SELINA Laboratory Impact Mass Spectrometry for Identification of Phospholipids and Other Organics in Dust from Enceladus

Ice dust emitted from Enceladus has been shown to be extremely informative source regarding the chemical composition of the subsurface ocean and its geochemical processes. When sampled by an impact ionization mass spectrometer during a flyby - without the need for a surface landing - these grains provide a means for particle-by-particle characterization. This approach avoids bulk sample dilution, allowing even rare molecules concentrated within a single particle to be detected, as exemplified by Cassini’s Cosmic Dust Analyzer (CDA). In preparation for ESA’s L4 mission, next-generation impact mass spectrometers are being considered to explore ocean chemistry, habitability, and astrobiology. The development and validation of these instruments require laboratory accelerated ice particles with archetypical biomolecules.

In this study, the Selected Ice Nanoparticle Accelerator (SELINA), coupled with a commercial Time-of-Flight (ToF) mass spectrometer, was used to accelerate 100–150 nm ice particles to terminal velocities of up to 3.6 km/s. Ice grains with embedded phospholipid membrane produced distinct single-impact mass spectra in negative mode. Mass assignment and mass-to-charge (m/z) calibration were supported by high resolution Orbitrap collision-induced dissociation (CID) studies of bare phospholipid and fatty acid ions.

At studied velocities the most intense peak corresponded to the PO₃^- ion with other prominent peaks included H₂PO₄^-, various phosphate fragments, fatty acids related ions, and phospholipid adduct. Additionally, impact ionization spectra of particles containing solvated fatty acids, phosphates, and acetates were recorded under similar conditions to better understand impact dynamics, specifically the notably low abundance of water clusters observed in negative mode.

The obtained results illustrate the imperative demand for high-resolution (Orbitrap-class)  impact ionization mass spectrometry probes, supported by extensive analogue databases, if complex biomolecules are to be successfully discovered during  ESA’s L4 mission.