Analysis of Polyethylene by Using Cyclopentadienyl Cobalt Chemical Ionization Combined with Laser-Induced Acoustic Desorption/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

A new mass spectrometric method has been developed for the analysis of low molecular weight polyethylene (PE). Laser-induced acoustic desorption (LIAD), combined with chemical ionization by the cyclopentadienyl cobalt radical cation (CpCo^•+) in a Fourier transform ion cyclotron resonance mass spectrometer, produces predominantly a quasimolecular ion, (R + CpCo − 2H₂)^•+, for each PE oligomer (R). An examination of artificial alkane mixtures revealed no mass bias for alkanes of differing molecular weights. However, the success of the LIAD/CpCo^•+ CI technique depends greatly upon the LIAD sample preparation method used. Several sample preparation methods were evaluated, and pneumatically assisted spin coating was concluded to provide the best mass spectra as a result of its ability to provide uniform PE coverage on the LIAD foils. The molecular weight distributions measured for several low molecular weight PE samples (200−655) were found to be in good agreement with manufacturers' values as determined by gel permeation chromatography.