Mechanisms of ionisation and atomisation of barium in graphite furnace atomic absorption spectrometry

Abstract

The mechanisms of ionisation and atomisation of barium in graphite furnace atomic absorption spectrometry were studied, from the viewpoint of activation energies for ion and atom formation, and the effect of additions of alkali metals to the analyte solution on sensitivity of barium depending on pyrolytic graphite (PG) and non-pyrolytic graphite (NPG) tubes. It was found that the addition of caesium chloride to the barium solution suppresses the barium ionisation for the NPG tube but not for the PG tube as the caesium in the PG is dissipated before the ionisation of barium begins, while for the NPG tube it was present in the tube during the ionisation of barium. An activation energy (E_a) of 770 kJ mol^–1 for the ionisation of barium was obtained and it was attributed to an ionisation mechanism involving barium carbide, unlike the atomisation with an E_a value of 540 kj mol^–1 which correlates with the breakage of the Ba–O bond. The reason why the ionisation suppressant does not lead to a consequent increase in the atomic absorption signal was explained in terms of the differing mechanisms for the atomisation and ionisation of barium.