Near Infra Red Spectroscopy

Abstract

NIR stands for Near Infrared and refers to the region of light immediately adjacent to the visible range, falling between 750 and 3,000 nanometers (nm = nanometers or 1/1000000000 of a meter) in wavelength. Most organic materials have well defined reflectance or transmittance features at these wavelengths. According to the principles of quantum physics, molecules may only assume discrete energy levels. Similar to the vibrating string of a musical instrument, the vibration of a molecule has a fundamental frequency, or wavelength, as well as a series of overtones. For molecules, the fundamental vibrations involve no change in the center of gravity of the molecule. The spectrum shape for any material is the result of these characteristic fundamentals and overtones. Near-infrared spectra are primarily the result of overtones, whereas there are many fundamentals in the mid and far infrared regions. Since the molecular structure of most compounds is very complex, the resulting spectra are actually the result of many overlapping peaks and valleys. Generally speaking, persons performing NIR analysis must then identify and characterize specific features in the spectra by means of statistical methods. Chemometrics software is designed to accomplish this task. The absorption of NIR radiation by organic molecules is due to overtone and combination bands primarily of O-H, C-H, N-H and C=O groups whose fundamental molecular stretching and bending absorb in the mid-IR region. These overtones are anharmonic, i.e., they do not behave in a simple fashion, making NIR spectra complex and not directly interpretable as in other spectral regions. Below is a graph depicting the prominent absorption bands as they relate to the overtone and combination bands of the fundamental vibrations occurring in the Mid IR region. To understand the types of measurements possible using NIR light, it is useful to understand several general properties of electromagnetic waves, as well as basics of classical molecular and atomic structure. EM radiation, is in the form of waves, and as such, has all the properties of a wave; including wavelength. Figure 1 graph is a typical wave. Wavelength is a distance between two points. Wavelength is particularly important to our discussion as it is closely connected to energy. Wavelength and energy are readily convertible from one to the other when speaking of EM waves. See figure 2 below They are related in the following manner