Determination of polymer type and comonomer content in polyethylenes by pyrolysis-photoionization mass spectrometry

Rapid microanalysis of a wide variety of polyolefins was performed using py rolysis-photoionization mass spectrometry (py-PI-MS). Solid samples (simila r to 10 mu g) were pyrolyzed on a heated probe in the source region of a ti me-of-flight mass spectrometer. Pyrolysates were "softly" ionized using coh erent vacuum ultraviolet radiation (118 nm). The resulting mass spectra wer e clearly different for low-density polyethylene, high-density polyethylene , and several ethylene/alpha-olefin copolymers. A combination of principal component analysis and linear discriminant analysis was used to classify po lyolefin samples directly from their photoionization mass spectra. The comp ositions of ethylene-butene and ethylene-octene copolymers were predicted u sing partial least-squares analyses. The values obtained using py-PI-MS wer e in good agreement with the measured C-13 NMR values. Samples of ethylene- octene containing 30 wt % carbon black and ethylene-butene containing 20 wt % silica were correctly classified and compositionally analyzed using py-P I-MS. Samples containing these additives are typically not amenable to stud y by solution-state C-13 NMR or IR.