Influence of surface oxidation on the quantification of polypropylene microplastics by pyrolysis gas chromatography mass spectrometry

The influence of photo-oxidation on the quantification of isotactic polypropylene by Pyrolysis Gas Chromatography/ Mass Spectrometry (Pyr-GC/MS) was assessed. Beads (oval shape, similar to 5 mm) and fragments (irregular shaped, 250-50 mu m and 500-1000 mu m) were subjected to relatively harsh simulated accelerated weathering conditions (using a filtered xenon-arc reproducing sunlight's full spectrum) for up to 37 and 80 days, respectively. Samples collected (n = 10 replicates for each treatment) at increasing number of weathering days were analysed by Fourier-transform infrared spectroscopy with Attenuated Total Reflection (FTIR-ATR), scanning electron microscopy, and differential scanning calorimetry in order to assess the extent and the rate of degradation. The rate of surface oxidation occurred faster for fragments compared to beads, probably due to their higher surface area. Quantification of the polypropylene trimer (2,4-dimethyl-1-heptene) via double shot Pyr-GC/MS, showed that the signal of the trimer relative to the mass of polypropylene was reduced through weathering with a degradation rate of 1:3 faster for fragments over beads. Signal reduction and carbonyl index were correlated to show that polypropylene with a carbonyl index of >= 13 has a significantly reduced 2,4-dimethyl-1-heptene signal when compared to virgin material. Consequently, the quantification of polypropylene subjected to weathering under harsh conditions may be underestimated by 42% (fragments, carbonyl index: 18) to 49% (beads, carbonyl index: 30) when quantified by Pyr-GC/MS and using virgin polypropylene calibration standards. Pyrolysis at a lower temperature (350 degrees C) identified six degradation specific markers (oxidation products) that increased in concentration with weathering. Further comparisons between virgin and weathered microplastics may need to be considered to avoid underestimation of microplastic concentrations in future studies. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study evaluated the influence of photo-oxidation on the quantification of isotactic polypropylene by Pyrolysis Gas Chromatography/Mass Spectrometry. The degradation rate of fragments was found to be three times faster than beads, likely due to their larger surface area. Additionally, polypropylene subjected to harsh weathering conditions may be underestimated by 42% to 49% when quantified using Pyr-GC/MS with virgin polypropylene calibration standards.