Diagnostic studies of ion beam formation in inductively coupled plasma mass spectrometry with the collision reaction interface
This dissertation describes a variety of studies focused on the plasma and the ion beam in inductively coupled plasma mass spectrometry (ICP-MS). The ability to use ICP-MS for measurements of trace elements in samples requires the analytes to be efficiently ionized. Updated ionization efficiency tables are discussed for ionization temperatures of 6500 K and 7000 K with an electron density of 1 1015 cm-3. These values are reflective of the current operating parameters of ICP-MS instruments. Calculations are also discussed for doubly charged (M2+) ion formation, neutral metal oxide (MO) ionization, and metal oxide (MO+) ion dissociation for similar plasma temperature values. Ionization efficiency results for neutral MO molecules in the ICP have not been reported previously.
The collision reaction interface (CRI) removes polyatomic ions in ICP-MS by introducing a collision or reaction gas directly into the plasma expansion as ions are extracted into the mass spectrometer. The main gases implemented in CRI are helium and hydrogen. Experiments are described for the determination of the collision cross sections of singly charged cations by employing helium in a hexapole collision cell. Based on those determined cross sections, general characteristics of the CRI collision region are determined and discussed using helium as a collision gas. One concern with implementing H2 as a reaction gas via the CRI is the possible formation of monohydride ions (MH+, M = atomic ion). The formation of MH+ ions and the influence of increasing H2 gas flow through the CRI skimmer cone are discussed to provide insight into the source of MH+ ion formation in the CRI and plasma expansion regions.