Trace Detection of Di-Isopropyl Methyl Phosphonate DIMP, a By-Product, Precursor, and Simulant of Sarin, Using Either Ion Mobility Spectrometry or GC-MS

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Di-isopropyl methyl phosphonate (DIMP) has no major commercial uses but is a by-product or a precursor in the synthesis of the nerve agent sarin (GB). Also, DIMP is utilized as a simulant compound for the chemical warfare agents sarin and soman in order to test and calibrate sensitive IMS instrumentation that warns against the deadly chemical weapons. DIMP was measured from 2 ppbv (15 μg m−3) to 500 ppbv in the air using a pocket-held ToF ion mobility spectrometer, model LCD-3.2E, with a non-radioactive ionization source and ammonia doping in positive ion mode. Excellent sensitivity (LoD of 0.24 ppbv and LoQ of 0.80 ppbv) was noticed; the linear response was up to 10 ppbv, while saturation occurred at >500 ppbv. DIMP identification by IMS relies on the formation of two distinct peaks: the monomer M·NH4+, with a reduced ion mobility K0 = 1.41 cm2 V−1 s−1, and the dimer M2·NH4+, with K0 = 1.04 cm2 V−1 s−1 (where M is the DIMP molecule); positive reactant ions (Pos RIP) have K0 = 2.31 cm2 V−1 s−1. Quantification of DIMP at trace levels was also achieved by GC-MS over the concentration range of 1.5 to 150 μg mL−1; using a capillary column (30 m × 0.25 mm × 0.25 μm) with a TG-5 SilMS stationary phase and temperature programming from 60 to 110 °C, DIMP retention time (RT) was ca. 8.5 min. The lowest amount of DIMP measured by GC-MS was 1.5 ng, with an LoD of 0.21 μg mL−1 and an LoQ of 0.62 μg mL−1 DIMP. Our results demonstrate that these methods provide robust tools for both on-site and off-site detection and quantification of DIMP at trace levels, a finding which has significant implications for forensic investigations of chemical agent use and for environmental monitoring of contamination by organophosphorus compounds.