Direct polyethylene fingerprinting by pyrolysis-compound specific isotope analysis (Py-CSIA)

Abstract

A low-density green polyethylene was studied using conventional analytical pyrolysis (Py-GC/MS), bulk C, N, H, O isotopic ratio (TC/EA-IRMS) and direct pyrolysis compound specific isotope analysis (Py-CSIA) of stable light elements C, N, and H (Py-GC- (FID)-C/TC-IRMS). Py-GC/MS (500 ºC) released series of n-alkane, α-alkene and α,ω-diene (C8 to C37). A peak at c. min. 6 was identified as o-Chloroaniline (1-Amino-2-chlorobenzene) corresponding to the plastic’s green dye. Bulk plastic isotopic values (TC/EA-IRMS) were δ13C=-30.16±0.60 ‰ δD=-79.23±0.28 ‰ and δ18O=+15.33±0.59 ‰ δ15N was not detected due to a relative low signal contribution from the sole N bearing dye compound. Py-CSIA confirmed a similar δ13C (-30.13±0.60 ‰) for the three homologous series (n-alkane -29.48±0.69 ‰ α-alkene -29.86±0. 36 ‰; α,ω-diene -30.26±0.54 ‰) but different from that o-Choloroaniline (-27.13±0.56 ‰). Only for this molecule δ15N could be measured (+21.87±1.97 ‰). These signatures are in line with tabulated values for synthetic materials but indicate that polyolefin and dye are probably from two distinct origins. In the range where it was possible to unambiguously measure isotope ratio (no co-elution zone between C12 and C19), Py-CSIA for hydrogen revealed significant different compound specific δD values for the three structures (n-alkane -98.87±5.40 ‰; α-alkene -74.74±1.50 ‰; α,ω-diene -67.38±2.98 ‰). A significant enrichment in the heavy isotope with chain length was observed for n-alkanes, best explained by the quadratic equation δD=0.59Cx2-16.38Cx+11.01(r2=0.959). The original structure of the casted plastic have tertiary carbons with higher polarity, our results suggests that these may act as deuterium concentrators. The incorporation of the most probable isotope (light 1H) into the n-alkane chains when cracking may be the cause of the observed decrease of the hydrogen isotopic values in short chain molecules. This effect is less obvious with increasing chain length due to a “dilution” effect and long chain n-alkanes tend to the isotopic signature of the unsaturates α,ω-dienes and α-alkenes. This finding is to be taken in consideration when measuring compound specific δD in polymeric materials using this new hyphenated Py-GC-TC-IRMS technique.