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PTR-MS applications in food

9 December 2015  •  Author(s): Alex Webbe, Laboratory Manager, Reading Scientific Services Ltd (RSSL) / Donna Byrom, Technical Specialist, Reading Scientific Services Ltd (RSSL)

Mass spectrometry has been a powerful tool used in food analysis for many years. In fact, the technique as we recognise it today is almost 100 years old, having its roots in the work of Dempster (1918) and Aston (1919), and there is no better technique for identifying the wide range of molecules that are found in food (or any other matrix).

PTR-MS applications in food

Hence, mass spectrometry already has many uses in food analysis, such as identifying the trace contaminants that gives rise to taints and offflavours, identifying chemical markers that are relevant to ingredient authenticity/purity, identifying molecules that are important in flavour, or that might be food toxins, and so on.

New developments

In the mass spectrometer, molecules are ionised (given an electric charge), and then identified on the basis of their absolute mass, and on the mass of their fragmentation products when the molecule is broken down into smaller pieces. The combination of mass data from any given molecule, whole and in pieces, is usually characteristic of that molecule and hence will lead to a positive identification. Two molecules may have the same absolute mass, they may even have the same constituent atoms, but when they have a different way of joining these atoms together (structure) the fragmentation products will be different. It is this difference that ultimately allows the original molecules to be distinguished and identified.

Incremental improvements in the technology have produced mass spectrometers that are now much more powerful and more sophisticated than the earliest devices, but the basic principle as described above remains the same.

One of the more interesting developments in recent years has been the introduction of Proton Transfer Reaction-Mass Spectrometry (PTR-MS), a variant not on the way that ionised particles are detected, but on the way they are produced. The PTR-MS technique is more ‘gentle’ in the way it applies a charge to molecules, and hence gives rise to fewer fragments. Whilst this makes PTR-MS less able to distinguish between molecules having the same molecular mass, (an issue that can be mitigated using a Time of Flight MS) it does mean that samples don’t need much preparation and importantly allows for real-time analysis of volatile organic compounds (VOCs) in the air.

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