Mass spectral characterisation of cyclic oligoesters in a biodegradable mulch film.

Rationale: Plastic mulch film manages weed growth and moisture loss on the surface of cropping beds. The chemical components of such plastics include polymer(s), additives and non-intentionally added substances (NIASs). The unknown chemical nature and behaviours of these constituents require investigation due to their potential to add to the anthropogenic chemical burden in the agrifood system.

Methods: Solvent extracts of a commercial 15% polylactic acid (PLA)/85% poly(butylene adipate-co-terephthalate) mulch film were investigated using gas chromatography-mass spectrometry (GC-MS) with electron ionisation to characterise the additive and NIAS components. The obscurity of some of the NIASs meant their identification was not readily achieved through routine MS library comparisons. As such, the identification of several polymer-derived compounds required interpretation of the MS data and re-application of the derived fragmentation patterns with reference to the wider literature. Unknowns were confirmed using commercially available compounds.

Results: Unknown NIASs were identified as cyclic oligoesters comprised of the monomeric building blocks of the polymer system. Cyclic structures derived from the monomers of polybutylene adipate (PBA) and polybutylene terephthalate (PBT) fragmented through a primary pathway involving 1,5- and 1,3-H transfers at ester linkages. Characteristic ions at m/z 111, 129, 183 and 201 for PBA-derived cyclic oligoesters and m/z 104, 132, 149 and 221 for PBT-derived cyclic oligoesters were assigned in the mass spectra of unknowns. Cyclic oligoesters containing sebacate moieties were also identified, indicating the presence of polybutylene sebacate as an unexpected component of the mulch.

Conclusions: Systematic analyses of the sort reported here are valuable for providing alternative approaches for the identification of plastic-related chemicals. Open publication of MS spectral data is required to build a greater understanding of the mulch film chemical components contributing to the environmental chemical load introduced to agroecosystems.