Fundamental studies of the plasma extraction and ion beam formation processes in inductively coupled plasma mass spectrometry

摘要

The fundamental and practical aspects are described for extracting ions from atmospheric pressure plasma sources into an analytical mass spectrometer. Methodologies and basic concepts of inductively coupled plasma mass spectrometry (ICP-MS) are emphasized in the discussion, including ion source, sampling interface, supersonic expansion, skimming process, ion optics and beam focusing, and vacuum considerations. Some new developments and innovative designs are introduced;The plasma extraction process in ICP-MS was investigated by Langmuir measurements in the region between the skimmer and first ion lens. Electron temperature (T e) is in the range 2000-11000 K and changes with probe position inside an aerosol gas flow. Electron density (n e) is in the range 108-1010 cm-3 near the skimmer tip and drops abruptly to 106-108 cm-3 downstream 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;Axially resolved distributions of electron number density and electron temperature were obtained to characterize the ion beam at a variety of plasma operating conditions. The electron density dropped by a factor of 103 along the centerline between the sampler and skimmer cones in the first stage and continued to drop by factors of 104-105 downstream of skimmer to the entrance of ion lens. The electron density in the beam expansion behind sampler cone exhibited a 1/z2 intensity fall-off (z is the axial position). A second beam expansion originated from the skimmer entrance, and the beam flow underwent with another 1/z2 fall-off behind the skimmer. Skimmer interactions play an important role in plasma extraction in the ICP-MS instrument;The measured floating voltages (V f) are in the range of +2 to +5 volts. V f generally decreases downstream of sampler and through the skimmer. Negative values of Vf were measured at downstream of the skimmer. RF amplitude shows a net positive potential in supersonic expansion. The plasma \u22rectification effect\u22 and the \u22calorelectric effect\u22 on the plasma potential measurements were discussed. The drop in V f near the skimmer may be caused by a disturbance in front of the skimmer. ftn1This work was performed at the Ames Laboratory under contract No. W-7405-ENG-82 with the U.S. Department of Energy. The United States government has assigned the DOE report no. IS-T 1713 to this dissertation.