Two-Dimensional Vibration Spectroscopy. V: Correlation of Mid- and near Infrared of Hard Red Winter and Spring Wheats

Abstract

Two-dimensional correlation spectroscopy across the near infrared (NIR) and mid-infrared (MIR) regions have been used to explain the NIR spectra of hard red winter and spring wheat and provide additional confidence in analytical models developed with empirical data. Recent studies have shown that the major C–H stretching vibrations and some of the aromatic C–H and ring stretching vibrations and the minor vibrations in the “fingerprint” region are correlated also. The technique has been expanded to include Raman spectra. The Raman spectra were enhanced with Maximum Likelihood methods to improve signal-to-noise (S/N) while maintaining resolution. This was necessary to eliminate the effects of fluorescence which degrades S/N. The use of NIR lasers at 1.1 μm generally eliminates fluorescence as a problem, but it is still quite prevalent in agricultural materials. The original study did not show any significant correlations to aromatic functionality. However, the band at 1552 nm correlates to the Raman and not to the MIR. This band has shown up in NIR spectroscopy models for the determination of lignin, but is not readily observed in the MIR. Thus it correlates to a Raman active rather than a MIR active band. The same phenomena are observed for the amide I, II and III bands for wheat. The interesting features from NIR and MIR are that there are correlations that distinguish winter from spring wheat. These, and the Raman spectra of wheat, will be shown. These studies show that multiple regions of the electromagnetic spectrum can be, and in deed need to be, used to interpret adequately the spectral and statistical results we have traditionally obtained in the NIR.