Effect of cation-exchange pretreatment of aqueous soil extracts on the gas chromatographic–mass spectrometric determination of nerve agent hydrolysis products after tert.-butyldimethylsilylation

Abstract

The efficency of pretreatment of aqueous soil extracts using a cation-exchange resin has been investigated by gas chromatographic–mass spectrometric (GC–MS) determination of nerve agent hydrolysis products after tert.-butyldimethylsilyl (TBDMS) derivatization. An aqueous solution containing methylphosphonic acid (MPA) and its monoalkyl esters, ethyl methylphosphonic acid, isopropyl methylphosphonic acid and pinacolyl methylphosphonic acid, was dried, and these phosphonic acids were derivatized with N-methyl- N-( tert.-butyldimethylsilyl)trifluoro-acetamide and analyzed by GC–MS. The yields of TBDMS derivatives were significantly decreased by the addition of calcium and magnesium ions to an aqueous solution (≈0.5 m M) before derivatization. The extent of lowered yields was related to the hydrophilicity of phosphonic acids. MPA and its monoalkyl esters were spiked into soil samples (sand, alluvial soil and volcanic ash soil), extracted with distilled water, dried, silylated and applied to GC–MS. The yields of TBDMS derivatives of monoalkyl esters from soil samples were low (3–42%) and MPA derivative was scarcely detected (yield: <0.7%). By desalting the aqueous soil extract by passage through a strong cation-exchange resin, the yields of TBDMS derivatives of monoalkyl esters were significantly improved (12–69%) and MPA derivative was detected (yield: 2–36%). The extent of improved yields was related to the concentrations of divalent metal cations in aqueous soil extracts. In combination with desalting by the cation-exchange resin, GC–MS after TBDMS derivatization enables detection of nerve agent hydrolysis products in soils at sub-ppm (0.2 μg/g) concentrations.