Direct Analysis in Real Time Mass Spectrometry (DART-MS) Study of Amino Acids in the Vacuum Stable Liquids EMI-BF$_4$, EMI-Acetate, DEMA-OTf, DEMA-MsO, and Glycerol

Abstract

The detection of organics and potential biomolecules is an important goal of planetary science and astrobiology spacecraft missions, and many types of analytical chemistry for studying organics depend upon liquid solvents as working fluids. However, most liquids are difficult to maintain in the vacuum of space, complicating their use and increasing spacecraft mass and cost. Here we present a study using Direct Analysis in Real Time mass spectrometry (DART-MS) as a rapid screen for the detectability of histidine, glycine, arginine, lysine, and aspartic acid at mole fractions of about $5\times10^{-4}$ to $10^{-3}$ in five vacuum-stable liquids: EMI-BF$_4$, EMI-acetate, DEMA-MsO, DEMA-OTf, and glycerol. In all of the ionic liquids, the solvent cation dominated the spectra and no amino acid was detected, apart from a weak histidine signal in EMI-BF$_4$ at 350$^\circ$ C (median 5\% of the normalized peak intensity). The protic DEMA ionic liquids did not enhance detection, and their ionic components instead dominated the mass spectra. Aspartic acid was not detected in any liquid. In glycerol solutions, histidine, glycine, arginine, and lysine were all detected, and histidine was frequently a dominant line. We thus find glycerol to be the most promising of the liquids tested for organic detection with an ideal operating DART helium temperature of 350--400 $^\circ$C. We also found no clear evidence of water uptake or aqueous alteration in the ionic liquids, which was a secondary goal relevant to ionic liquid spacecraft electric propulsion.