The applicability of the band-stripping and complementary matching method has been demonstrated by the analysis of temperaturedependent near-infrared (NIR) absorption spectra in the 7500-6500 cm - 1 region of oleic acid ( cis -9-octadecenoic acid) in the pure liquid state. This method is based on first derivative-second derivative pair (D1-D2) plots and a new concept called the <i>complementary</i> band, cBD<sub><i>i</i></sub>, created by subtracting all the rest of the bands, exclusive of the <i>i</i>th estimated band, eBD<sub><i>i</i></sub>, from an experimental spectrum. The degree of coincidence of both band shapes provides a suitable measure for the quality of fit for each individual component band. It has been confirmed from the present analysis of the NIR spectra of oleic acid measured over a temperature range of 16-79 °C that the change of the peak intensity of the component band at around 6915 cm<sup>-1</sup> due to the first overtone of an O-H stretching vibration of the monomer has two transition points around 35 and 55 °C. Moreover, the present study has provided new insight into the analysis of temperature-dependent spectral variations of oleic acid. Among the three temperature ranges, 16-35 °C, 35-55 °C, and 55-79 °C, in the first range the band near 6915 cm<sup>-1</sup> shows a slight increase and in the second range it has a linear intensity change with a slope of 0.002 a.u./°C. In the third range, a rapid increase of the peak intensity is observed. This band exists even at 15 °C (just below the melting point) and shows a shift from 6910 to 6915 cm<sup>-1</sup> and a band narrowing from 85 to 80 cm<sup>-1</sup> (full width at half-height) over a temperature range of 16 to 79 °C. Furthermore, it has been found that there are two broad bands at around 6835 and 6778 cm<sup>-1</sup>.
Katsue Koashi, Makio Iwahashi, and Yukihiro Ozaki, "Band Analysis of Temperature-Dependent Near-Infrared Spectra of Oleic Acid in the Pure Liquid State by the Analytic Geometric Approach," Appl. Spectrosc. 57, 1539-1550 (2003)