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Dairy detection: monitoring melamine in milk


12 December 2008

Two leading groups of mass spectrometrists have applied their expertise to improve melamine detection in milk. 

They were responding to the demand for a simple, fast and cheap melamine detection technique after the industrial chemical was found to be present in Chinese milk in September 2008. Tainted milk powders were blamed for the deaths of four babies, and for illnesses affecting tens of thousands of infants.

Melamine, commonly used as a fire retardant and a plastic resin, was added to milk during processing to artificially boost its apparent protein content as assayed by total nitrogen content analysis. 

 

Infant drinking milk

Tainted milk powders were blamed for the deaths of four babies, and for illnesses affecting tens of thousands of infants

 

The two new techniques share the advantages of being highly specific, accurate, simple and quick. Both use ambient ionisation - the samples are ionised in their native environment. This means they have potential to be developed into a portable detection kit for use in product quality control. The two group's techniques differ in the details of the sample ionisation. 

"Ultrasonic nebulisation for EESI sample delivery is extremely simple and extremely rapid"
- Renato Zenobi
Renato Zenobi, ETH Zurich, Switzerland, and colleagues used ultrasound to turn the melamine-spiked liquid milk samples into a fine spray (nebulise). The spray was then ionised by extractive electrospray ionisation (EESI) and analysed using tandem mass spectrometry.1 The method is said to take 30 seconds per sample allowing a high sample throughput. The lower limit of detection of melamine is in the range of a few nanograms of melamine per gram of milk. 

Zenobi comments on his technique saying 'ultrasonic nebulisation for EESI sample delivery is extremely simple and extremely rapid, while maintaining a reasonable sensitivity.'

Graham Cooks, Purdue University, West Lafayette, US, and his coworkers used a low temperature plasma probe to ionise their samples and, using the same type of mass spectrometry, achieved a similar speed and limit of detection. The detection limits seen by both groups are well below the minimum level at which melamine becomes toxic to humans. 

"We took it as a challenge to use simpler instrumentation and develop a faster method"
- Graham Cooks

Cooks says that the existing technique for melamine determination is comparatively complex, 'the newspapers carried extensive discussions on the melamine tampering scandal and reported on the accepted triple quadrupole liquid chromatography-mass spectrometry methodology for its detection. We took it as a challenge to use simpler instrumentation and develop a faster method based on ambient ionisation.'

David Muddiman, professor of mass spectrometry at North Carolina State University, Raleigh, US, describes the techniques as 'marvellous examples of how innovative, direct analysis ionisation methods, when coupled with mass spectrometry have the ability to address contemporary problems facing the world. They have removed all the major obstacles allowing for mass spectrometry not only to compete, but to take the lead in these types of analyses.'

James Hodge

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References

1. L Zhu, G Gamez, H Chen, K Chingin, and R Zenobi, Chem. Commun., 2009, DOI: 10.1039/B818541G
2. G Huang, Z Ouyang and RG Cooks, Chem. Commun., 2009, DOI: 10.1039/B818059H

Link to journal article

Rapid detection of melamine in untreated milk and wheat gluten by ultrasound-assisted extractive electrospray ionization mass spectrometry (EESI-MS)
Liang Zhu, Gerardo Gamez, Huanwen Chen, Konstantin Chingin and Renato Zenobi, Chem. Commun., 2009, 559
DOI: 10.1039/b818541g

High-throughput trace melamine analysis in complex mixtures
Guangming Huang, Zheng Ouyang and R. Graham Cooks, Chem. Commun., 2009, 556
DOI: 10.1039/b818059h

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