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Towards reliable detection of nanoparticles in food

Samples used for the intercomparison on dry ice
©EU, 2017
Oct 12 2017

The JRC,  together with  scientists from Dutch, German and Danish organisations, organised an interlaboratory study to investigate the readiness of single-particle ICP-MS for the determination of the median diameter of silver nanoparticles in food. The study revealed that significant improvements are needed before the method can be reliably used for official control purposes.

Nanoparticles are particles with at least one dimension in the range of 1 – 100 nanometres (1 billionth of a metre). These materials may have different properties from the same substance in bulk: for example, bulk gold is shiny yellow, whereas gold in its nano-form is of a bright red which was already used in medieval times to colour glass. As these different properties may also lead to different interactions with humans or the environment, nanomaterials have come under intense scrutiny over the recent years. To respond to perceived consumer concerns, the EU mandated that producers must inform consumers if their food or cosmetic products contain substances in their nano-form (Regulations 1223/2009/EU and 1169/2011/EU).

Accurate measurements are necessary to confirm the correctness of such labelling. Single-particle inductively-coupled plasma mass spectrometry (sp-ICP-MS) is a method that shows promise not only to detect nanoparticles, but also to determine their size number concentration (number of particles per g of material).

To assess the capability of the method, JRC scientists, in collaboration with the Dutch Wageningen University and the Danish Technical University, organised an international interlaboratory comparison for the determination of silver nanoparticles in chicken meat. Silver nanoparticles were added to minced chicken meet, frozen and sent to 10 laboratories from 7 different countries. The laboratories then analysed the samples and reported back to the JRC, which evaluated the data.
The study found that each laboratory could determine the median particle diameter with high precision (repeatability standard deviation 2-5 %), but that significant differences between laboratories existed (reproducibility standard deviation between 15 and 25 %). In addition, the diameters determined by the laboratories were about 60 % larger than the diameter of the particles that were added, indicating that some agglomeration had occurred either during sample processing or sample preparation. Nevertheless, the method shows promise as screening tool for the determination of particle size.
Significant improvements are needed before the method can be reliably used for the determination of the particle number concentration: the reproducibility standard deviation reached 70 – 90 %, which is insufficiently accurate.

Improvement in sample preparation should lead to less variation between laboratories, both for the median diameter as well as for the particle number concentration.
The study also pointed out that – similarly to most other fields of analytical chemistry – it is unlikely that a simple measurement protocol applicable for all nanoparticles in all materials can be developed, as the interaction between particles and material strongly influence the analytical behaviour.

Read more in: S. Weigel et al.: "Results of an interlaboratory method performance study for the size determination and quantification of silver nanoparticles in chicken meat by single-particle inductively coupled plasma mass spectrometry (sp-ICP-MS)", Anal Bioanal Chem 409 (2017) 4839–4848, doi: 10.1007/s00216-017-0427-2

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