Chromatogram-Kendrick Mass (CKM) Plot for Comprehensive Visualization of Pyrolysis Products in Polyolefin Blends

This study introduces the chromatogram-Kendrick mass (CKM) plot, a novel visualization method that integrates four-dimensional data sets acquired by pyrolysis gas chromatography/time-of-flight mass spectrometry (Py-GC/TOFMS) using field ionization, comprising chromatographic retention time, peak intensity, mass-to-charge ratio (m/z), and Kendrick mass defect (KMD) values. The capability of freely rotating the plot to any viewing angle allows intuitive interpretation of the numerous pyrolysis products derived from complex polymer mixtures, leading to more efficient structural identification. To demonstrate its utility, a CKM plot was applied to polypropylene (PP)/polyethylene (PE) blends subjected to thermo-oxidative degradation for 2, 4, and 6 h. The plot reveals that untreated PP/PE primarily produces hydrocarbon ions without heteroatoms, whereas degraded PP/PE generates abundant oxygen-containing ions. Top-down views of the plot clearly separated the two ion series: ions at shorter retention times originated from PP, whereas those at longer retention times were derived from PE. Thus, the CKM plot enabled visualization and analysis of structural isomers with identical m/z but different retention times, which are difficult to comprehensively detect using conventional one-dimensional chromatography and mass spectrum analysis. Quantitative evaluation of the oxidative isomers indicated that PP underwent preferential oxidation over PE in the PP/PE blends. This difference was attributed to the greater lability of tertiary hydrogen atoms in PP, which facilitated the abstraction of hydrogen and accelerated radical propagation. Overall, the CKM plot provides a powerful new approach for distinguishing between PP- and PE-derived pyrolysis products, elucidating degradation trends in polyolefin mixtures, and evaluating recycled polymer materials.