Comprehensive Insights into Organic Matter from Astrophysical Ice Analogues by Multimodal Ionisation High-Resolution Mass Spectrometry

Advancing the limits of molecular characterisation is essential in astrochemistry to elucidate the mechanisms involved in the formation and evolution of organic matter during the early stages of the Solar System. In this work, we employed a multimodal ionisation approach to extend the description of the organic diversity present in residues formed during the VUV processing of interstellar ices analogues composed of H2O-CH3OH-NH3 mixtures. The soluble organic fraction was extracted with methanol and analysed with high-resolution FT-ICR mass spectrometry, using electrospray ionisation (ESI) and atmospheric pressure photoionisation (APPI), both in positive- and negative-modes. About 15,000 distinct molecular formulas were detected, enhancing the characterisation of such samples from a factor of three. While ESI offered the most comprehensive molecular coverage, detecting CHO, CHN, and CHNO species, APPI in positive-mode revealed additional, previously undetected compounds. These newly observed assignments were mainly heteroatom-poor and unsaturated molecules, including low-oxidised and aromatic CHO structures, as well as CHN molecular formulas consistent with N-heterocyclic features. In contrast, APPI (-) proved to be limited for characterising these samples. On average, the residue exhibits typical characteristics: H/C = 1.68; O/C = 0.33; N/C = 0.31; m/z 419.76046; Formula = C19H31N5O5; Double Bond Equivalent (DBE) = 7; Aromaticity Equivalent (Xc) = 1.93. Present multimodal ionisation approach provides new insights into the chemical diversity of soluble organic matter analogues and highlights the complementary strengths and limitations of each ionisation mode, thereby contributing to enhance our understanding of the organic diversity observed in astrophysical environments.