Background
As part of the European Green Deal and the Circular Economy Action Plan, the European Union has taken measures to reduce microplastic pollution in the environment by 30 per cent by 2030. However, there is still a lack of reliable data for a comprehensive risk assessment. This is because identifying and quantifying microplastics in the environment remains a challenge: in addition to validated analysis methods, standard operating procedures and harmonisation to ensure that reliable, meaningful data that is comparable with other study results can be collected, reference materials are of central importance. Dr Korinna Altmann is an expert in this field at the Federal Institute for Materials Research and Testing (BAM). We asked her about the current state of research and development of reference materials for microplastics research.

 

Dr Korinna Altmann
Dr Korinna Altmann studied chemistry at the Free University of Berlin and obtained her doctorate at the Federal Institute for Materials Research and Testing (BAM) in the field of polymer surfaces. Since 2015, Dr Altmann has been researching microplastics at BAM and working on establishing and further developing methods for analysing microplastic samples in environmental media and on developing realistic reference materials for microplastic analysis. At BAM, Dr Altmann heads the work on the analysis and measurement methodology of microplastics and nanoplastics as part of the EU projects ‘POLYRISK’ and ‘PlasticsFatE’ and the ‘PlasticTrace’ project, which aims to develop and harmonise methods for the chemical identification, physical characterisation and quantification of released micro- and nanoplastics in drinking water, food and environmental matrices. Dr Altmann led the working group on international ring trials with the aim of harmonisation in the European-funded CUSP cluster and is also project manager for the development of a standard in an international standardisation committee on reference materials for microplastics with the aim of supporting the regulatory efforts of the European Commission.
 
Our questions
 
Dr Altmann, in the PlasticTrace project, you are leading the research activities on the production of reference materials for microplastics and nanoplastics analysis. Why are reference materials so important and what are the general challenges in developing such reference materials? To what extent are reference materials a prerequisite for validating measurement methods and realistically interpreting measurements in scientific studies on the occurrence and content of microplastics and nanoplastics in the environment, for example in food, soil, air or water?
 
Basically, reference materials are materials in which at least one specific property is particularly well characterised and identical for all subsamples, i.e. homogeneous. If the material is not only homogeneous but also stable beyond a defined period of use, it may be called a reference material. In addition, there are certified reference materials for which the measurement results of the specific property are additionally corroborated by measurements in other laboratories, i.e. with the aid of a ring test or comparative test. The measurement result becomes metrologically traceable.
 
Such (certified) reference materials are essential if the accuracy and reliability of methods are to be ensured. The use of a reference material in the validation of the developed method makes it possible to determine the recovery rate. If this approaches 100% recovery, the suitability of the method for the respective application is confirmed. With the help of the reference material, each individual step in microplastic analysis, e.g. sampling, sample preparation and detection, can be evaluated individually. This allows an uncertainty (i.e. deviation) from the reference value to be determined for the measurement result, or the method itself can be optimised to achieve a higher recovery rate. In addition, possible sources of contamination can also be identified and eliminated.
 
There are now three major challenges in microplastics research: the material, the concentrations in real compartments and the analysis methods themselves.
 
Reference materials are always produced for a specific application. This raises the question of which reference material should be developed if its production involves considerable costs and time. I have been working at BAM since 2018 in the field of reference materials. We have decided to focus on the four types of polymers that are most frequently detected in the environment and also most commonly produced by industry: polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET). These are frequently used in the packaging industry, meaning that we humans are in constant contact with them. Depending on the issue at hand, other important polymers include polyamides (PA), polyvinyl chloride (PVC) or polymethyl methacrylate (PMMA), as well as the biodegradable polymers polylactic acid (PLA) or polybutyrate adipate terephthalate (PBAT) and styrene-butadiene rubber (SBR) as an elastomer for evaluating tyre abrasion.
 
The first major challenge now is to turn the polymer material into a reference material. There are various initiatives: one is the above-mentioned concept study from science and research: Development of new reference particles at the University of Bayreuth. Another comes from the working group led by Prof. Harre at the Dresden University of Applied Sciences. Both produce particles of a defined number with sizes greater than 125 µm. The particles can serve as model particles with a minimal number and uniform size, but they are not realistic. In the environment, most microplastics are created by weather-induced degradation of plastic products that are disposed of improperly. The resulting particles are classified according to their size: 1-5 mm (large microplastics), 1-1000 µm (microplastics) and < 1 µm (nanoplastics) (ISO/TR 21960:2020). In order to produce a realistic reference material for microplastics research, the material must therefore have an irregular shape of particle fragments, a broad size distribution between 1 and 1000 µm and consist mainly of one of the above-mentioned polymer materials. Such particles can be produced by cryogenic grinding or other comminution techniques.
 
