A microchannel thermalization inlet design to reduce molecular fragmentation in orbital and flyby closed-source mass spectrometers. (August 2019) - Turner, Brandon M
Abstract: Closed-source mass spectrometers rely on thermalization of neutral molecules, which are intercepted at a high velocity relative to the spacecraft. However, encountered molecules generally impact with enough kinetic energy to drive chemical modification, obscuring the identity of native compounds. We describe a novel inlet design that reduces dissociation and other chemical changes to sampled species by quenching the impact energy faster than the dissociation process. The inlet consists of a parallel array of microchannels. Impinging molecules experience the same number and type of thermalizing collisions as in a conventional closed-source inlet, but the process is several orders of magnitude faster than it is in prior designs due to the short distance between successive impacts. Preliminary calculations using the representative molecule hexane show that the lowest pathway to dissociation is breaking one of the carbon-carbon bonds and that vibrationally excited neutrals survive intact only a short time after the initial impact. The ab initio and density functional theory calculations described here show that lifetimes of impact-induced, vibrationally excited states depend on impact velocity, molecular weight, and molecular bonding and for hexane are in the range of 10 −4 to 10 −10 s for encounter velocities of 9–13 km/s, assuming a translation-to-vibration energy conversion of 14%. For all molecules, the microchannel thermalization inlet allows encounter velocities at least 1.25 times higher than a conventional thermalization inlet for a given level of fragmentation. With hexane, for instance, fragmentation in the microchannel inlet is negligible at velocities of 11 km/s, whereas a conventional inlet starts experiencing fragmentation of hexane at 8.5 km/s. Ram pressure enhancement is maintained using this novel inlet, preserving the improved sensitivity of closed-source designs. The microchannel thermalization inlet reduces all types of impact-induced chemical changes, including racemization, isomerization, and rearrangement, for any encounter velocity. Highlights: High vibrational energy in neutral molecules causes widespread fragmentation. Antechambers in spacecraft closed ion sources do not dissipate this energy quickly. The new microchannel inlet has a shorter time between successive collisions. These rapid collisions thermalize energetically excited neutrals quickly. This rapid dissipation of energy causes reduced fragmentation of neutral molecules.