The study was conducted in the pesticide analytical laboratory and experimental field of Entomology Division, Bangladesh Agricultural Research Institute, Joydebpur, Gazipur during 2011-12 seasons. The standard for Fenvalerate and Acephate was obtained from Sigma-Aldrich Laborchemikalien, Gmbh P O Box-100262 D30918, Seelze, Germany via Bangladesh Scientific Pvt. Ltd. Dhaka, Bangladesh. Standards of both insecticides contained 99.6% purity. Marketable sizes of hyacinth bean and tomato were collected from two different supervised field trials at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15 days after spray (DAS) which were sprayed with Fenvalerate @1.0 ml/L of water and Acephate @1.0gm/L of water. The formulated products of those were Fenfen 20EC and Asataf 75SP. The purity of formulated insecticides was tested in the laboratory and found to be 100% pure.
2.1. Extraction and separation Field collected samples (250g) were grounded thoroughly with the meat grinder (Handmixer M-122, Bamix, Switzerland). A sub-sample of 20g was taken into a wide mouth jar then 100 ml of hexane was added to it. Sodium sulphate (Na2SO4) was also added with the sample until the water was removed from the sample. The mixture was then macerated with a high-speed homogenizer (Ultraturax, IKA T18 basic, Germany) for 2 minutes. The homogenized material was then poured into 250 ml conical flask and placed into the shaker (Orbital Shaking Incubator, Rexmed, Sweden) for 12hrs continuous shaking. After shaking, the slurry was filtered through a Buchner funnel with suction. The flask and filter cakes were rinsed with 25 ml of hexane each. The filtrate was then transferred into 250 ml round bottom flask and was dried to 5 ml by evaporation using a rotary vacuum evaporator (Laborota-4001, Heidolph, Germany). The concentrated filtrate was then transferred into 500 ml separatory funnel making 10 ml in volume. Around 20 ml methanol was added with 10 ml filtrate and shaked vigorously for 5 minutes. After shaking, the separatory funnel was set on a stand and kept undisturbed for 5 minutes. Then the clear part of the solution from the bottom of the separatory funnel was collected in a vial which was then centrifuged at 1200 rpm for 5 minutes (Laboratory Centrifuges, Sigma-3K30, Germany). After centrifuge, the supernatant was collected for injection in GC.
2.2. Detection and Quantification of pesticide residue in samples The concentrated extracts were subjected to analysis by GC-2010 (Shimadzu). For Acephate FTD (Flame Thirm ionized Detector) and for Fenvalerate ECD (Electron Capture Detector) were used. The capillary column used in FTD was ATTM-1, length 30m, Inner Diameter (ID) 0.25mm and film thickness 0.25µm and in case of ECD it was Optima 1 and length, ID and film thickness was same. Nitrogen was used as a carrier and made up gas in ECD and in FTD it was Helium.
Prior to the injection of the sample extract, standard solutions of different concentrations of both pesticide groups were prepared and injected with the above instrument parameters. The samples were calibrated (retention time, peak area etc.) against three to four pointed calibration curves of a standard solution of the concerned pesticide. Each peak was characterized by its retention time. Sample results were expressed in ppm automatically by the GC software which represented the concentration of the final volume injected. From this value the actual amount of pesticide residue present in the sample was determined by using the following formula:
Residue in the sample (ppm) = Conc. obtained in injected volume (ppm) X Quantity of final volume (L) / Amount of sample taken (kg)