Gas chromatographic analysis of the thermally unstable dimethyl methylphosphonate carbanion via trimethylsilyl derivatization

A sensitive gas chromatographic method was developed to monitor the reaction of lithium diisopropylamide (LDA) with dimethyl methylphosphonate (DMMP) to generate the phosphonate carbanion (DMMPA). Analysis of the DMMPA was complicated due to its thermal instability and lack of a chromophore. To overcome these problems, samples were derivatized with trimethylsilylchloride (TMSCl) to form DMMPA–TMS which was sufficiently volatile and thermally stable for GC analysis. Work-up of the derivatized solution with 10 vol.% 2-propanol in hexanes was necessary to quench residual TMSCl prior to GC analysis. The presence of DMMPA–TMS and other sample components was confirmed by GC–MS analysis. This method was utilized to profile the synthesis of DMMPA as DMMP was added to LDA and then aged at −78 °C. Method precision for DMMPA–TMS of less than 0.2% RSD was achieved for repeat injections after normalization of the response with n-dodecane contained in the sample. Due to the thermal instability of the DMMPA, subambient derivatization temperatures were essential to the stability, and consequently, accurate quantification. Under optimized conditions, this derivatization was successfully utilized as a process monitoring tool.