Direct analysis of milk using the Agilent 4100 Microwave Plasma-Atomic Emission Spectrometer (MP-AES)

Significance of the Topic

Accurate determination of trace and major elements in milk is critical for food safety, nutritional labeling and quality control. Milk’s complex colloidal structure—comprising fat emulsions, casein micelles and aqueous phases—poses analytical challenges due to matrix interferences, poor nebulization and atomization inefficiencies.

Objectives and Study Overview

This application note presents a rapid, direct method for quantifying Al, Cr, Cu, Fe, Mg, Mn and Zn in non-fat liquid milk and milk powder reference material using the Agilent 4100 Microwave Plasma-Atomic Emission Spectrometer (MP-AES). The study evaluates detection limits, accuracy and precision under simple dilution conditions without extensive sample digestion.

Methodology and Instrumentation

Sample Preparation:

• Milk samples diluted 1:20 v/v in an aqueous solution containing 10 % v/v tertiary amine mixture (CFA-C), pH adjusted to 8.0.
• Non-fat milk powder (NIST SRM 1549) dissolved in the same diluent; aliquots vortexed to clarity.

Instrumental Setup:

• Agilent 4100 MP-AES with nitrogen plasma supplied by compressor or on-site generator.
• OneNeb nebulizer featuring Flow Blurring technology coupled to a double-pass cyclonic spray chamber.
• MP Expert software for automatic background correction, optimization of nebulizer pressure, viewing position and acquisition parameters.
• Key operating parameters: reading time 5 s, three replicates, stabilization time 15 s, auto background subtraction.
• Selected analytical wavelengths ranged from 213.857 nm (Zn) to 396.152 nm (Al).

Main Results and Discussion

Detection Limits and Sensitivity:

• Instrumental LODs in solution between 0.4 and 76 µg/L, LOQs from 1.5 to 250 µg/L.
• Considering sample dilution, LODs in milk ranged from 8 µg/L (Fe) to 1500 µg/L (Mg).
• Performance comparable to ICP-OES and markedly superior to FAAS, especially for Al and Cr without oxidizing gases.

Accuracy and Precision:

• Recoveries in milk powder reference material: 100 % for Al, 108 % for Mg using external calibration.
• Other elements below LOD in reference but spiked liquid milk achieved 90–112 % recoveries via external calibration or method of standard additions (required for Al, Cr, Mn, Zn to compensate matrix effects).

Matrix Considerations:

• Tertiary amine diluent effectively dissociates casein micelles and stabilizes cations in the aqueous phase.
• Flow Blurring nebulization reduces droplet size distribution, improving precision and reducing matrix suppression.

Benefits and Practical Applications

• Minimal sample preparation—direct dilution in tertiary amine buffer.
• Lower operational and maintenance costs using nitrogen plasma from an air compressor or generator.
• High throughput with sequential multi-element determination under optimized conditions.
• Suitable for routine QA/QC in dairy processing, regulatory compliance and nutritional labeling.

Future Trends and Potential Applications

Expanding the use of MP-AES with matrix-tolerant nebulization could enable direct analysis of other complex foodstuffs (juices, wines, plant extracts). Integration with automated sample handling and on-line calibration methods will further streamline multi-element profiling. Adoption in decentralized or resource-limited laboratories is facilitated by lower gas and consumable requirements.

Conclusion

The combination of simple dilution in tertiary amine buffer, OneNeb Flow Blurring nebulization and Agilent 4100 MP-AES provides a robust, cost-effective approach for accurate multi-element analysis of milk. Detection limits rival ICP-OES and surpass FAAS, with recoveries within 90–110 % for key trace metals, supporting its implementation in routine food testing workflows.

References

1. R. G. Jensen, Handbook of Milk Composition, Academic Press, London, 1995.
2. P. Pohl and B. Prusisz, Determination of Ca, Mg, Fe and Zn partitioning in UHT cow milks by two-column ion exchange and flame atomic absorption spectrometry, Talanta 71 (2007) 715–721.
3. S. E. Emmett, Analysis of liquid milk by inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 3 (1988) 1145–1146.
4. E. Coni, A. Stacchini, S. Caroli and P. Falconieri, Analytical approach to obtaining reference values for minor and trace elements in human milk, J. Anal. At. Spectrom. 5 (1990) 581–586.
5. J. A. Nóbrega et al., Direct determination of major and trace elements in milk by ICP-AES and ICP-MS, J. Anal. At. Spectrom. 12 (1997) 1243–1246.
6. C. C. Nascentes et al., Direct determination of Cu and Zn in fruit juices and bovine milk by thermospray flame furnace AAS, Talanta 64 (2004) 912–917.
7. A. M. Gañán-Calvo, Enhanced liquid atomization: from flow-focusing to Flow Blurring, Appl. Phys. Lett. 86 (2005) 214101.
8. M. A. Aguirre et al., Compensation for matrix effects on ICP-OES by on-line calibration methods using a new multi-nebulizer based on Flow Blurring, J. Anal. At. Spectrom. 25 (2010) 1724–1732.