Harmonisation of Sample Preparation for the Microplastic Analysis: Recovery Rates of Tablets containing Microplastics as Reference Material for Food and Environmental Matrix

Microplastic (MP) are everywhere and their impact on health and environment is still discussed, based on the precautionary principle, their presence should be monitored and emission should be reduced. The crucial point is the reduction of MP pollution but also the introduction of legally binding, standardized procedures for their analysis. The limited availability of certified reference material as well as absence of standard operating procedures to identify, characterize and quantify MP according to number and mass have generated lack of comparability between analytical methods and laboratories.First, we developed tablets of a water-soluble matrix, containing defined numbers of MP particles or mass contents. These can be solved in water and handled as reference materials in drinking water without additional sample preparation. These samples are measured using mass-based methods as thermo-analytical methods like thermal desorption-gas chromatography-mass spectrometry (TED-GC/MS) or pyrolysis gas chromatography mass spectrometry (Py-GC/MS) and using number-based methods as vibrational spectroscopy like Infrared and Raman after filtration. Our aim was to develop standard operating procedure to measure MP in complex matrices and to harmonise and compare different detection and characterization methods. These matrices require treatments to show a reliable quantification and identification of MP. We selected baby milk powder, as food matrix, which have a high content of water soluble and organic compounds. For the identification and quantification of small microplastics (SMPs; 10-100 µm) with TED-GC/MS and Py-GC/MS, a simple and fast pre-treatment with citric acid was applied. For the assessment of the number of SMPs, with vibrational spectroscopy (µ-Raman, µ-Infrared) and with electron microscopy (SEM), a multi-enzymatic digestion and a microwave alkaline hydrolysis was tested. As environmental sample, we selected surface water. The suspended particulate matter contained in the water have a high content of inorganic compounds. In this case, no sample preparation was used for the thermoanalytical methods and a sample preparation including density separation and oxidative digestion was tested to identify and quantify SMPs with vibrational spectroscopy. Depending on matrix and on the detection procedure, sample preparations influence significantly recovery rates. This work shows various analytical results and discuss technical aspects of the different procedures.