Constraining oxygen isotope exchange kinetics between organic compounds and water using electrospray ionization Orbitrap mass spectrometry, and implications for the oxygen isotopic compositions of meteoritic organics

Abstract

Oxygen isotopic composition of organic compounds is an important tracer of processes and environmental conditions, with applications ranging from prebiotic organic synthesis in the early solar system to paleoclimate reconstruction. To meaningfully interpret the oxygen isotopic signature of such molecules, we need to consider their rate of oxygen exchange with water, e.g., during aqueous alteration on asteroidal parent bodies or residence on the Earth. In this study, we experimentally constrained the kinetics of oxygen isotope exchange between water and ketones (acetone, cyclopentanone) or carboxylates (acetate, butyrate) near neutral pH between 274.7–373 K. We incubated them in ¹⁸O rich water, and measured their ¹⁸O/¹⁶O ratios after incubation using an electrospray ionization Orbitrap mass spectrometer. While ketones completely exchanged oxygen with water within hours, carboxylates achieved only up to 8 % oxygen exchange after days of incubation. The time evolution of their ¹⁸O/¹⁶O were consistent with first order kinetics, with exchange rate constants of 1.99 (±0.1) × 10⁻⁴ s⁻¹ (acetone), 6.55 (±0.23) × 10⁻⁵ s⁻¹ (cyclopentanone), 3.9 (±0.1) × 10⁻⁸ s⁻¹ (acetate), 3.9 (±0.2) × 10⁻⁸ s⁻¹ (butyrate) at 298 ± 1 K and activation energies of 14.47 ± 0.43 kcal/mol (acetone), 20.35 ± 1.47 kcal/mol (cyclopentanone), 3.53 ± 0.26 kcal/mol (acetate), 2.81 ± 0.43 kcal/mol (butyrate). Rapid exchange for ketones is explained by their hydration in aqueous media, and implies that ketones extracted from meteorites should have completely re-equilibrated with the last water they were in contact with over periods of hours or more (possibly at asteroidal parent bodies, or by exposure to terrestrial meteoric or laboratory waters). Carboxylic acids are resistant to exchange due to electrostatic repulsion between the hydroxyl ion and carboxylate ion in basic media. Furthermore, we predict that significant oxygen isotopic exchange between meteoritic insoluble organic matter (IOM) and aqueous fluid is likely during aqueous alteration, based on rate constants reported in this study and previous studies and constraints on the bonding environments of oxygen in IOM.