Detection of 2-Chloroethyl Ethyl Sulfide and Sulfonium Ion Degradation Products on Environmental Surfaces Using Static SIMS

2-Chloroethyl ethyl sulfide (CEES) is a simultant for the chemical warfare agent bis(2-chloroethyl)sulfide (also known as HD or mustard), and both molecules undergo hydrolysis and subsequent condensation in aqueous solution to form stable sulfonium ions. The sulfonium ions derived from CEES are directly detected on quartzic surfaces using static SIMS instrumentation, which employs a molecular ReO₄⁻ (250 D) primary ion and pulsed secondary ion extraction. Pulsed extraction mitigates surface charging, and the ReO₄⁻ primary particle is efficient at sputtering molecular surface species into the gas phase. CEES eliminates CI− to form an ethyl thiiranium intermediate, which is susceptible to nucleophilic attack by water and methanol to form 2-hydroxyethyl ethyl sulfide and 2-methoxyethyl ethyl sulfide. These two products and unhydrolyzed CEES also function as nucleophiles that condense with the ethyl thiiranium intermediate, resulting in the formation of sulfonium ion aggregates that are observable using SIMS. The previously unreported methoxy-substituted sulfonium ion suggests that a variety of derivatives are possible if different nucleophiles are present in the vicinity of the ethyl thiiranium intermediate. This work demonstrates that the sulfonium ion aggregates are stable on mineral surfaces and also demonstrates the potential value of SIMS for the detection of unanticipated ionic species in monitoring applications where mustard and its degradation products are suspected.