A Brief Review of the near Infrared Measurement Technique

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Note that the term frequency refers to the number of vibrations per unit of time, designated by the Greek letter v (nu) and generally specified in units of S-1) or Hertz (Hz). Amplitude is defined by the interatomic distance covered at the extremes of the vibrating dipole, and is dependent upon the amount of energy absorbed by the infrared active bond. When incoming photons from a spectrophotometer source lamp strike different molecules in a sample, two direct results may occur: (1) the frequency of the disturbing energy does not match the natural vibrational frequency of the molecule, or (2) the disturbing frequency does match the vibrational frequency of the molecule. When there is a match between the disturbing frequency of the illumination energy and the natural vibrational frequency of a molecule in the sample, the molecule absorbs this energy, which in turn increases the vibrational amplitude of the absorbing dipoles. However, regardless of the increase in amplitude, the frequency of the absorbing vibration remains constant. Another name for the dipole model from Figure 1 is an ideal harmonic oscillator. The frequency at which the dipole (or ideal harmonic oscillator) vibrates (stretches or bends) is dependent upon the bond strength and the masses of the atoms bonded together. When the harmonic oscillator (HO) vibrates, the vibrational energy is continuously changing from kinetic to potential and back again. The total energy in the bond is proportional to the frequency of the vibration. The use of Hooke's law (in our case referring to the elasticity properties of the HO), is applied to illustrate the properties of the two atoms with a spring-like bond between them. The natural frequency of vibration for a bond (or any two So we have light from a spectrophotometer striking a matrix of various molecules. If the molecules do not interact with the light then the light passes through the matrix with no interaction whatsoever (never mind about such matters as scattering, refractions, reflectance and absorption losses; we will get to those next year). However, if molecules interact with the light in very specific ways we refer to them as active or infrared active. (In the case of NIR we are most interested in X-H bonds, i.e. N-H, C-H and O-H hydroxyl stretching and bending.) Infrared active molecules can be seen as consisting of mechanical models with vibrating dipoles. Each dipole model vibrates with a specific frequency and amplitude as shown in the simple model below in Figure 1. where 1i = Planck's constant (or 6.6256 x 10-27 erg-sec; and v is the frequency of light (or the number of vibrations per second or in units of S-1). Thus the energy for any specific photon can be quantified (don't act so surprised!), and it is this energy which interacts with the vibrating bonds within infrared active molecules.