Characterization of a PET reference material with exact-matching double isotope dilution MS and qNMR

Polyethylene terephthalate (PET) microplastics are pervasive pollutants, yet reliable quantification is hampered by the scarcity of standardized protocols and lack of certified reference materials. To address this, the Joint Research Centre of the European Commission developed a liquid chromatography-exact-matching double isotope dilution mass spectrometry (LC-EMD-IDMS) method for characterizing a mass value of the PET reference material EURM-060. PET particles (< 300 µm) embedded in a NaCl carrier were hydrolyzed (110  $_{}^{\circ }$ C, 120 min) to terephthalic acid (TPA). A perdeuterated TPA isotopologue (d $_4^{}$ -TPA) served as a spike, and calibration blends were prepared to meet the exact-matching criterion. MS acquisition in negative SRM mode provided isotope ratios, which, together with a rigorously derived measurement equation and a full uncertainty budget, yielded the PET mass in each unit. Ten randomly selected test units of EURM-060 gave masses between 451 and 586  $\mu$ g with relative standard uncertainties of 1.3-1.6 %. The average value is $X_a$ =516  $\mu$ g±15  $\mu$ g (k=2), with a relative expanded uncertainty of 2.9 %. Independent verification by quantitative $_{}^1$ H-NMR on three additional test units (526  $\mu$ g±56  $\mu$ g) matched the LC-EMD-IDMS result within the expanded uncertainty range. The characterization meets the requirements of ISO 33405, providing a SI-traceable value of PET mass and a reference material that can reduce inter-laboratory variability in PET microplastic analysis.