Abstract
Spectral reconstitution (SR) is a dilution technique developed to facilitate
the rapid, automated, and quantitative analysis of viscous oil samples by Fourier
transform infrared spectroscopy (FT-IR). This technique involves determining the
dilution factor through measurement of an absorption band of a suitable spectral
marker added to the diluent, and then spectrally removing the diluent from the
sample and multiplying the resulting spectrum to compensate for the effect of
dilution on the band intensities. The facsimile spectrum of the neat oil thus
obtained can then be qualitatively or quantitatively analyzed for the parameter(s)
of interest. The quantitative performance of the SR technique was examined with two
transition-metal carbonyl complexes as spectral markers, chromium hexacarbonyl and
methylcyclopentadienyl manganese tricarbonyl. The estimation of the volume fraction
(VF) of the diluent in a model system, consisting of canola oil diluted to various
extents with odorless mineral spirits, served as the basis for assessment of these
markers. The relationship between the VF estimates and the true volume fraction
(VF<sub>t</sub>) was found to be strongly dependent on the dilution ratio and
also depended, to a lesser extent, on the spectral resolution. These dependences are
attributable to the effect of changes in matrix polarity on the bandwidth of the
ν(CO) marker bands. Excellent VF<sub>t</sub> estimates were obtained by making a
polarity correction devised with a variance-spectrum-delineated correction equation.
In the absence of such a correction, SR was shown to introduce only a minor and
constant bias, provided that polarity differences among all the diluted samples
analyzed were minimal. This bias can be built into the calibration of a quantitative
FT-IR analytical method by subjecting appropriate calibration standards to the same
SR procedure as the samples to be analyzed. The primary purpose of the SR technique
is to simplify preparation of diluted samples such that only approximate proportions
need to be adhered to, rather than using exact weights or volumes, the marker
accounting for minor variations. Additional applications discussed include the use
of the SR technique in extraction-based, quantitative, automated FT-IR methods for
the determination of moisture, acid number, and base number in lubricating oils, as
well as of moisture content in edible oils.
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