Zinc Phosphide Residue Determination in Alfalfa (Medicago sativa)

An analytical method for the determination of zinc phosphide (Zn3P2) residues in alfalfa ranging from 10 μg g-1 to 100 mg g-1 is described. Using a suspension of Zn3P2 in propylene glycol, Zn3P2 standards were prepared and midharvest alfalfa and dried hay control samples fortified to assess method recovery. Addition of 1% phosphoric acid to Zn3P2-containing samples produced phosphine (PH3) gas for headspace gas chromatography/flame photometric detection analysis. Response linearity was assessed for (8.9 × 10-5)−0.027 μg of Zn3P2 mL-1 headspace (r 2 = 0.9914) and 0.027−2.02 μg of Zn3P2 mL-1 headspace (r 2 = 0.9998) concentration ranges. Analyte recovery exceeded 81% for Zn3P2 fortification levels ranging from 0.010 to 100 μg g-1, and the method limit of detection was 2.1 × 10-3 μg g-1 in midharvest alfalfa and 3.9 × 10-3 μg g-1 in dried hay. Keywords: Zinc phosphide; phosphine; residues; Medicago sativa; alfalfa; gas chromatography/flame photometric detection