BUCHI NIRCal - Manual (Page 10)

NIRCal 5.5 Software Manual
10 NIRCal 5.5 Manual, Version A
1
Introduction
1.1 Introduction to NIR
1.1.1 Spectroscopy
Light is a fast time dependent sequence of electric and magnetic fields propagating in space.
Light can be characterised with physical properties, like:
frequency:
wavelength:
wavenumber:
velocity of
propagation:
energy:
A summary of the electromagnetic spectrum can be seen in the Picture below. NIR lies between the
visible and middle infrared range. With this light, the molecular vibrations are activated, similar to the
infrared range.
Most of the molecule vibrations take place at a characteristic frequency which lies between 10
12
to
10
14
Hz. The osculating molecule interacts with the electrical field or light when the frequencies are the
same.
Hz Infrared Light
Not only are the basic vibrations absorbed with very high degree of excitation in the infrared range, but
also frequencies 2 or 3 times higher. The harmonic frequencies are absorbed with a lower degree of
excitation and lie in the near infrared range.
Using an infrared spectrum, the characterisations of molecules is possible. In the NIR spectra region
mainly the overtones and combination tones of -CH, -OH and -NH groups are absorbed, so NIR is
suitable mainly for organic substances.
The absorption bands are very broad and they often overlap, which can cause problems by direct
interpretation of the spectra. The most often used evaluation method in spectroscopy, the Lambert-