Determination of Methyl Mercury in Water and Soil by HPLC-ICP-MS

Significance of the Topic

Mercury species such as methylmercury (MeHg) pose severe environmental and health risks due to their high toxicity and bioaccumulation potential.
The ability to separate and quantify inorganic and organic mercury directly in water and soil informs risk assessment and regulatory compliance.

Objectives and Study Overview

This study aimed to develop a sensitive, specific method for determining MeHg, ethylmercury (EtHg), and inorganic Hg2+ in water and soil.
The approach combined high-performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometry (ICP-MS) to achieve low detection limits and robust speciation analysis.

Methodology and Used Instrumentation

Sample preparation:

• Water: direct injection after minimal dilution; detection limit better than 10 ng/L.
• Soil: extraction with 7.6% HCl and 0.1% 2-mercaptoethanol, ultrasonic agitation, centrifugation, pH adjustment, and filtration.

Chromatographic separation:

• Column: ZORBAX Eclipse XDB-C18 (2.1 × 50 mm, 5 µm).
• Mobile phase: 0.06 M ammonium acetate, 5% methanol, 0.1% 2-mercaptoethanol, pH 6.8 at 0.4 mL/min.
• Injection volumes: 20 µL (standard ≤ 100 ng/L), 100 µL (low-level).

ICP-MS detection:

• Agilent 7500a, RF power 1550 W; nebulizer flow 0.75 L/min; chilled spray chamber; one-piece Fassel torch.
• Argon plasma enables compound-independent calibration for Hg detection.

Main Results and Discussion

Chromatographic performance:

• Retention times: MeHg ~2.5 min; Hg2+ ~3.2 min; EtHg ~6.5 min.
• Good peak resolution and minimal drift (ISTD RSD <5% over 10 h).

Analytical figures:

• Detection limits <10 ng/L for all species in ultrapure water; potential for sub-ng/L with higher reagent purity.
• Linear dynamic range over four orders of magnitude (10 ng/L to 100 µg/L) with R² >0.998.
• High-matrix tolerance: recoveries 90–110% in 3% NaCl.
• Soil extracts spiked at 90 pg (Hg) yielded recoveries between 80% and 120%.

Benefits and Practical Applications of the Method

The HPLC-ICP-MS technique offers:

• Element-specific detection with minimal interferences compared to GC-ECD methods.
• Simplified sample preparation for water and soil with robust recoveries.
• Capability for direct analysis of environmental samples without extensive preconcentration.

This method supports routine monitoring of mercury species in environmental water, wastewater, and contaminated soils.

Future Trends and Applications

Advances may include:

• Integration of automated sample preparation workflows to increase throughput.
• Improved ICP-MS sensitivity via collision/reaction cell technologies for sub-ng/L detection.
• Application to broader matrices such as biological tissues or sediments.
• Coupling with high-resolution mass spectrometry for identification of unknown organomercury species.

Conclusion

The combined HPLC-ICP-MS method delivers rapid, accurate speciation of MeHg, EtHg, and Hg2+ in water and soil.
It meets stringent detection limits and regulatory criteria while requiring straightforward sample handling.
This technique is well suited for environmental monitoring, risk assessment, and regulatory compliance.

References

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• National Academy of Sciences, An Assessment of Mercury in the Environment, NRC, 1978.
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• Felton J. S. et al., Heavy metal poisoning: mercury and lead, Ann. Intern. Med., 1972, 76, 779–792.
• GB 18918-2002, Discharge standard of pollutants for municipal wastewater treatment plant of China.
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