Speciation analysis of methylmercury via species specific isotope dilution GC-ICP-MS

Importance of the Topic

Mercury is a pervasive environmental contaminant whose toxicity and bioaccumulation depend on its chemical form. Methylmercury (MeHg+) in particular poses significant risks to human health and ecosystems due to its biomagnification in food chains. Accurate speciation at trace and ultra-trace levels is therefore essential for environmental monitoring, risk assessment, and regulatory compliance.

Objectives and Study Overview

This study demonstrates the use of a Thermo Scientific Trace 1310 gas chromatograph coupled via a GCI 200 heated interface to a Thermo Scientific Element 2 high-resolution ICP-MS for species-specific isotope dilution GC-ICP-MS analysis of inorganic mercury and methylmercury. The goal was to validate performance in diverse matrices and establish sensitivity, precision, and accuracy benchmarks.

Methodology and Instrumentation

Instrumentation Used:

• Trace 1310 GC with splitless injector and CP-Sil 5CB column (15 m×0.25 mm, 0.25 µm)
• GCI 200 heated transfer line at 240 °C
• Element 2 HR-ICP-MS with Pt tipped sampler and skimmer, low resolution, RF power 1150 W
• Carrier and transfer gases: Ar, make-up gas as required

Sample Preparation:

1. Spike with Me201Hg+ enriched tracer to achieve 201Hg/202Hg ≈ 1
2. Biological samples: alkaline digestion (25% KOH in MeOH at 60 °C), acidify to pH 3.9, buffer, propylation with Na-tetrapropylborate, extract into n-hexane
3. Water samples: direct buffering, propylation, hexane extraction, optional preconcentration by Ar blow-down
4. Sediment: selective acidic extraction of organic Hg, tracer spike, propylation, hexane extraction
5. GC separation: MeHgPr at 1.90 min, HgPr2 at 2.70 min; data acquisition for isotopes 200Hg, 201Hg, 202Hg

Data Analysis:
Isotope peaks were integrated and corrected for baseline. Concentrations of MeHg+ were calculated via isotope dilution considering sample mass, spike concentration, and measured isotope ratios.

Main Results and Discussion

Isotope ratio accuracy was within 1% deviation for natural ratios; the enriched 202Hg/201Hg ratio showed 2.7% deviation due to low 202Hg abundance. Certified reference material DORM-2 (4.47±0.32 mg/kg MeHg) yielded recoveries ≥98% with RSD ≤0.27% across triplicate injections and 1% across independent digests. Detection in ultrapure water down to 1 ng/L MeHg (1 pg injection) was demonstrated, with peak areas increasing from ~5 000 to ~70 000 for improved precision after evaporation preconcentration. Field water from the Weser River exhibited MeHg <0.1 ng/L with RSD up to 28%. CRM ERM-CC580 sediment (75.5±4 µg/kg MeHg) matched measured 74.9±0.75 µg/kg. Natural sediments showed MeHg levels of 0.6–2.3 µg/kg, representing 0.3–0.9% of total Hg.

Benefits and Practical Applications

This GC-HR-ICP-MS approach delivers rapid (<4 min) separation, species-specific quantification, and isotope verification, outperforming fluorescence-only methods by correcting for recovery losses and detecting side-reactions. It is applicable to diverse environmental and biological matrices for QA/QC, research, and regulatory monitoring.

Future Trends and Potential Applications

Advancements may include on-line microextraction, higher-throughput automation, broader organomercury speciation (e.g., ethylmercury), coupling with miniaturized sample preconcentration, and real-time field deployable systems for comprehensive environmental surveillance.

Conclusion

Species-specific isotope dilution GC-ICP-MS using the Trace 1310 GC, GCI 200 interface, and Element 2 HR-ICP-MS provides robust, accurate, and sensitive methylmercury speciation across varied sample types. Its precision, speed, and detection capabilities make it a gold standard for environmental mercury analysis.

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