Knudsen cell mass spectrometry using restricted molecular beam collimation. I. Optimization of the beam from the...

This study analyzes molecular beam sampling by mass spectrometry by the Knudsen method using the so-called " restricted collimation device" . This device, defined by the field and source apertures, was proposed in order to eliminate any additional contribution to the genuine molecular beam of surface vaporizations coming from the vicinity of the effusion orifice as usually detected by the ion source of a mass spectrometer.

The molecular transmission of the " restricted collimation device" was calculated using a vaporization law under vacuum taking into account the real surface where the molecules are emitted, i.e., the sample evaporation surface in the Knudsen cell or the effusion orifice section, towards the ion source inlet by integration of elementary solid angles.

An optimum is observed depending on the pair of selected apertures that define the restricted collimation device, i.e., the field and source apertures. This optimum is different from that previously calculated when taking into account only the solid angle, as defined by the restricted collimation device.

This difference is attributed to the previously approximate assumption that optimizing the restricted collimation solid angle automatically optimizes the sampling of the effused beam included in the restricted collimation angle. Moreover, the location of the evaporating surface for molecules traveling through the collimation device towards the ionization chamber remains an important factor: the distance between the sample evaporation surface and the field aperture is of paramount importance as it changes the molecular transmission to the mass spectrometer or to any target collection device in the conventional Knudsen method.