New coil geometries for the study of secondary discharge in atmospheric-pressure helium inductively coupled plasma mass spectrometry

Several coil geometries are proposed to reduce the secondary discharge in atmospheric-pressure helium inductively coupled plasma mass spectrometry (He ICPMS) using a crystal-controlled 40-MHz generator. The effectiveness of the proposed geometries in controlling the plasma potential is investigated by the Langmuir probe method and ion kinetic energy (IKE) measurements. The influence of RF power on plasma gas temperature (Tg) is investigated through measurements of ICP-mass spectrometer interface pressure and IKEs for new and conventional coil geometries. Trends in plasma potential and Tg are well correlated, revealing that Tg is elevated (to ∼3500 K) at high power levels mainly as a consequence of the interaction between the ICP and the grounded sampler. The reduction of the secondary discharge results in lower Tg values (∼2600 K), necessitating the use of a membrane desolvator to remove water-related polyatomic interferences observed in the mass spectrum. Solvent removal also improves the sensitivity for bromine as a high-ionization-potential element by one order of magnitude compared to the previous studies.