Chemical ionization in the atmosphere? A model study on negatively charged “exotic” ions generated from Titan's tholins by ultrahigh resolution MS and MS/MS

Titan tholins generated by complex processes (including ion–molecule reactions) in a laboratory plasma were investigated by ultrahigh resolution MS and tandem MS/MS measurements. Titan has a special interest in astrobiology because “in situ” measurements by the Cassini–Huygens spacecraft indicate the presence of complex organic molecules of prebiotic interest. The present work focuses on negatively charged ions that have not been systematically studied by ultrahigh resolution MS and MS/MS. The negatively charged ions were generated from a tholin sample by both laser desorption ionization (LDI) and electrospray ionization (ESI). The chemical compositions determined for the negatively charged ions clearly indicate the presence of highly unsaturated (H/C < 1) species with high nitrogen content (presumably related to multiple cyano functionalities). This is characteristically different from the previously analyzed positively charged ions that are more saturated and contain amino and imino functionalities. Based on tandem MS/MS experiments and quantum chemical calculations we propose characteristic structural features for selected ions. They include open chain (C6N3−) and aromatic ring structures (C10N5−). The basic non-aromatic structural unit C2N3− seems to play an important role and several structural “families” can be derived as HCN, HCCH and H2 “adducts” of this ion.

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► A systematic FT-ICR and LTQ-Orbitrap ultrahigh resolution MS and MS/MS study is presented.
► ESI and LDI generated negative ions from “tholin” samples were studied.
► The negative ions are highly unsaturated and contain cyano functionality.
► Structurally related species (families) were proposed.
► The current study may have relevance to Titan's atmospheric chemistry.