Measuring Ultra-Trace Levels of Mercury

Significance of the Topic

Mercury remains widely used in industry and dentistry despite gradual replacement in some processes. Its high toxicity necessitates monitoring at ultra-trace levels in environmental and drinking water samples to meet tightening regulations that now demand detection at 0.05 ug/L or below.

Objectives and Study Overview

This study demonstrates the application of a mercury concentration accessory for atomic absorption spectrometry to achieve detection limits down to 0.01 ug Hg per liter. The goals include assessing signal enhancement via gold amalgamation, optimizing operational parameters, and validating method performance through recovery experiments.

Methodology and Instrumentation

A SpectrAA-400P atomic absorption spectrometer equipped with a VGA-76 cold vapor generator and the MCA-90 accessory was used. Mercury vapor is trapped on a gold-plated mercury amalgamation trap within a quartz tube, isolated by a rapid isolation valve, and thermally released for analysis. Key steps include a delay phase to stabilize vapor flow, a collection phase of defined duration, and a heating phase to desorb mercury. Sample preparation involves reduction with tin(II) chloride in acid and stabilization with nitric acid and potassium dichromate. Instrument parameters such as delay time (typically 25 s), collect time (up to 600 s), and heat time (around 40 s) are optimized for sensitivity and reproducibility.

Main Results and Discussion

Signal enhancement via amalgamation allows detection limits of 0.01 ug/L or lower. Calibration over 100 to 300 ng/L is linear, and extended collection times (e.g. 600 s) yield peak heights of about 1 absorbance unit with RSD below 5%. A drinking water recovery study at 200 ng/L spike level produced mean recoveries of 87.9% with RSD around 2%, meeting EPA guidelines. Method precision and memory effects are managed by thorough system conditioning and occasional trap cleaning.

Benefits and Practical Applications

• Sub-parts-per-trillion detection capability for mercury
• Automated control of trapping and desorption cycles
• High reproducibility with minimal memory effects
• Compliance with stringent regulatory limits for water quality

Future Trends and Opportunities

Advances may include integration of on-line real-time monitoring, development of portable field-deployable cold vapor systems, novel trap materials to further reduce memory effects, and coupling with data analytics for predictive maintenance and process control.

Conclusion

The MCA-90 accessory provides a robust solution for ultra-trace mercury quantification by atomic absorption, achieving regulatory detection limits while ensuring precision and reliability. Strict cleanliness protocols remain essential to control reagent and system contamination.

References

1. Department of Health NHMRC Desirable Quality for Drinking Water in Australia 1980
2. American Public Health Association Standard Methods for the Examination of Water and Wastewater 16th Edition 1985 p 143