High resolution imaging of barium ions and atoms near the sampling cone of an inductively coupled plasma mass spectrometer

Planar laser-induced fluorescence was used to map density distributions of ground state barium atoms, ground state barium ions, and excited-state barium ions in the region between the load coil and the sampling cone of an inductively coupled plasma mass spectrometer. The effects of power, nebulizer gas flow rate, and the addition of lithium to the sample on the distributions were studied. The maps reveal that the radial distributions of atomic species across the diameter of the plasma are compressed as the plasma is drawn into the sampling orifice, and that as a result of that compression, the distribution of ions across the 1-mm diameter of the sampling orifice is non-uniform. The distribution changes as conditions in the plasma change. The skimmer cone that separates the first and second stages of the differentially-pumped vacuum interface transmits ions exclusively from the center of the distribution that exists at the sampling cone. As a result, the overall efficiency with which ions are transmitted through the vacuum interface varies as conditions in the plasma change.