Comparison of Six Methods for Determining Aged Phosphine Residues in Wheat

Phosphine (PH3) and methyl bromide (ChhBr) are the main fumigants used on stored grains. Published multiresidue methods, including those based on solvent extraction and on purge-and-trap techniques, give poor recoveries of CH3Br and, in our hands, close to zero recovery of PH3 and carbonyl sulfide (COS), a potential fumigant. We examined factors influencing fumigant analysis, including stability of chemicals in leachates and partitioning of fumigant between leachate and air. The partition ratio, defined as the ratio of fumigant concentration in leachate to that in the headspace, varied between 1.1 for PH3 and >100 for ethylene dibromide (EDB). An official procedure involving solvent extraction followed by partitioning was modified by being performed in sealed flasks. This change raised the recovery of ChfoBr from 28 to 85%. Volatile fumigants, including PH3 and COS, were determined from concentrations in the headspace over the leachate (aqueous acidified acetone). Recoveries were nearly quantitative at levels down to 3 ng PH3/g and 16 ng COS/g, provided that fortified samples were used as analytical standards. Thus an existing multifumigant procedure was adapted to enable determination of the main fumigants used on staple foodstuffs.