Use of Global Analysis for Studying Drying Oils
Global analysis demonstrates several advantages over single wavelength
analysis for studying drying oils. Firstly, as demonstrated by the 2.72
mM Co(hfac)2 and 2.04 mM Co(oct)2 data,
diffraction and wavelength dependent baseline drift are intrinsically
addressed through the fitting procedure, allowing for analysis of data
without the need for pre-treatment and simplifying the overall fitting
procedure. Secondly, as demonstrated by the quality of fit to theA B C model, including the fingerprint region
of the IR in the modeling allows for the identification of intermediate
species formed during the drying process that may be difficult or
impossible to detect in other regions of the IR spectrum. Finally, the
use of global analysis provides a visual representation of the quality
of the final fit in the form of residuals, which aids in the
identification of places where the model can be further improved.
Both A and C in these models correspond to those
previously identified as the starting and ending species in previous IR
studies of linseed oil drying. Specifically, A corresponds to
bis-allylic C-H sites found in linoleic and linolenic fatty acids (Fig.
1) as evidenced by the peaks at 3010 and 723 cm-1, andC corresponds to isolated trans-alkenes formed after
isomerization as evidenced by the peak at 970 cm-1.
The intermediate species B has peaks at 1117 and 985
cm-1, neither of which are present in speciesA or C . These peaks are consistently present
regardless of the catalyst used but have varying estimated molar
absorptivities depending on the identity and amount of catalyst used.
This results from limitations of global analysis as described above in
the section “Selection of the Model for Global Analysis”.