Determination of Mercury in Microwave Digests of Foodstuffs by ICP-MS

Significance of the Topic

The accurate measurement of ultratrace mercury in food is critical for public health monitoring due to mercury’s neurotoxic effects and its bioaccumulation in the aquatic food chain. Regulatory limits are stringent, and robust analytical protocols are essential to ensure food safety and compliance.

Objectives and Study Overview

This study demonstrates a method for quantifying mercury in microwave-digested food matrices at the parts-per-trillion level using an Agilent 7500i ICP-MS. Emphasis is placed on minimizing memory effects, achieving system stability over extended runs, and validating performance with certified reference materials.

Methodology and Instruments

Microwave Digestion

• Samples (0.2–0.6 g) underwent wet oxidation in heavy-duty vessels with 10 mL HNO₃ and 0.2 mL HCl, ramped to 180 °C for 10 min.
• Digests were diluted to 100 g with ultrapure water.

Mercury Stabilization and Sample Preparation

• Ten-milliliter aliquots of each digest received 20 µL of 1000 ppm Au(III) solution for a final gold concentration of 5 ppm to prevent mercury adsorption and volatilization.
• Calibration standards (0–100 ppb Hg) and blanks were prepared in 10% HNO₃ containing 5 ppm Au.
• 250 ppb Tl was introduced online as an internal standard.

Used Instrumentation

• ICP-MS: Agilent 7500i with a micro-flow concentric nebulizer and cooled double-pass spray chamber at 2 °C.
• ISIS high-throughput sample introduction with two rotary pumps for rapid sample flush and reduced memory effects.
• RF power: 1400 W; plasma gas: 16 L/min; carrier gas: 0.85 L/min; make-up gas: 0.14 L/min.
• Acquisition: isotopes ^199Hg, ^200Hg, ^201Hg, ^202Hg and ^205Tl; total cycle time 131 s per replicate.

Main Results and Discussion

Calibration over a continuous 36-hour period showed high linearity (R² ≈ 0.998) and consistent sensitivities at 10–30 ppt levels, confirming effective elimination of memory effects. Analysis of 120 diverse food digests yielded stable responses, while 10 evaluations of an external QC standard (1010 ppt) delivered a mean recovery of 106.9% (±8%). Two certified reference materials (LGC 7160 Crab Paste at 0.096 mg/kg and NIST 1547 Peach Leaves at 0.031 mg/kg) analyzed 14 times each over a month produced results within certified uncertainty limits.

Benefits and Practical Applications

• Rapid, high-throughput quantitation of mercury in complex food matrices with minimal sample carryover.
• Low detection limits in the mid-ppt range enable compliance testing and surveillance of trace contaminants.
• Robustness over extended operation reduces downtime and maintenance.

Future Trends and Opportunities

Emerging developments may include coupling ICP-MS with collision/reaction cell technology to further minimize interferences, automation of sample digestion workflows, and advancement of rinse chemistries to reduce gold use. Expanding the approach to speciation analysis of methylmercury could provide deeper insights into dietary exposure risks.

Conclusion

The described protocol using microwave digestion, gold-based stabilization, and high-throughput ICP-MS offers a reliable and efficient solution for ultratrace mercury determination in foodstuffs. Stability over prolonged runs and excellent agreement with certified values demonstrate its suitability for routine quality control and regulatory monitoring.

Reference

Entwisle J. Determination of Mercury in Microwave Digests of Foodstuffs by ICP-MS. Agilent Technologies Application Note, 2004.