Identification and quantification of polyethylene terephthalate (PET) polyesters and microplastics by combining mild chemical depolymerization with dimethylaminopropylamine (DMAPA) and MALDI FTICR & LC ESI Orbitrap mass spectrometry analysis

Polyethylene terephthalate polyesters (PET) formed of terephthalic acid and ethylene glycol units are one of the most commonly found plastics in our daily life and environment. Micro- and nano-PET occurs since their fabrication as they have been detected in water of PET-bottles and in marine ecosystems due to weathering degradation. The identification of PET microplastics modification is still very challenging. They are principally identified by optical spectroscopy and by Py-GC-MS. Unfortunately, these techniques are not efficient for identifying and quantifying PET at molecular level and they provide very few structural information on their modifications. In this work, PET was depolymerized in soft conditions combined and analyzed by complementary mass spectrometry approaches. After pre-solubilization with 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), a direct transamidation using N,N-dimethyl-1,3-propanediamine (DMAPA) at soft temperature was performed to break ester bonds and improve MS ionization allowing the detection of acids and aromatic di- and poly-acids. The resulting products were analyzed by ultra-high resolution mass spectrometry using MALDI ionization on a 9.4 Tesla FTICR MS and ESI ionization on an Orbitrap MS hyphenated to liquid chromatography. Daily life PET samples from different countries, brands and contents were studied and their depolymerized products were identified and quantified. A specific fingerprinting for each sample was attributed. Dimers, oligomers, scission products, oxygenated species, crosslinking structures and additives were successfully identified with high accuracy and resolution. This strategy can be applied to other polyesters and co-polyesters.