FUNDAMENTAL-STUDIES OF THE SAMPLING PROCESS IN AN INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETER .4. REPLACEMENT OF THE INDUCTIVELY COUPLED PLASMA WITH A HELIUM MICROWAVE-INDUCED PLASMA

A fundamental investigation was performed to assess the feasibility of replacing the argon source in an inductively coupled plasma mass spectrometer (ICP-MS) with a helium plasma source. For this study an argon ICP-MS interface was modified to sample a helium plasma. However, despite efforts to increase pumping efficiency in the mass spectrometer first stage, the conventional ICP-MS interface could not be reconfigured for optimal skimming of the first-stage free-jet expansion as gas dynamic theory advocates. Moreover, coulombic forces produced by ion-beam space charge after the skimmer cone were found to influence ion movement more strongly than gas-kinetic forces. These conclusions were based on kinetic energies measured across the mass range for ions as they exited the first vacuum stage of the interface. Because of the importance of these coulombic forces on ion movement, gas-kinetic factors need not be a primary consideration in skimming a helium free-jet expansion.