Determination of Mercury With On-line Addition of Stannous Chloride Using an Axial ICP-OES

Significance of the Topic

Mercury is among the most toxic heavy metals in the environment, posing serious risks to human health and ecosystems. Reliable monitoring of mercury at sub-microgram per liter levels is critical for water quality assessment across drinking, surface, ground, sea, and industrial waste streams. Conventional direct aspiration ICP-OES methods lack sufficient sensitivity, while established cold vapor techniques impose additional hardware requirements

Objectives and Study Overview

This application note demonstrates a straightforward approach to enhance mercury detection in the microgram-per-liter range by on-line addition of stannous chloride (SnCl₂) as a reductant, using an axially-viewing ICP-OES system. The goal is to achieve significant signal intensification and lower detection limits without the need for a dedicated vapor generation accessory

Instrumentation Used

• Agilent 720-ES axial-viewing ICP-OES with Echelle polychromator and CCD detector
• Four-channel peristaltic pump and mass flow controller for sample and reductant delivery
• Concentric glass nebulizer with cyclonic spray chamber and Y-piece mixer
• Agilent ICP Expert II software for instrument control and data acquisition

Methodology

Sample and stannous chloride solutions were pumped separately and combined via a Y-piece immediately before nebulization. Operating conditions included 1.2 kW RF power, 15 L/min plasma gas, 1.5 L/min auxiliary gas, and 0.65 L/min nebulizer flow. SnCl₂ solutions were prepared at 15 % (w/v) in 20 % HCl to prevent hydrolysis and oxidation. Calibration curves were constructed using aqueous mercury standards and NIST 1641D certified reference material diluted in 2 % v/v HNO₃

Results and Discussion

On-line addition of SnCl₂ produced more than a 30-fold increase in mercury signal intensity at emission lines 184.887 nm, 194.164 nm, and 253.652 nm. Signal enhancement plateaus at approximately 10–15 % SnCl₂, balancing maximized reduction with minimal Sn spectral interference. Detection limits improved by roughly an order of magnitude, dropping from 2–3 µg/L without reductant to 0.2–0.4 µg/L with SnCl₂. Validation against NIST 1641D yielded results within certified uncertainty

Benefits and Practical Applications

• Enhanced sensitivity in the µg/L range without specialized vapor generation hardware
• Simple retrofit to existing axial ICP-OES systems
• Rapid sample throughput with on-line reagent addition
• Applicable to a variety of water matrices, including drinking, surface, and industrial waste waters

Future Trends and Potential Applications

Integration of on-line reductant addition with automated sampling and data processing platforms could further streamline trace mercury monitoring. Coupling with multi-element speciation techniques and development of portable ICP-OES systems may expand field applications. Advances in reagent delivery and interferent correction algorithms will enhance robustness for complex matrices

Conclusion

On-line addition of stannous chloride to an axially-viewing ICP-OES offers a cost-effective and user-friendly method for mercury determination at microgram-per-liter levels. Signal enhancement of over 30× and detection limits near 0.2 µg/L demonstrate performance approaching dedicated vapor generation techniques, with straightforward implementation

References

• US EPA Method 245.1, "Determination of Mercury in Water and Wastes by Cold Vapor Atomic Absorption" (1979).
• P. Doidge, "Determination of Mercury in a Certified Reference Sludge Material Using the Varian 710-ES," Agilent Application Note.
• NIST SRM 1641D, Mercury in Water Standard Reference Material.