Mass Spectral Studies of N,N-Dialkylacetimidamides Using ESI-HRMS and MS/MS

For global safety, an effective control and usage of chemical toxins related to chemical warfare agents (CWAs) is of critical importance. Rapid identification of these toxins, their degradation products, and synthetic precursors across various sample matrices is essential. This study focuses on the swift and trace-level identification of N,N-dialkylacetimidamides (amidines), which serve as both precursors and degradation products of Novichoks. These chemicals are listed as nonscheduled reportable chemicals (Schedule 1.A.13 and 1.A.14) under the Chemical Weapon Convention (CWC) by the Organisation for the Prohibition of Chemical Weapons (OPCW).

A high-resolution positive ion mode electrospray ionization tandem mass spectrometry (ESI-HRMS and MS/MS) is employed for the identification and structural characterization of a series of amidines (1-10).

The product ion spectra of all the amidines showed common fragment ions corresponding to [C₂H₄N]⁺, [MH-C₂H₃N]⁺, and [MH-R₂NH₂]⁺. Characterization of isomeric compounds was accomplished by unique fragment ions and differences in the relative intensity of common fragment ions at a collision energy (20 eV). These data have been successfully utilized for the identification of N,N-diisopropylacetimidamide and N-methyl-N-propylacetimidamide in the 56th and 57th OPCW official proficiency test samples, respectively.

A series of amidines (1-10) including three isomeric sets (3-5, 6-7, and 8-10) were synthesized and characterized by ESI-HRMS and MS/MS, and nuclear magnetic resonance (¹H NMR) spectroscopic methods. All the isomeric compounds were differentiated based on their tandem mass spectral data. This method was successfully demonstrated to identify spiked amidines in the 56th and 57th OPCW official proficiency test samples.