Comparison of six methods for determining aged phosphine residues in wheat

Phosphine (PH3) residues on wheat were determined by gas chromatography (GC) in a blind experiment using 6 methods to assess whether results from different methods are comparable. For one method, 2 different chromatographic conditions also were used. Samples were fumigated for 7 days and aired for 2 or 7 days to obtain "aged" residues. The methods involved determination of PH3 in the headspace over aqueous acidified acetone plus wheat (method a), over ground wheat (method b), over ground wheat plus headspace concentrations over ground wheat in acetone after transfer to another flask (method c), over ground wheat in acetone (method d), over whole wheat after microwave heating (method e), and over water after dilution of 0.5 mL aqueous acetone leachate with 10 mL buffered sodium sulfate solution (method f). For each method, gas-tight flasks and syringes were used. Data to support new methods c, d, and f are discussed. Each procedure correctly identified the order of PH3 residues as highest, medium, and absent. Quantitative results from all methods were similar, except that results from method b were on average 44% lower than the average results from other procedures. Thus each method, except method b, is suitable for determination of PH3 residues. The relative advantages of each procedure are discussed