On the analysis of microplastic particles in foodstuffs

On the analysis of microplastic particles in foodstuffs

03.09.2020

Microplastic particles, i.e. particles with a size of 1 to 1,000 µm, in foodstuffs is becoming a keenly debated topic of discussion. Initial studies are being published that prove their presence. One example is the study on microplastic in mineral water commissioned by the magazine Öko-Test, about which we report in this issue. But how is the presence of such particles actually proved? We asked Dr. Ulrike Braun, a scientist at the Federal Institute for Material Research and Testing (BAM), about the problems involved with the analysis. Dr. Braun is also the coordinator of the joint research project RUSEKU (Representative Investigation Strategies for an Integrative System Approach to Specific Emissions of Plastics into the Environment), which is funded by the BMBF as part of the research program "Plastics in the environment". The aim of RUSEKU is to optimize the procedures for taking representative samples from water and waste water.

Dr. Braun, there have been many reports recently about microplastic in fish, lettuce, drinking water and mineral water. The layman is naturally curious: How can microplastic be detected and determined in foodstuffs and beverages?
Analyzing microplastic in environmental samples, but particularly also in foodstuffs, certainly is a big challenge for analysts. The chemical structure of synthetic polymers – the main constituent of microplastic – differs only very slightly from that of the generally far more common natural particles. Apart from that, particles in the lower micrometer range are very difficult indeed to detect because of the process involved. Microplastic analyses are susceptible to error, especially through the contamination of e.g. air-borne particles or the loss of particles through the complicated preparation procedures that are frequently needed.
Processes for analyzing microplastics must therefore not only be able to unmistakably identify the chemical "fingerprint" of the microplastic particles, they must also be very sensitive over a wide size range and, at the end of it all, be capable of providing representative conclusions. They must therefore be fast, valid and comparable.
Microplastics can be determined by volume or by the number of particles. Do validated processes or methods exist in both these areas?
In the analysis of microplastics, a distinction is made between processes that determine the number of particles – the microscopic-spectroscopic methods – and processes that determine the mass contents. The latter include the thermoanalytical and chemical processes, which, with the help of thermally or chemically extracted information, allow conclusions to be drawn about the total content of microplastic in the sample. The information from both types of process – i.e. particle size/number and mass content – cannot be converted from one to another because the particles are present in highly variable forms: Irregular or spherical particles, fibers, film fragments etc.
As part of the BMBF research program "Plastics in the environment", the team is evaluating the present processes with regard to their information value and limitations. This is proving to be very difficult because realistic reference materials and test systems with acceptable homogeneity are very difficult indeed to obtain.
Generally speaking, however, it has been found that spectroscopic processes are more time-consuming. Thermoanalytical processes can, because of the higher sample throughput and the minimum requirements on sample preparation, deliver results with an acceptable amount of scatter.
How is it possible to tell whether the correct method was selected with regard to the respective problem, and whether this was then correctly applied?
The first consideration is: what is the targeted result and what requirements are made on the measurements for this? Based on the answers, the methodology must be selected, also taking into account the medium to be sampled. The regulatory monitoring of production plants or foodstuffs, for example, makes differing demands on representativity, comparability and analytical depth of information, as, for example, the claim to describe the effect of an individual particle on an organism.
The omnipresence of plastics in our day-to-day life means that microplastic can be found in virtually every sample. Previous results on the occurrence of microplastic particles in environmental media should, however, be critically questioned. The various methods for the detection and quantification of microplastic are in some cases still at the optimization phase, and a validation of the data quality does not yet exist.
(August 2020)
Photo: © BAM

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