Characterization of the supersonic expansion in the vacuum interface of an inductively coupled plasma mass spectrometer by high-resolution diode laser spectroscopy

The supersonic expansion in the first vacuum stage of an inductively coupled plasma mass spectrometer has been characterized by laser-induced fluorescence of metastable argon atoms in the expansion. Atom velocities and temperatures were determined from Doppler shifts and linewidths, respectively, in the excitation spectra of the argon atoms. Shock structures characteristic of a supersonic expansion, the barrel shock and the Mach disk, were manifest as bimodal velocity distributions. The terminal velocities reached by the atoms were characteristic of conditions in the plasma source upstream from the entrance to the vacuum interface.