Testing of Ion Exchange Solid Phase Extraction Media for Extraterrestrial In Situ Sample Preparation on Liquid Samples

Abstract

The search for life on other bodies in our solar system is currently focused on Ocean Worlds, those bodies known to contain liquid water, as water is one of the requirements for life as we understand it. In the search for organic biosignatures that would indicate the presence of past or current life, liquid samples from these bodies would utilize an initial sample preparation step in which interfering substances such as salts can be removed before analysis. Previous work on potential sample preparation techniques for these samples evaluated solid phase extraction (SPE) on cation exchange media, but only explored amino acid analytes at low salt to analyte ratios and used high concentration eluents. This work utilized mixed-mode ion exchange solid phases, developing methods for elutions in low concentration solutions and evaluating polar analytes with a range of functionalities. Method development revealed the need for a pre-elution step in the process to decrease the elution volume required. Both anion and cation exchange media were evaluated for the capture of analytes from solutions that simulated Earth’s oceans. The Oasis MCX and Strata X-C cation exchange media provided the best results, with >90% retention of all analytes including amino acids, organic amines, nucleotides, peptides, and an oligonucleotide. These cation exchange media retained even anionic components, including glutamic acid and organic acids, with >90% efficiency. These analytes were released in the wash step, but salt ion removal was completed before release, allowing this technique to be used for desalting of these analytes. Extraction of a 14 component mixed analyte solution also showed retention of all analytes, with testing of analyte concentrations down to 100 nM in 35 g/L simulated ocean solution. Oasis MAX and Strata X-A anion exchange media did not retain glutamic acid, fumaric acid, or dipicolinic acid when the salt concentration was high; these anionic analytes were easily extracted from low salt solutions. The anion exchange media showed a range of functionality for extracting other analytes from the simulated ocean water, capturing >90% of tryptophan and phenylalanine, but retaining <50% of the valine and >20% of the glycine. Irradiation exposure, as a model for the solar irradiation expected during deployment to extraterrestrial locations, did not affect the performance of any ion exchange media tested. A reversed-phase SPE column was directly coupled to a cation exchange column, to investigate removal of nonpolar compounds that might bind to and block the mixed-mode ion exchange media.