GC‑EI‑MS and GC‑CI‑MS characterization of n-Alkenyl mono- and diesters of Alkylphosphonic acids with retention index correlations.

Microscale syntheses of thirty‑two n-alkenyl derivatives of alkylphosphonic acids - sixteen symmetric O,O-dialkenyl diesters and sixteen O-alkenyl monoesters (analyzed as their O-trimethylsilyl derivatives) were carried out for comparative mass‑spectrometric and chromatographic studies. These compounds fall under Schedule 2.B.04 of the Chemical Weapons Convention (CWC); however, twenty‑four of them are absent from major mass‑spectral libraries (NIST 23, OCAD v28_2026, VGWD_2026) and lack published retention index (RI) values. GC‑EI‑MS and GC‑CI‑MS analyses were performed, and methane PCI conditions yielded abundant protonated molecules [M+H]+, which frequently constituted the base peak and provided reliable confirmation of molecular weight. Distinct fragmentation behavior enabled classification of the compounds into two structural groups: derivatives with terminal double bonds and crotyl derivatives, the latter showing suppressed [M+H]+ intensities and altered adduct formation. For all homologous series, RI values showed linear correlations with the number of carbon atoms in the alkenyl chain or, in the case of crotyl derivatives, with the alkyl group attached to phosphorus. The generated EI and PCI mass-spectral data, diagnostic ions, and experimental and predicted RI values expand the analytical basis for identification of CWC‑relevant compounds. All results have been submitted for incorporation into the OPCW Central Analytical Database (OCAD) to support chemical-weapons verification and non‑proliferation efforts.