Screening and Quantitation of Trace Metals in Milk by using ICP-MS

Significance of the Topic

Milk is a staple dietary component worldwide, providing essential nutrients but also vulnerable to contamination by toxic metals. Ensuring accurate detection of trace metals in milk supports food safety, regulatory compliance and public health protection.

Objectives and Study Overview

This study aimed to develop and validate a robust multi-element screening and quantification method for 18 trace metals in milk. The method combines microwave-assisted closed-vessel acid digestion with inductively coupled plasma mass spectrometry. Validation followed AOAC 2015.01 guidelines and targeted compliance with FSSAI and EC maximum residue limits.

Methodology and Instrumentation

The sample preparation protocol used 2.5 grams of milk digested with a tri-acid mixture of nitric acid, hydrogen peroxide and hydrochloric acid under a controlled microwave program (ramp to 190 oC, hold 15 minutes). After cooling, digests were diluted to 25 mL, spiked with a multi-element internal standard mix (Sc, Ge, Y, In, Tb, Rh, Tl, Ir, Bi at 20 μg/L) and 100 μg/L gold for mercury stabilization. Isopropanol (2%, v/v) improved ionization efficiency.

The analysis employed a Thermo Fisher iCAP-RQ ICP-MS with Qtegra ISDS software. The kinetic energy discrimination mode used helium as cell gas to remove polyatomic interferences. Key operating conditions included:

• RF power 1550 W
• Nebulizer gas flow 1.06 L/min, auxiliary gas 0.8 L/min, cool gas 14 L/min
• Sample uptake/wash 45 s, dwell time 0.03 s, three readings per aspiration
• Total acquisition time 16 s, sampling depth 5 mm

Calibration used five-point curves for each element, achieving R2>0.995. Method validation included spiking for limit of quantitation (0.5–80 μg/L), recovery tests and precision assessments.

Key Results and Discussion

Linearity for all target elements exceeded R2 of 0.999. Limits of quantitation ranged from 0.5 to 40 μg/L depending on the element. Average recoveries fell between 80 and 115 with relative standard deviations below 10. The kinetic energy discrimination mode effectively suppressed polyatomic interferences without compromising sensitivity. The validated method achieved compliance with AOAC 2015.01 performance criteria and met FSSAI and EC maximum residue limits for heavy metals in milk.

Benefits and Practical Applications of the Method

The described approach offers high throughput and reliable quantification of multiple trace metals in a single run. It is suitable for routine food safety laboratories conducting quality control and regulatory monitoring. The use of closed-vessel digestion and helium collision cell ensures robust interference removal and consistent results.

Future Trends and Potential Applications

Advances may include integration with automation for sample preparation, higher throughput autosamplers and hyphenated techniques such as speciation analysis. Emerging portable ICP-MS instruments and data analytics platforms could extend trace metal screening to field and supply-chain checkpoints. Continued tightening of regulatory limits will drive further method optimization and adoption.

Conclusion

A validated microwave digestion and ICP-MS protocol provides a sensitive, selective and precise solution for multi-element trace metal analysis in milk. It meets international performance standards and supports food safety requirements across diverse laboratory settings.

References

1. FAO Infographic on Milk Production Infographics 2019
2. Commission Regulation EC No 1881/2006 on Maximum Levels for Contaminants in Foodstuffs
3. FSSAI Manual for Food Safety, 17th Edition, 2017
4. Briscoe M, Determination of Heavy Metals in Food by ICP-MS, J AOAC Int 98(4):1113-1120, 2015