Baseline Shift
In several datasets, a wavelength-dependent baseline shift can be
observed at the start of the reaction (Fig. 8). This
wavelength-dependent increase in absorption occurs during the induction
period of autoxidation, and is consistent with light scattering,
possibly indicating the formation of nanoparticulate structures in the
oil. The extent to which this scattering occurred did not obey any
discernable pattern with respect to catalyst identity or concentration,
suggesting a complex interplay of factors leads to this light
scattering. Possible sources include: a) Co nanoparticles formed from
the drier; b) reverse micelles formed from polar oxidation products of
the natural antioxidants present in the oil; or c) a mixture of both a)
and b). It is not possible to distinguish between these possibilities
based on IR data alone, and further analysis of these possibilities is
currently underway using model systems. Attempts to normalize the
baseline using linear baseline corrections or a penalized least squares
smoother(Eilers 2003) were made, but did not lead to any improvement in
the global analysis. Despite the interference, global analysis fitting
generated reasonable SAS and rate constants, demonstrating the
advantages of including data from a wider variety of wavelengths.