Optical emission spectrometry: spectral theory
ICP-OES: inductively coupled plasma – optical spectrometry

Each element in the periodic table has a given number of electrons. Each electron, according to quantum theory, has a probability of being located on one of the layers and sub-layers around the nucleus. This theory was introduced by Planck, who assumed that electromagnetic energy was emitted (or absorbed) in discrete values, meaning that energy is discontinuous. In the case of electrons in a free atom, four quantum numbers are used to define the energy state of an electron:

• the principal quantum number n (n = 1 to 7 for atoms in the ground state) which corresponds to the layers called K, L, M,..., Q ;

• the orbital angular momentum $$