The first major challenge now is to turn the polymer material into a reference material. There are various initiatives: one is the above-mentioned concept study from science and research: Development of new reference particles at the University of Bayreuth. Another comes from the working group led by Prof. Harre at the Dresden University of Applied Sciences. Both produce particles of a defined number with sizes greater than 125 µm. The particles can serve as model particles with a minimal number and uniform size, but they are not realistic. In the environment, most microplastics are created by weather-induced degradation of plastic products that are disposed of improperly. The resulting particles are classified according to their size: 1-5 mm (large microplastics), 1-1000 µm (microplastics) and < 1 µm (nanoplastics) (ISO/TR 21960:2020). In order to produce a realistic reference material for microplastics research, the material must therefore have an irregular shape of particle fragments, a broad size distribution between 1 and 1000 µm and consist mainly of one of the above-mentioned polymer materials. Such particles can be produced by cryogenic grinding or other comminution techniques.
 
The third challenge arises from the complexity of environmental samples. Before the plastic particles can be detected, the sample must be taken, homogenised and the matrix largely reduced to avoid false positive results. Throughout the entire analytical process chain, care must be taken to avoid contamination, e.g. through the use of plastic objects. Some water samples can be prepared simply by filtration, whereas soils or compost, for example, require much more intensive sample preparation by reducing the organic or inorganic content. However, this must not lead to any change in the plastic particles themselves or their number or mass.
 

Microplastics research uses methods that determine the number of particles in a sample or use thermal, chemical or physical methods to estimate the total mass of microplastics in a sample. Are there now promising methods that can be used to validly determine both the number of particles and the mass content of microplastic particles in environmental samples?
 

The requirements for microplastic detection vary greatly. Members of the general public in particular want to know how great the risk of microplastics and nanoplastics is to human health. We are exposed to microplastics every day, whether through inhalation or food consumption. In this context, it is very important to know the particle size, number and shape. This can be determined using microscopic methods. However, when it comes to regulatory approaches, mass concentrations are the more interesting values. Limit values in guidelines, regulations or laws are traditionally expressed in terms of mass. Simulation models for predicting transport pathways or distributions also work predominantly with mass. Thermal methods are also well suited as routine methods, if only to quickly assess whether relevant amounts of microplastics are present in the sample. The same applies to chemical methods, which are, however, limited to certain types of polymers (e.g. PET). PVC can be detected well using ion chromatography.
 
Naturally, it is certainly difficult to find a method that can determine both the particle count and the mass quantitatively, as each method has its advantages and disadvantages. However, work is already underway to combine the methods. For example, the sample could first be analysed microscopically to determine the particle count before the particles are thermally pyrolysed and detected for mass. This requires sample preparation that works for both methods.
 
Personally, I don't think we'll find a method in the near future that can accurately and quantitatively determine both the particle count and the mass content. However, we have a good pool of methods that we can use to reliably answer the questions at hand.
 

The results of the POLYRISK and PlasticTrace projects, as well as the PlasticFatE project, will be incorporated into recommendations for standards and legislation for the scientific assessment of impacts and risks to humans and the environment. Can you say anything about the results of the projects to date and the importance of reference materials for microplastics research? What is the current status of standard development and harmonisation of research methods?
 

The aim of these projects, which involve large research networks, is to conduct scientific research and, through knowledge transfer, to support possible regulation should this be necessary. The precautionary principle applies to microplastics and nanoplastics, which is why the EU Commission is already making initial efforts to regulate them, for example in the Drinking Water Directive (EU) 2020/2184 and the Waste Water Directive (EU) 2024/3019. Both directives explicitly require the monitoring of microplastics.
 
The EU projects mentioned above focused in particular on researching suitable, realistic materials that can be used as reference materials. BAM now offers the first microplastic reference materials, with more in the certification process. In addition, the projects focused on developing methods for detection. Only by validating the methods using reference materials can we determine correct and accurate particle counts and masses in the various compartments, which in turn are a prerequisite for realistic toxicological studies on the hazard potential of microplastics and nanoplastics.
 
The tablets developed as reference materials in PlasticTrace were used in PlasticsFatE for an international comparison test under the umbrella of VAMAS (Versailles Project on Advanced Materials and Standards). Over 50 laboratories from all continents took part. It was shown that we are on the right track. However, there are still uncertainties and room for improvement, so we need to conduct such tests regularly in order to harmonise micro- and nanoplastics analysis worldwide. Only if we have reference material and uniformly coordinated standard operating procedures in the regulatory context will we be able to achieve consistent results worldwide.
 
Dr Altmann, thank you very much for the interview!
 

Further information:
 
The results of the projects will be transferred to various standardisation committees. These deal with detection methods, sample preparation of compost and reference material:

• ISO/TC 147/SC 2/JWG1
  ISO/DIS 16094-2 Water quality — Analysis of microplastic in water; Part 2: Vibrational spectroscopy methods for waters with low content of sus-pended solids including drinking water
  ISO/DIS 16094-3 Water quality — Analysis of microplastic in water; Part 3: Thermo-analytical methods for waters with low content of suspended solids including drinking water
• ISO/TC 61/SC 14/WG 4
  ISO/WD 24899 Plastics – A method for extraction of microplastics from compost samples
  ISO/NP 25654 Plastics — Reference materials for the validation of microplastic detection Methods
  
  

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