Determination of Phosphine Residues in Wheat and Yellow Corn with a New Developed Method Using Headspace and SIM Mode GC-MS

Phosphine is considered the main fumigant material used for long time for controlling insect pests in stored grains. This research was focused on the determination of phosphine residues in cereal matrixes (mainly wheat and corn) after the fumigation process, using a new procedure developed to reduce the number of analytical steps and improve the chromatographic separation, identification, and quantitation of analyte, thus leading to enhanced total efficiency and sensitivity. This method, using gastight vials for extraction with 5% sulfuric acid, a heating extraction sequence, and injection with headspace and GC-MS in selected-ion monitoring mode, gave clean separation and accurate results. Repeatability was achieved for both wheat and corn after spiking samples at the 0.01 mg/kg level, with RSD values of 7.6 and 6.3%, respectively. The LOD and LOQ values were 0.46 and 1.38 µg/kg, respectively. The mean value of phosphine residue in wheat was 8.43 µg/kg, with an RSD of 8.17%, whereas it was 8.09 µg/kg in corn, with an RSD of 7.75%. All residues detected in all the replicates were below the estimated maximum residue limit for wheat and corn (0.1 mg/kg). The highest residue value for wheat was 9.85 µg/kg and the lowest was 7.70 µg/kg, whereas for corn, the highest value was 9.03 µg/kg and the lowest 7.30 µg/kg.

Evaluation of Headspace Solid-Phase Microextraction for Analysis of Phosphine Residues in Wheat

In headspace (HS) analysis, a fumigant is released from a commodity into a gas-tight container by grinding, heating, or microwaves. A new technique uses HS-solid-phase microextraction (SPME) for additional preconcentration of fumigant. HS-SPME was tested for detection of phosphine (PH3), chosen for examination because of its wide use on stored commodities. PH3 was applied to 50 g wheat in separate 250 mL sealed flasks, which were equipped either with a septum for conventional HS analysis or with one of four HS-SPME fibers [100 μm polydimethylsiloxane (PDMS), 85 μm carboxen (CAR)/PDMS, 75 μm CAR/PDMS, and 65 μm PDMS/divinylbenzene (DVB)]. The wheat was heated at 45°C for 20 min. In conventional HS analysis, a gaseous aliquot (80 μL) was taken from the HS and injected into the GC instrument. In the HS-SPME procedure, the fiber was removed from the HS and exposed in the heated injection port of the GC instrument. In all cases, PH3 was determined under the same chromatographic conditions with a GC pulsed flame photometric detector. In a comparison of the efficacy of the fibers, the bipolar fibers (CAR/PDMS and PDMS/DVB) contained more PH3 than the aliquot in the conventional HS analysis; larger size bipolar fibers extracted PH3 more efficiently than smaller fibers (e.g., 85 > 75 > 65 μm). The nonpolar fiber (PDMS) contained no PH3. Four fortification levels of PH3 on wheat were tested: 0.01, 0.05, 0.1, and 0.3 μg/g. The response of each bipolar fiber increased with the fortification levels, but the conventional HS analysis detected no fumigant at the lowest fortification level of 0.01 mg/g. Under the conditions of the validation study, the LOD was in the range of 0.005–0.01 ng PH3/g wheat.

Determination of Phosphine Residues in Whole Grains and Soybeans by Ion Chromatography via Conversion to Phosphate

An ion chromatographic (IC) method was developed for determining phosphine (PH3) in whole grains (barley, corn, oats, rice, rye, and wheat) and soybeans. The method converts phosphine to phosphate (i.e., orthophosphate) and isolates the phosphate by IC with eluent-suppressed conductivity detection. Recoveries of unbound phosphine by the method were similar to those obtained by an established colorimetric method for 7 different products fortified at 3 levels. Mean recoveries were low (i.e., 30-60%) and varied with product type and level of fortification. Recoveries of PH3 from previously fumigated products fortified with aluminum phosphide ranged from 19.0% for barley fortified at 0.734 ppm to 88.3% for corn fortified at 1.691 ppm. Precision data from 3 products based on replicate analyses (n = 4 or 5) gave relative standard deviations of 1.78-4.66% for mean laboratory-fumigated PH3 levels of 0.679-1.309 ppm. Estimated limits of detection (LOD) and quantitation (LOQ) for PH3 were 0.010 μg/g (10 ppb) and 0.0275 μg/g (27.5 ppb) at signal-to-noise ratios (S/N) of 4:1 and 10:1, respectively. These values were also determined for a nonchemically suppressed IC system with LOD of 0.02 μg/g (20 ppb) and LOQ of 0.055 μg/g (55 ppb) at S/N of 4:1 and 10:1, respectively. Phosphate response was linear over the concentration range equivalent to 0.30-10.0 fig P/mL, with a mean correlation coefficient of 0.9988 based on replicate standard curves. The relationship of product composition to recovery from various products was also examined.

Comparison of Six Methods for Determining Aged Phosphine Residues in Wheat

abstract 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.

Comparison of Six Methods for Determining Aged Phosphine Residues in Wheat

abstract 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.

Spectrophotometric Method for Determination of Phosphine Residues in Cashew Kernels

A spectrophotometric method reported for determination of phosphine (PH3) residues in wheat has been extended for determination of these residues in cashew kernels. Unlike the spectrum for wheat, the spectrum of PH3 residue-AgN03 chromophore from cashew kernels does not show an absorption maximum at 400 nm; nevertheless, reading the absorbance at 400 nm afforded good recoveries of 90- 98%. No interference occurred from crop materials, and crop controls showed low absorbance; the method can be applied for determinations as low as 0.01 ppm PH3 residue in cashew kernels