492. DOI: 10.1016/j.talanta.2021.122492

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last time modified: July 8, 2021

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