Chapter 1 - Principles of Infrared Spectroscopy

Abstract

Major food components are generally complex molecules resulting from the polymerization of monomers such as amino acids or carbohydrates. These monomers exhibit specific chemical groups such as carboxylic and amine functions in amino acids. As each chemical group may absorb in the infrared region, it appears useful in a first step to clearly identify the characteristic absorption bands of these groups in the near- and mid-infrared regions. C–H bonds, which are found in large quantities in organic molecules, show stretching vibrations between 2750 and 3320 cm–1 in the mid-infrared (MIR) region. The location of these bands is related to carbon hybridization. In the near-infrared (NIR) region, the first and second harmonics for C–H stretching vibrations are observed at about 1700 nm and 1200 nm, respectively. Combination bands involving stretching and bending of the C–H bond may be identified between 2000 and 2500 nm and, with a lower intensity, between 1300 and 1440 nm. In the NIR region, the second harmonics assigned to the stretching of C–H bonds give a weak absorption band at about 1200 nm. In this region, hexane shows two bands at 1186 and 1208 nm, whereas dodecane is characterized by a band at 1208 nm and a shoulder at 1186 nm.