Optimizing a Two-Step Laser Time-of-Flight Mass Spectrometer for In Situ Astrobiology Investigations

The ability to acquire spectral measurements of geologic samples on other terrestrial bodies in situ is becoming increasingly important, especially in instances where sample return is impractical. I conducted an investigation into the capabilities of a two-step laser desorption time-of-flight mass spectrometer (L2MS) for in situ organic detection, which could be used on future landed missions to planetary bodies including Mars, Europa, Enceladus, Titan, or NEOs, to inventory the surficial organic species. I used complementary data sets to inform and optimize the L2MS to build the phase space of optimal instrument configurations and organic species. This is a crucial step for the detection of biosignatures on planetary surfaces using this innovative and promising space technology for the detection of fragile biomarkers. Developing an L2MS for non-destructive organic detection has helped address NASA technology roadmap objectives through the evaluation of biomarker detection results of several complementary instrumentation techniques. During my NSTR fellowship, I tested and optimized an L2MS for organic and biosignature identification. I tested several desorption and ionization laser configurations on a suite of organic and inorganic sample classes to build the phase space of the optimal L2MS laser configurations. I also applied several statistical algorithms to the data I obtained with the L2MS and existing, complementary instrumentation available at NMSU, including an acousto-optic tunable filter (AOTF) IR reflectance spectrometer, a laser-induced breakdown spectrometer (LIBS), a scanning electron microscope (SEM), an energy dispersive x-ray spectrometer (ERS), and an X-Ray Diffraction (XRD) & X-Ray Fluorescence Spectrometer (XRF) (available at NASA GSFC) to quantify and compare the effectiveness of these instruments as biomarker identification tools. To determine the optimal configuration of the L2MS for a specific biosignature or class of prebiotic organic molecules, I measured a suite of samples using several IR desorption wavelengths, which correspond to absorption regions in IR reflectance spectra collected by the AOTF IR point spectrometer. Specifically, I addressed the following key questions: • What combination of in situ techniques is most effective in identifying endogenic and exogenic organics within samples? • How can a mass spectrometer be designed, implemented, and optimized to analyze key organic materials, including fragile biomarkers, at a planetary body, while simultaneously minimizing spacecraft resource requirements? • How can statistical algorithms be applied to data obtained with several complementary in situ instruments to infer the presence of extant or extinct life? I answered these scientific questions through the following technology research contributions: • laser configuration: I determined the ideal desorbing and ionizing laser configurations by optimizing the timing of the ionizing laser relative to the desorption laser. The energetics of the desorption process and spatial distributions of various complex organics within a plume can vary based on properties of the sample and of the molecule itself. An experimental effort to characterize gas-phase distributions at the LDMS inlet is critical in optimizing this instrument for biosignature detection. • laser wavelength: I determined the optimal wavelength of the UV and IR lasers used for ionization and desorption. The degree of fragmentation for a complex organic species is likely influenced by the energy of the ionizing and desorbing lasers. In addition to adjusting the pulse timing, I tested 2.7–3.1 μm and 3.0-3.4 µm tunable desorption lasers, a 3.45µm breadboard laser, and a 266 nm Nd:YAG ionization laser (commercial and breadboard) to determine which are best suited for this application. • systematic sample suite investigation: I expanded the suite of sample classes, to include synthetic PAHs, amino acids, nucleobases, peptides, and meteoritic powders, and field samples that are more closely analogous to some planetary surface materials. By collecting data on several minerals, containing both synthetic organic dopants and natural organic constituents, I was able to test the sensitivity of the optimized laser configuration to sample class, mineral matrix, and microstructural effects that may reflect details of formation or depositional processes on a parent body.