Challenges in the quantification of poly(ethylene terephthalate) microplastics via thermoanalytical methods posed by inorganic matrix components

Microplastics (MP) have been in the spotlight of environmental research for several years. Thermoanalytical methods, such as pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC-MS), are among the most promising techniques for the quantification of MP in complex environmental samples. Poly(ethylene terephthalate) (PET) is one of the most widely used polymers, but its quantification in environmental samples is a particular challenge. This study emphasizes various effects of the inorganic sample matrix on the pyrolysis of PET, and discusses several approaches to tackle these issues. Inorganic matrix constituents caused changes in the distribution of pyrolysis products, reactions with the analytes, or losses of intensity due to decomposition of the specific markers used for identification and quantification of PET. Therefore, MP need to be separated from the inorganic sample components. A promising approach to remove the inorganic matrix is pressurized liquid extraction. However, PET underwent a depolymerization reaction during the extraction, impacting recovery and reproducibility of the method. The use of TMAH, a frequently used derivatization agent in Py-GC-MS analysis, did not compensate for these matrix effects. Thus, a fast, reliable thermoanalytical method for a precise quantification of PET in complex environmental matrices can currently not be recommended, and extensive, time-consuming sample clean-up protocols seem to be unavoidable.