In
the case of the SR procedure, TPP is incorporated into the diluent,
and no weighing or precise delivery of the oil or diluent
is required since the dilution ratio can be determined precisely
from the absorption of the spectral marker, thereby allowing
the spectrum of the oil to be accurately reconstituted from the
spectrum of the diluted oil sample. In this context, it may be
noted that the spectroscopic observation of a slow conversion
over time of the TPP reagent present in the diluent to TPPO
prompted experiments in which varying amounts of TPPO were
added to the diluent but did not affect the FTIR PV determinations.