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

Significance of the topic

Mercury contamination in foodstuffs poses a significant risk due to its neurotoxicity and bioaccumulation in ecosystems. Reliable ultratrace analysis is essential for regulatory compliance, public health protection, and environmental monitoring.

Objectives and overview

This study aims to establish a robust method for quantifying mercury in microwave-digested food samples using Agilent 7500i ICP-MS. Key goals include minimizing memory effects, achieving low detection limits, and verifying accuracy against certified reference materials over extended analytical runs.

Methodology and instrumentation

The procedure combines wet microwave digestion with advanced ICP-MS techniques:

• Sample preparation: 0.2–0.6 g of food digested with ultra-pure HNO3 and HCl in a high-pressure microwave system (ramped to 180 °C, held 10 min).
• Memory suppression: Offline addition of Au(III) at 5 ppm to all standards, samples, and wash solutions to prevent mercury adsorption and volatilization.
• ICP-MS setup: Agilent 7500i equipped with two rotary pumps, high-throughput ISIS, micro-flow concentric nebulizer, cooled glass spray chamber (2 °C), and a second vacuum pump for improved interface stability.
• Internal standard: Online addition of Tl (250 ppb) via the ISIS system; 1:20 dilution factor.
• Operating parameters: Plasma gas 16 L/min, carrier gas 0.85 L/min, make-up flow 0.14 L/min, RF power 1400 W, sample tubing 0.89 mm ID.

Acquisition targeted Hg isotopes (199, 200, 201, 202) and Tl with appropriate dwell times, summing responses to enhance sensitivity.

Main results and discussion

• Detection limits in digests: 10–30 ppt for mercury due to efficient wash-out and memory suppression.
• Linearity and stability: Five calibration checks over 36 h showed consistent response and R² ≈ 0.998.
• Quality control: Aquacheck standard recovery 106.9% ± 8% (n = 10), confirming precision.
• Certified reference materials: LGC 7160 (crab paste, 0.096 mg/kg) and NIST 1547 (peach leaves, 0.031 mg/kg) analyzed 14 times over one month, consistently matching certified values.
• Throughput: Over 500 microwave digests analyzed in continuous runs exceeding 36 h without sensitivity loss.

Benefits and practical applications

• Rapid wash-out and minimal carry-over thanks to Au(III) and ISIS high-throughput mode.
• Low ppt detection limits suitable for stringent regulatory requirements.
• Extended linear calibration range reduces need for re-analysis and dilutions.
• Robustness supports high sample loads in routine food safety laboratories.

Future trends and possibilities

• Integration of online gold addition and automation for further throughput gains.
• Expansion to mercury speciation analysis via hyphenation with chromatography.
• Development of miniaturized and field-deployable ICP-MS systems for rapid on-site monitoring.
• Application to a broader range of environmental and biological matrices.

Conclusion

The described method using Agilent 7500i ICP-MS with Au(III) memory suppression and ISIS high-throughput sample introduction demonstrates reliable, low-level mercury quantification in food digests. It delivers robust stability, excellent accuracy against certified materials, and efficient workflow for routine monitoring.

References

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