Indirect Detection of Novichok Nerve Agents A-230, A-232, and A-234 by Ion Mobility Spectrometry: Limitations of Hydrolysis-Based Marker Strategies.

Ion mobility spectrometry (IMS) is widely used for on-site detection of chemical warfare agents due to its high sensitivity and rapid response times. While volatile G-series nerve agents can be detected directly in the gas phase, detection of low-volatility agents such as VX or A-series nerve agents (Novichoks) is considerably more challenging. For VX, indirect detection is often achieved through the volatile hydrolysis product VX-thiol, which can dominate the vapor phase above VX samples. Whether comparable volatile marker compounds exist for A-series agents has remained largely unexplored. In this study, the vapor-phase composition above small droplets of the Novichok agents A-230, A-232, and A-234 was investigated under controlled humidity conditions using IMS. Reference spectra of potential marker compounds, including N,N-diethylacetamidine and N,N-diethylacetamide, were recorded to support signal assignment. For VX, increasing humidity resulted in an increased VX-thiol response, consistent with enhanced hydrolytic formation of this volatile marker. In contrast, the expected amidine marker showed only very limited vapor-phase availability for the investigated A-series agents. Weak N,N-diethylacetamidine signals were detected for A-230 and A-232 at an elevated humidity level of 80% RH, whereas no corresponding signal was observed for A-234. N,N-Diethylacetamide was consistently detected shortly after introduction of the respective A-series samples. Pre-experimental GC-MS analysis showed that this latter compound was already present in the agent stocks, whereas N,N-diethylacetamidine remained undetected in stock samples. The initial amide signal therefore most likely reflects a preexisting volatile constituent associated with synthesis and/or storage rather than formation during the IMS experiment. Overall, the results indicate that a hydrolysis-based marker strategy analogous to VX-thiol is considerably less effective for A-230, A-232, and A-234 under the investigated conditions. No comparably robust hydrolysis-derived volatile marker was identified for the A-series agents, while preexisting volatile constituents may influence rapid gas-phase screening. The findings highlight both the potential and the limitations of extending existing IMS-based gas monitoring approaches to low-volatility nerve agents.