Internal combustion
engine lubricants are subject to thermo-oxidative
degradation during use and must be designed to withstand oxidation
in order to extend their useful life. Understanding the complex chemical
process of thermo-oxidative degradation is essential to designing
higher performing engine lubricants. In this study base oil samples
composed of a Group II base oil, doped with three different levels
of biodiesel (B0, B15, and B100), were subjected to benchtop oxidation
testing of up to 168 h, which mimics the conditions experienced in
an internal combustion engine. The resulting samples were analyzed
by Fourier transform ion cyclotron resonance mass spectrometry (FTICR
MS) for ultrahigh-resolution characterization to monitor oxidation
as a function of time and biofuel content. Both negative-ion nanoelectrospray
ionization and positive-ion atmospheric pressure photoionization were
utilized. Most of the oxidation products were found to be polyoxygenated
species containing 1–8 oxygen atoms, with the number of detected
species increasing with oxidation time. Assessment of the maximum
carbon number of protonated classes indicated the involvement of oligomerization
reactions; additionally, modeling of mean double bond equivalents
(DBE) for each protonated class suggests increasing carbonyl content
for each particular class with increasing oxidation time. The oxidations
of B15 and B100 doped samples were compared to that of B0. B15 samples
were found to correspond closely to B0 samples, with a similar number
of species detected. B100 samples showed a significant increase in
number of species generated at 24–72 h relative to B0 and B15;
however, a similar number of species were observed at 168 h for all
samples, indicating a similar level of base oil oxidation at the final
oxidation point. FTICR MS is shown to afford new insights into base
oil oxidation as a function of time and biofuel content.