A new possibility of the generalized two-dimensional correlation spectroscopy. 2. Sample-sample and wavenumber-wavenumber correlations of temperature-dependent near-infrared spectra of oleic acid in the pure liquid state

This paper reports an example of the application of sample-sample correlation spectroscopy, the novel possibility of the generalized two-dimensional (2D) correlation spectroscopy, proposed in the preceding paper. Temperature-dependent near-infrared (NIR) spectral variations of oleic acid (cis-9-octadecenoic acid) in the pure liquid state have been analyzed in the region 7600-6600 cm(-1) by two kinds of 2D correlation analyses, wavenumber-wavenumber and sample-sample correlation. The analyses have been made for the NIR spectra after two kinds of different spectral pretreatments and also for the raw spectra. First, the spectra have been offseted on the higher wavenumber side (around 7500 cm(-1)) and adjusted with respect to the intensity of the band at 7182 cm(-1) measured at 15 degrees C. In the secund pretreatment both wavenumber sides (the 7600-7400 and 6700-6600 cm(-1)) have been baseline corrected and only C-H combination bands and a band due to the first overtone of the monomer O-H group have been considered. In the last ease the original spectra without any pretreatment have been analyzed. The 2D wavenumber-wavenumber cross-product matrices, i.e., synchronous spectra, show that Fur the First and second cases the spectral variations concentrate on the C-H and O-H bands. The newly proposed sample-sample correlation verifies convincingly the existence of two phase transition temperatures at 32 and 55 degrees C that are important in the process of monomerization of dimers of oleic acid. The analysis based upon the raw spectra gives the strongest evidence for these temperatures in the sample-sample correlation pattern.