Natural levels of dimethyl sulfide in rough rice and its products.

Natural levels of dimethyl sulfide (DMS) in rough rice and its products (polished rice, brown rice, and broken rice) were determined by a gas chromatograph equipped with a flame photometric detector and sulfur mode, after extraction with 25% KBr solution in a sealed system. DMS was found to occur naturally in nine newly harvested and stored Australian varieties of rough rice and its products and decreased during storage after harvesting. Natural levels of DMS in rough rice and its products varied with variety, fraction, and period of storage. The order of levels of DMS was rough rice = brown rice > polished rice = broken rice. The range of values was 0.002-30 mg kg(-1) (ppm, w/w).

Fate of (14)C-labeled carbonyl sulfide on grains and grain fractions.

14C-Labeled carbonyl sulfide (COS) was used to measure the amount of sorbed fumigant and alteration products on grains. Wheat, paddy rice, polished rice, mungbean, and safflower were exposed to a 60 mg L(-)(1) of (14)COS for 7 days and then aired for 5 days. Carbonyl sulfide and/or alteration products in sugars, protein, starch, amino acids, protopectines, and hemicelluloses were undetectable. The total uptakes of radioactivity determined after fractionation and also by extraction were in the range of 36-53 ng g(-)(1) COS equiv. The total radioactivity determined by autoradiography was below the detection limit of 70 ng g(-)(1) COS equiv. Radioactivity in the commodities was less than 0.003% of all the radioactivity applied. The amount of retained radiolabel was measured in three ways. First, biochemical fractions such as lipids and amino acids were separated by chromatography, and the activity was determined in each component. Second, commodities were crushed and extracted in aqueous acetone until the maximum amount of radiolabel was extracted. Third, autoradiography was carried out on commodity kernels.

Release of fumigant residues from grain by microwave irradiation.

Multiresidue analysis of fumigants is important because of their widespread use on staple foodstuffs, such as grain. Fumigants are usually extracted from grain either by solvent extraction or by purge-and-trap techniques. In this paper, fumigant residues in wheat were "extracted" by a microwave procedure. Wheat, in gas-tight Erlenmeyer flasks, was placed in a domestic microwave oven, and fumigants were released into the headspace by microwave irradiation. Power settings for maximum release of fumigants were determined for CH3Br, PH3, CS2, and COS. Recoveries of fortified samples were > 90%. Completeness of extraction was assessed from the amount of fumigant retained by the microwave-irradiated wheat. This amount, determined from both solvent extraction and from further microwave irradiation, was always small (< 5% of the amount obtained from the initial procedure). Limits of quantitation were < 1 ng/g for CH3Br, PH3, and CS2. These low limits were essentially due to the absence of interference from solvents. The microwave method is rapid and solvent-free. However, care is required in selecting the appropriate power setting. The safety implications of heating sealed flasks in microwave ovens should be noted.

Statistical analysis of decay of organophosphorus and pyrethroid insecticides and carbaryl on paddy rice, maize, sunflowers and field peas.

A model is developed to describe the decay of seven insecticides on stored paddy rice, maize, sunflowers and peas, and is shown to be more precise than each of six literature models on pesticide decay. It relates residues (R), as a ratio of applied concentration (R(0)), to time after insecticide application (t), modifying the usual first-order kinetics equation lnR=lnR(0)-kt by assuming that the rate constant k has a mixture of values according to a gamma distribution with mean K. The resulting equation is lnR=lnR(0)-Kcln(1+t/c), where c is a constant determined empirically. For large c, this model reduces to the first-order model with K=k. The parameter K is further modelled as linear in temperature for maize and rice. The rate of loss of protectants was found not to be seriously affected by co-application with other insecticides, and this is a useful finding for future studies on rates of loss of grain protectants. The apparent rate of loss was also not seriously affected by the method of analysis, whether by gas chromatography or high-performance liquid chromatography, and this finding supports previous validation of the methods.

A rapid, colorimetric, procedure for determination of fenitrothion.

Fenitrothion in formulations and on grains is determined colorimetrically as 4-nitro-3-methylphenoxide, after hydrolysis at room temperature catalysed by alkaline peroxide. Interferences in determination of residue levels are removed by filtration of commodity extract through basic alumina or by barium chloride plus excess oxidant.

Determination of pyrethroid residues on paddy rice by reversed-phase high-performance liquid chromatography.

Several synthetic pyrethroids and the synergist piperonyl butoxide have been determined as aged residues on paddy rice by reversed-phase high-performance liquid chromatography with detection at 225 nm. These compounds are commonly used as protectants for stored grains. Studies on the comparative rates of extraction of both the pesticides and interfering material from the grain were conducted with acetone, methanol, and hexane as extracting solvents. Acetone was the best of these solvents because it provided quantitative extraction of the pesticides over a 48-h period, and did not give high levels of ballast material. Pyrethroids present in the extract at levels in excess of 0.5 microgram/ml could be determined by direct injection, but at lower concentrations, clean-up and preconcentration were required. Clean-up of acetone extracts was accomplished with either Florisil or alumina pre-columns, and up to a tenfold preconcentration was achieved by adsorption of the pesticide on a C18 pre-column, or by concentrating the extract through evaporation of the solvent. These approaches gave good recoveries and linear calibration plots. Detection limits were of the order of 0.05 microgram/ml.