FUNDAMENTAL-STUDIES OF THE SAMPLING PROCESS IN AN INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETER .3. MONITORING THE ION-BEAM

The influence of gas dynamic and electrostatic forces on the ion beam that passes through an ICP-MS interface has been studied. Results from these experiments show that electrostatic forces overpower gas-kinetic processes in the molecular-flow regions of the interface. Because ion movement in these regions cannot be predicted by gas dynamic theory alone, optimal skimming conditions must be determined on an empirical basis. Those parameters that were found to influence ion-beam formation are first-stage pressure, solvent load, and inner-gas flow rate. Also studied was the influence of atomic mass on dispersion within the ion beam. Elements of mass greater than the bath-gas species (argon) were found to travel with the ion beam, whereas lighter elements tended to be dispersed off axis. Importantly, however, dispersion of these lighter ions could be eliminated by reducing ion-beam space charge.