Detection of the amino acid histidine and its breakup products in hypervelocity impact ice spectra

Impact ionization time of flight mass spectrometry (TOF-MS) instruments study molecular composition of space-borne dust grains by impacting them at several km/s. The kinetic energy of the impact ionizes molecules from the dust grain, allowing them to be characterized with TOF-MS. The ability of these instruments to assess the inventory and distribution of organics throughout the solar system is important for understanding habitability across planetary bodies and the origins of terrestrial life. In particular, it would be beneficial to know whether this type of instrument can successfully detect amino acids, or other organics necessary for life that may be found on potentially habitable ocean worlds such as Europa. However, there have been questions about whether the fast impact ionization processes can shatter complex organic molecules before they can be studied, and if there is some critical flyby velocity below which fragmentation can be mitigated. Here we describe a set of experiments using a novel airbrushing technique for ice creation at the Colorado Dust Accelerator at the Institute for Modeling Plasmas, Atmospheres, and Cosmic Dust (IMPACT). Experiment 1 used a surface of pure histidine-monohydrochloride, and Experiment 2 used an identical histidine sample, except with a 60 nm water ice layer vapor deposited on top. These surfaces were kept at ∼80 Kelvin and impacted by <2μm-diameter iron dust particles at velocities ¿ 3 km/s. The resulting impact plumes were studied using TOF-MS, and the ion yields of fragmentation products relative to the parent molecule were measured. Direct comparison of the breakup products for each experiment show that water ice layers significantly reduce fragmentation rates, both in absolute terms and as functions of velocity. We find that for the bare amino acid, the fragmentation rate rises significantly beyond 6.1 km/s, while the ice-shielded amino acid shows significant increases beyond 8.5 km/s. Furthermore, observed fragmentation species correlate with those produced by published results from electron ionization experiments. In particular, we observe characteristic breakup products for histidine at masses 81–83 and 110 AMU, the dominant breakup products associated with histidine in the existing literature.