Characterization of Amidine-Based Degradation Products of Novichok A-Series Nerve Agents Using Single-Quadrupole...

The identification of Novichok A-series nerve agents depends primarily on the detection of stable degradation products, as intact agents are rarely encountered in environmental or forensic samples. However, limited GC-MS reference data exist for amidine-based degradation products of A-series nerve agents relevant to Chemical Weapons Convention (CWC) verification. This study aims to address a critical analytical gap supporting forensic attribution and international chemical weapons verification.

A series of eight N,N-dialkylethanimidamide compounds relevant to predicted Novichok degradation pathways was synthesized under controlled conditions. Samples were analyzed by GC-MS using electron ionization (EI) and positive chemical ionization (PCI) with isobutane reagent gas. Spectral data, comprising major fragment ions, protonated molecules, and adduct ions, were acquired using a GC-MS system under optimized analytical conditions. GCRI were determined using the Van den Dool and Kratz method with n-alkane standards.

EI mass spectra showed reproducible fragmentation dominated by α-cleavage, producing characteristic low-mass ions, particularly m/z 42 [C₂H₄N]⁺ and m/z 71 [C₃H₇N₂]⁺, with isomer dependent base peaks and relative abundances. As a complementary technique, PCI with isobutane yielded spectra with molecular mass confirmation via [M+H]⁺ ions, with minimal adduct formation (≤ 12%). RI ranged from 938 to 1145 and increased systematically with alkyl substitution, enabling discrimination of structural isomers. The combined EI and PCI data provide reliable spectral fingerprints and retention behavior for amidine-based degradation products of A-series nerve agents.

This work establishes comprehensive GC-MS reference data for amidine-based degradation products associated with Novichok A-series nerve agents. The developed method enabled the successful identification of spiked amidine compounds in the 56th and 57th OPCW proficiency test (PT) samples. These results will enhance the capability of forensic and verification laboratories to detect, confirm, and interpret degradation markers, thereby supporting OPCW PT, chemical weapons attribution, and compliance with the CWC.