The present paper is a review of recent works relating to the optical emission spectroscopy applied to the determination of the ground state densities of N, O and B atoms in Ar-N-2, Ar-O-2 and Ar-BCl3 flowing post-discharges. New results on the B-atom destruction probability (gamma = 0.4-0.5) on quartz walls are presented. The effect of small quantities of H-2 into Ar-N-2 and Ar-O-2 discharge on N and O atoms densities within the respective post-discharge has been analysed. Then, the authors demonstrate that the optical emission of Na in impurity may be applied to the determination of the vibrational temperature of N-2 (X, v) states at the usual temperatures for nitriding of metals in Ar-N-2 post discharges. From the diagnostic of Ar-N-2 post-discharges, it is clearly specified that the nitriding of iron base alloys in a flowing post discharge reactor is coming from N atoms, especially as a few H-2 is admired into the Ar-N-2 discharge. Finally, correlation between N-atom density and the thickness of iron nitrided layers are given as H-2 is introduced into the Ar-N-2 discharge.