**1. Introduction**

Spectroscopy measures and interprets phenomena of absorption, dispersion or emission of electromagnetic radiation that occur in atoms, molecules and other chemical species. Absorption or emission is related to the energy state changes of the interacting chemical species which characterise them, which is why spectroscopy may be used in qualitative and quantitative analysis.

The application of spectroscopy to chemical analysis means considering electromagnetic radiation as being made up of discrete particles or quanta called photons which move at the speed of light. The energy of the photon is related to the wavelength and the frequency by Plank's constant (h = 6.62 x 10-34 J second) and the speed of light in a vacuum (c = 3 x 108 m/s) according to the following equation (Skoog et al., 2001):
