Characterization of polyalphaolefins using halogen anion attachment in atmospheric pressure photoionization coupled with ion mobility spectrometry-mass spectrometry

Abstract : Polyalphaolefins (PAOs) are saturated alpha olefin oligomers used as a base stock oil for synthetic lubricants. The synthetic base stocks are manufactured from linear alpha olefins by catalytic oligomerization processes. The aim of this work was the characterization of different PAO grades, synthesized from different linear alpha olefins using two oligomerization processes, acid and metallocene catalyses. Negative ion atmospheric pressure photoionization (APPI) coupled with ion mobility spectrometry-mass spectrometry (IMS-MS) permitted the detection of intact PAO adducts with either chloride, bromide or iodide ions using halogenated solvents (e.g. dichloromethane, dibromomethane and diiodomethane) and toluene as the dopant. The best signal-to-noise ratio was obtained with dichloromethane. The APPI mass spectra displayed characteristic ion distributions for high viscosity PAO grades. The mass shift between two adjacent ions permitted the identification of repeating units and consequently the monomers of alpha olefins used to manufacture the PAO. For low PAO grades, the halide anion adducts were not detected as they are less stable. The IMS-MS data, as well as the correlated variables, i.e. the drift time and full width at half maximum (FWHM) of the IMS peaks, can be used to differentiate polyalphaolefins of the same grade but differently synthesized.