MODIFICATIONS OF A METHOD FOR DETERMINING MULTIFUMIGANT RESIDUES

Phosphine (PH3) and methyl bromide (CH3Br) are the main fumigants used on stored grains. Published multiresidue methods, including those bas ed on solvent extraction and on purge-and-trap techniques, give poor r ecoveries of CH3Br and, in our hands, close to zero recovery of PH3 an d carbonyl sulfide (COS), a potential fumigant, We examined factors in fluencing fumigant analysis, including stability of chemicals in leach ates and partitioning of fumigant between leachate and air. The partit ion ratio, defined as the ratio of fumigant concentration in leachate to that in the headspace, varied between 1.1 for PH3 and >100 for ethy lene dibromide (EDB). An official procedure involving solvent extracti on followed by partitioning was modified by being performed in sealed flasks. This change raised the recovery of CH3Br from 28 to 85%. Volat ile fumigants, including PH3 and COS, were determined from concentrati ons in the headspace over the leachate (aqueous acidified acetone). Re coveries were nearly quantitative at levels down to 3 ng PH3/g and 16 ng COS/g, provided that fortified samples were used as analytical stan dards. Thus an existing multifumigant procedure was adapted to enable determination of the main fumigants used on staple foodstuffs.