1.1 ,This procedure covers the determination of (Tri-n-butyl)-n-tetradecylphosphonium chloride (TTPC) in a soil matrix by extraction with acetone, filtration, dilution with water, and analysis by liquid chromatography/tandem mass spectrometry. TTPC is a biocide that strongly adsorbs to soils.2 The sample extracts are prepared in a solution of 75 ,% acetone and 25 ,% water because TTPC has an affinity for surfaces and particles. The reporting range for this method is from 250 to 10 ,000 ng/kg. This analyte is qualitatively and quantitatively determined by this method. This method adheres to multiple reaction monitoring (MRM) mass spectrometry.