Mass spectrometers have proven to be vital to understanding the Solar System and the planets within it. Spacecraft containing mass spectrometers have been sent to numerous remote places and have determined important information about the atmospheric composition of Venus, Earth, Mars, Jupiter, and Saturn, along with other celestial bodies. Such results have shown a variety of small neutral molecules, such as CH4 NH3, H2O, CO2, and CO, neutral radicals such as atomic O, H, and N, and a host of small ions, such as H+, N+, and NH4+. Closed ion source inlets, which allow for the detection of these small neutral molecules, contain a spherical antechamber that allows the neutrals to thermalize with the walls of the chamber through many successive collisions before they are introduced into the ionization region of the spacecraft mass spectrometer. These collisions, however, energetically excite neutral molecules and lead to many chemical changes, such as racemization, ionization, or even dissociation. When these changes occur, smaller neutrals can be produced, even if they were not in the original sample from the atmosphere or surface. As a result, the determination of the true composition of an atmosphere or a surface is cast into doubt. Herein is given a brief description of mass spectrometry in space research and how the closed ion source has greatly assisted this process. Dissociation and other chemical changes caused by the high velocity impacts that occur in closed source antechambers is also addressed. A theoretical approach to understanding such dissociative processes that occur after high energy collisions in closed source antechambers is described and undertaken. Chapter 2 describes a proof-of-concept study using hexane as a representative molecule and determines the velocity at which widespread dissociation of hexane molecules is likely to occur in closed source antechambers. This same theoretical process is then utilized in Chapter 3 with many more members of the n-alkane family to probe what effect molecular weight has on the amount of dissociation. Alkanes of both higher and lower molecular weight than hexane (C6H14) are used to show the effect as a function of molecular weight. In all cases, it was found that the velocity at which half of the incoming neutral n-alkane molecules dissociate is roughly the same for all molecular weights studied. This result is then applied to current and future space research through a proposed hardware solution, which will reduce the amount of dissociation and a discussion of how this effect may be seen in the results obtained from future mission instruments. Lastly, future work with different molecular weights and with successive collisions (the second, third, fourth, etc.) is described. This future work will further expand the present study to show how different functional groups, which may be partly responsible for higher-than-expected levels of NH3 and CO2, are affected after a high velocity, high energy impact.
College and Department
Physical and Mathematical Sciences; Chemistry and Biochemistry
BYU ScholarsArchive Citation
Turner, Brandon M., "High-Velocity Impact Dissociation of Molecular Species in Spacecraft-Based Mass Spectrometers" (2022). Theses and Dissertations. 9672.
homolytic bond cleavage, radical formation, high-velocity impacts, closed ion source inlets, spacecraft mass spectrometers