Capillary Electrophoresis Coupled to Mass Spectrometry for the Detection of Organics in High Salinity Samples Relevant to Ocean Worlds

Ocean worlds in our Solar System have captivated the attention of scientists due to the presence of liquid water that could make it possible for these worlds to harbor life. Because all life on Earth is built from a selected set of organic molecules, clear patterns appear in the relative distribution of organics when a sample has a biotic origin.  A powerful approach in the search for life involves seeking for such chemical patterns. The liquid-based separation technique of capillary electrophoresis (CE) holds unique promise for this task. CE is a high-resolution separation technique for molecules in solution that allows the analysis of a broad range of compounds using a relatively simple instrumental set up. CE separations occur within small diameter glass capillaries (25-100 mm I.D.) filled with a background electrolyte. CE is an ideal candidate for in situ planetary missions, especially to areas where aqueous analysis is required. 

Although CE can be coupled to multiple detectors, mass spectrometry (MS) is particularly attractive for planetary exploration because it adds another separation dimension based on mass-to-charge (m/z) ratios. Although there are multiple ionization techniques to couple CE to MS, the most common one is electrospray ionization (ESI). With ESI, the compounds that are separated by CE can be efficiently transferred from the liquid phase into the gas-phase. The coupling of CE and MS allows detailed characterization of biomolecules, and more importantly the identification of unknowns in complex mixtures. We have recently reported on the development of a CE instrument that can be coupled to multiple detection systems, including MS ¹. Other detectors include laser-induced fluorescence for sensitive analysis of amino acids and contactless conductivity detection for analysis of inorganic ions and organic acids. This system is under development for biosignature detection as part of the Europan Molecular Indicators of Life Instrument (EMILI)² and the Ocean Worlds Life Surveyor (OWLS).

Based on the major constituents potentially expected in the oceans of Enceladus and Europa, we used NaCl and MgSO₄ salts to evaluate the effect of Na⁺, Mg²⁺, Cl^-, and SO₄^2- on the detection of a wide range of organics by CE-MS using a sheathless interface ³. We have selected a mixture of amino acids, peptides, nucleosides, and nucleobases for this study, all of which are building blocks of the main polymers of terrestrial biology and are associated with at least one of the rungs of the Ladder of Life. We demonstrate CE-MS limits of detection for these organics ranging from 0.05 to 1 mM (8 to 8 ppb), in the absence of salts. More importantly, organics in the low mM range (1 to 50 mM) are detected by CE-MS in the presence of 3 M NaCl without desalting, preconcentration or derivatization ³. The applicability of CE-MS for analysis of challenging natural samples was demonstrated by analysis of samples from Mono Lake. Multiple organics were detected in the sample despite the presence of a salt front. These results demonstrate the potential of CE-MS for in situ organic analysis on future missions to ocean worlds.