Determination of Melamine in Milk by Ion Chromatography with UV Detection

Importance of the Topic

The detection of melamine in milk and dairy products is crucial for consumer safety and regulatory compliance. Melamine adulteration can falsely elevate protein measurements and has led to serious health incidents. A reliable, sensitive, and selective analytical method is needed to screen liquid milk, powdered milk and milk-based foods.

Objectives and Study Overview

This study aimed to develop and validate an ion chromatography method with ultraviolet detection for quantifying melamine in various dairy matrices. The approach leverages a cation-exchange column, an inline concentrator, and a regenerating suppressor to achieve low detection limits and robust performance. Sample preparation, calibration strategy and method precision were evaluated for liquid milk, milk powder and a milk-based candy.

Methodology and Instrumentation

Equipment and Chromatographic Setup

• Dionex ICS-3000 system with dual pump and dual temperature detector module
• AM Automation Manager with high-pressure injection valve and sample trap concentrator (IonPac TCC-LP1, 4 x 35 mm)
• IonPac CS17 analytical column (4 x 250 mm) with CG17 guard column (4 x 50 mm)
• Eluent generator EGC II MSA with CR-CTC cartridge, delivering 10–30 mM methanesulfonic acid gradient (0–10 min) at 1.0 mL/min
• UV detection at 240 nm; injection volumes of 10 µL for milk samples and 100 µL for candy analysis

Sample Preparation

• Liquid milk: dilute with water, acidify with 3% acetic acid, filter and clean up on OnGuard RP cartridge
• Milk powder and candy: dissolve sample in water, dilute, filter and apply same cleanup procedure
• Calibration standards prepared from 1 mg/L secondary stock over 12.5–800 µg/L range

Main Results and Discussion

Calibration of melamine was linear (r2 > 0.9997) over 12.5–800 µg/L. The method detection limit was estimated at 4.4 µg/L based on seven replicate injections of 25 µg/L. Analysis of unspiked liquid milk and milk powder showed no detectable melamine. Spiked recoveries exceeded 90% with RSD below 10% for both matrices. A milk-based candy sample contained 13.8 µg/L in solution, corresponding to 0.27 µg/g in the solid; spiked recoveries were 88–91%. For the candy, coeluting peaks were resolved by switching to an isocratic 5 mM MSA eluent for 20 minutes without manual eluent preparation using the RFIC system.

Benefits and Practical Applications

This ion chromatography approach offers

• High selectivity for cationic melamine in complex dairy matrices
• Low detection limits and accurate quantitation with minimal matrix interference
• Automated eluent generation for reproducibility and ease of use
• Flexible method adjustment via software without chemical preparation

Future Trends and Applications

Advances may include coupling with mass spectrometry for enhanced specificity, further miniaturization of inline concentrators, expanded multi-analyte capabilities, and integration of sample prep modules for streamlined workflows in food safety laboratories.

Conclusion

A robust RFIC-UV method was established for melamine determination in milk, milk powder and milk-based candy. The protocol demonstrated low detection limits, high recovery, and adaptability to different sample types. Automated eluent generation and inline cleanup enhance throughput and reliability for routine screening of dairy products.

References

• Rapid Determination of Melamine in Liquid Milk and Milk Powder by HPLC on the Acclaim Mixed-Mode WCX-1 Column with UV Detection Application Note 221 Dionex Corporation 2009
• Determination of Melamine in Milk Powder by Reversed-Phase HPLC with UV Detection Application Note 224 Dionex Corporation 2009
• ICS-3000 Ion Chromatography System Operator’s Manual Document No 065031-03 Dionex Corporation
• Determination of Iodide in Milk Products Application Note 37 Dionex Corporation 2004