Chemical interferences on the atomization yield of high reduction potential elements : Signal suppression in the plasma source spectrometry
In inductively coupled plasma spectrometry, signal suppression by acids has been recognized for a long time. It is always explained as the physical effects on the sampling processes, plasma conditions, and aerosol properties. In some cases, signal suppression apparently relates to the chemical properties of element. However, there was no publication discussed this phenomenon. In this work, it has been proven that mercury signal suppression depends on the plasma properties and nitric acid concentration. It relates to the atomization yield of mercury compounds, which is governed by the red-ox equilibrium in the central channel of plasma. Mercury complex formation equilibrium has been proven to exert in the heated droplets in plasma. The equilibrium state is governed by the complex formation with ammonium nitrate, the complex dissociation by nitric acid, and the heat obtained from plasma. Mercury halogen complex is not sensitive to the changes in acidity. Therefore, it is able to keep mercury free from nitric acid interferences and unify mercury atomization in the central channel of plasma. Similar to mercury, the uncertain signal and memory effect of platinum group elements and gold mainly arises from the chemical interference on the atomization efficiency. It not only depends on nitric acid concentration, but also on the standard reduction potential (Eo) of precious metal species in solution. Chemical interference on the spectrometric signals is governed by the robustness of the plasma, the capabilities to ingest aerosol and to protect foreign disturbance. The robustness of plasma is not equal to the excitation conditions. It has been observed that nitric acid has no significant interferences on mercury signal intensity of MIP-OES. When wet aerosol enters into the resonant cavity, it is completely desolvated in the powerful electromagnetic field. Liquid phase instantly disappears from the plasma observation zone. No doubt, the chemical equilibrium does not exist. In conclusion, chemical reactions exist in the plasma and interference the spectrometric signal. The best way to overcome the chemical matrix interferences in the plasma source spectrometry is to realize the controllable atomization.