Ketalization of phosphonium ions by 1,4-dioxane: Selective detection of the chemical warfare agent simulant DMMP in mixtures using ion/molecule reactions

Phosphonium ions CH₃P(O)OCH₃⁺ (93 Th) and CH₃OP(O)OCH₃⁺ (109 Th) react with 1,4-dioxane to form unique cyclic ketalization products, 1,3,2-dioxaphospholanium ions. By contrast, a variety of other types of ions having multiple bonds, including the acylium ions CH₃CO⁺ (43 Th), CH₃OCO⁺ (59 Th), (CH₃)₂NCO⁺ (72 Th), and PhCO⁺ (105 Th), the iminium ion H₂C=NHC₂H₅⁺ (58 Th) and the carbosulfonium ion H₂C=SC₂H₅⁺ (75 Th) do not react with 1,4-dioxane under the same conditions. The characteristic ketalization reaction can also be observed when CH₃P(OH)(OCH₃)₂⁺, viz. protonated dimethyl methylphosphonate (DMMP), collides with 1,4-dioxane, as a result of fragmentation to yield the reactive phosphonium ion CH₃P(O)OCH₃⁺ (93 Th). This novel ion/molecule reaction is highly selective to phosphonium ions and can be applied to identify DMMP selectively in the presence of ketone, ester, and amide compounds using a neutral gain MS/MS scan. This method of DMMP analysis can be applied to aqueous solutions using electrospray ionization; it shows a detection limit in the low ppb range and a linear response over the range 10 to 500 ppb.