Mass spectral fingerprinting of chemical weapons convention-relevant sulfur–oxygen heterocyclic alkylphosphonate esters by EI and PCI GC-MS

Reliable detection and unequivocal identification of chemical warfare agents, their precursors, and degradation products are central to verification activities under the Chemical Weapons Convention (CWC). Cyclic alkylphosphonate esters containing phosphorus-sulfur-oxygen heterocyclic frameworks are particularly underrepresented in mass spectral libraries, hindering confident identification. In this study, six CWC-relevant cyclic alkylphosphonate esters-2-alkyl-1,3,6,7,2-dioxadithiaphosphonane-2-oxides and 2-alkyl-1,3,6,8,2-dioxathiamethanphosphodane-2-oxides bearing methyl, ethyl, and isopropyl substituents were microsynthesized and systematically characterized using gas chromatography-mass spectrometry under electron ionization (EI) and positive chemical ionization (PCI) conditions. Under EI conditions, all compounds exhibited low-abundance molecular ions (2-8%) accompanied by highly reproducible and structure-specific fragmentation. Nine-membered disulfide derivatives consistently produced a dominant base peak at m/z 85, attributed to stabilized dihydrothiophene fragments, whereas methylene-bridged ten-membered analogs showed characteristic base peaks at m/z 123 or 125, reflecting enhanced ring stability. Across the series, prominent fragment ions at m/z 118 (58-70%) and alkyl-dependent ions confirmed homologous fragmentation behavior. Positive chemical ionization using isobutane generated clear protonated molecular ions ([M + H]⁺) at m/z 215-257 for all six compounds, enabling unambiguous molecular weight confirmation with minimal fragmentation. Gas chromatographic retention indices ranged from 1774 to 2043 and increased systematically with alkyl chain length, providing complementary chromatographic identifiers. The combined EI/PCI mass spectral fingerprints and retention index data generated in this work significantly expand reference datasets for cyclic alkylphosphonate esters and directly support reliable identification of CWC-related compounds in environmental, forensic, and Organization for Prohibition of Chemical Weapons proficiency-testing contexts.