LANGMUIR PROBE MEASUREMENTS OF ELECTRON-TEMPERATURE AND ELECTRON-DENSITY BEHIND THE SKIMMER OF AN INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETER

A single Langmuir probe is inserted axially into the region between the skimmer and first ion lens of an inductively coupled plasma mass spectrometer (ICP-MS). Reasonable current-voltage (I-V) curves are obtained with an axial spatial resolution of approximately 5 mm. Electron temperature (T(c)) is in the range 2000-11 000 K and changes with aerosol gas flow rate and water loading in a similar way as did previous T(e) measurements with the probe inside the supersonic jet. Electron density (n(c)) is in the range 10(8)-10(10) cm-3 near the skimmer tip and drops abruptly to 10(6)-10(8) cm-3 as the probe is retracted further behind the skimmer. Electron density in the beam leaving the skimmer also depends on water loading and on the presence and mass of matrix elements. These results support the general premise that the properties of the ion beam are affected by a matrix element, which is thought to cause matrix interferences in ICP-MS.