MASS-SPECTROMETRIC SAMPLING OF IONS FROM ATMOSPHERIC-PRESSURE FLAMES .4. SCATTERING PROCESSES IN MOLECULAR-BEAMS FROM SUPERSONIC EXPANSIONS

In a study of sampling, ions have been extracted from a variety of flames and analysed mass spectrometrically. For this, a flame sample was expanded continuously and supersonically into a vacuum, and an ion beam formed for mass analysis. Observations of ion currents for various pressures in the first chamber demonstrate that ions are scattered there by neutrals after the sample has changed from continuum to molecular flow. The loss of ions is greatest for large weakly-bonded ones and least for small atomic ions, but can be as high as 95%. However, this often does not lead to a significant distortion of a mass spectrum and the instrument only becomes noticeably less sensitive to the larger ions. The attenuation is also found to be greatest with large sampling holes. Two distinct processes, viz., beam and background scattering, are found to responsible. In the former, ions are scattered by those neutrals that enter the vacuum chamber with the ions; in the latter, which is less important, neutrals from the background gas penetrate and collide with species in the ionic/molecular beam. We conclude that scattering cross-sections of flame ions and neutrals have gas kinetic magnitudes. These experiments have also provided information on the molecular beams resulting from the sampling of atmospheric pressure flames. For instance, it is clear that the transition from continuum to molecular flow is not characterised by a particular Knudsen number, such an unity, being attained. Instead, the transition surface on the axis is always located some ten orifice diameters downstream of the nozzle throat, i.e., at a fixed terminal Mach number, and so is probably characterised by the collision frequency becoming too low for energy transfer to occur. Also, it proved possible to establish that ambient molecules penetrate the beam at roughly 145 nozzle diameters down the expansion, resulting in background scattering. © 1979.