Plasma temperature from ion kinetic energies and implications for the source of diatomic oxide ions in inductively coupled plasma mass spectrometry
Ion kinetic energies have been measured for a suite of atomic ions and diatomic oxide ions as a function of nebulizer flow for an inductively coupled plasma mass spectrometer. The mass dependence of the ion energies is interpreted to yield estimates for the gas kinetic plasma temperature. The results provide a temperature profile for the plasma through the region of significant ion density. The plasma temperature derived from the atomic ion kinetic energies falls through this region from a high of about 6800 K at low nebulizer flows to about 3300 K at high flows. The plasma temperature derived from the oxide ion energies is lower than that derived from the atomic ion energies by about 500 K at most nebulizer flow rates. The results are discussed in terms of the source of the oxide ions. It is argued that the dominant source of the observed oxide ions is not within the interface which extracts the plasma into the mass spectrometer. The results are consistent with a model that envisages the plasma as a turbulent reactor containing pockets of cooler gas in which oxide ion formation and persistence are promoted. They are also consistent with a model that associates the oxide ions with the cooler region in the vicinity of vaporizing droplets and particles.