Volatile organic compounds identification and specific stable isotopic analysis (delta C-13) in microplastics by purge and trap gas chromatography coupled to mass spectrometry and combustion isotope ratio mass spectrometry (PT-GC-MS-C-IRMS)

Microplastics (MPs) have become one of the major global environmental issues in recent decades due to their ubiquity in the environment. Understanding MPs source origin and reactivity is urgently needed to better constrain their fate and budget. Despite improvements in analytical methods to characterize MPs, new tools are needed to help understand their sources and reactivity in a complex environment. In this work, we developed and applied an original Purge-&-Trap system coupled to a GC-MS-C-IRMS to explore the delta C-13 compound-specific stable isotope analysis (CSIA) of volatile organic compounds (VOC) embedded in MPs. The method consists of heating and purging MP samples, with VOCs being cryo-trapped on a Tenax sorbent, followed by GC-MS-C-IRMS analysis. The method was developed using a polystyrene plastic material showing that sample mass and heating temperature increased the sensitivity while not influencing VOC delta C-13 values. This robust, precise, and accurate methodology allows VOC identification and delta C-13 CSIA in plastic materials in the low nanogram concentration range. Results show that the monomer styrene displays a different delta C-13 value (- 22.2 +/- 0.2 parts per thousand), compared to the delta C-13 value of the bulk polymer sample (- 27.8 +/- 0.2 parts per thousand). This difference could be related to the synthesis procedure and/or diffusion processes. The analysis of complementary plastic materials such as polyethylene terephthalate, and polylactic acid displayed unique VOC delta C-13 patterns, with toluene showing specific delta C-13 values for polystyrene (- 25.9 +/- 0.1 parts per thousand), polyethylene terephthalate (- 28.4 +/- 0.5 parts per thousand), and polylactic acid (- 38.7 +/- 0.5 parts per thousand). These results illustrate the potential of VOC delta C-13 CSIA in MP research to fingerprint plastic materials, and to improve our understanding of their source cycle. Further studies in the laboratory are needed to determine the main mechanisms responsible for MPs VOC stable isotopic fractionation.