Sampling: Six most popular dry fish samples namely Bombay duck (Loittya), Ribbon fish (Chhuri), Anchovy (Teilla phasa), Croaker (Poa), Chinese pomfret (Rupchanda) and Indian salmon (Lakhua) were collected from Karwan Bazaar of Dhaka (a whole sale market), Bangladesh for proximate biochemical analysis and screening of harmful additives. The control samples (for screening of harmful additives) of six different fishes were collected from drying yards of Sonadia island (a fish processing zone of Bangladesh) that are known sample treated with no insecticides and taken in to account as blank. A. Proximate biochemical analysis:- Proximate biochemical analysis was done according to the standard methods (Bhuiyan, et. al., 2003). B. Screening of DDT and Heptachlor:- Apparatus: Mincer fish chopper (Weisser No. 81 K), Soxhlet extractor, separatory funnels (500 ml & 200 ml), Chromatographic tube (20 mm i.d. 50 cm long), sample concentrator (Techne dry block DB.3), round bottomed flask (500 ml & 100 ml), volumetric flask (50 ml & 10 ml), Gas Chromatograph (GC-14B, Shimadzu), syringe(10 μl, Hamilton Co.). Reagents: Acetone, Diethyl ether, Dimethyl Formamide saturated with petroleum ether, n-hexane, petroleum ether (30- 60OC), petroleum ether (30-60OC) saturated with Dimethyl Formamide, eluting mixture 1 (petroleum ether + diethyl ether 94:6 v/v), standard solutions , eosin solution (2 mg in 100 ml), sodium sulphate solution (2 g/100 ml Na2SO4 10 H2O), sodium sulphate anhydrous ( heated for at least 2 h at 550OC), Florisil 60-100 mesh (heated for at least 2 h at 550OC, cool & stored in tightly stoppered container, prior to use heated for at least 5 h at 130OC, cool & add 5% w/w water, shake this mixture for at least 20 min and stored in a container for at least 10 h), cotton wool. All the solvents used for the analysis purchased from MERCK, Germany. DDT & Heptachlor standards were obtained from Sigma Chemicals. Sample preparation: All the samples are finely comminuted in a mincer; heating of the samples during comminuting is avoided by briefly chopping several times (Peter and Zeumer, 1987). Extraction: Triturate a sample of 25 gm, with anhydrous Sodium Sulphate to dry, powdery mixture, with the aid of an extraction thimble; extract the mixture exhaustively with Petroleum Ether in Soxhlet apparatus. Concentrate just to dryness the extract solution by a concentrator and dilute to 25 ml with Petroleum Ether saturated with Dimethyl Formamide (Peter and Zeumer, 1987). Clean up: Clean up was done in two steps- (Peter and Zeumer, 1987).
Analytical quality control Gas chromatograph equipped with ECD was checked for linearity. Instrumental limit of detection for GC-ECD was 1.0 μg/l for organochlorine pesticides. An aliquot of dry fish samples which were collected as blank and treated exactly as a sample including exposure to all glassware, equipment, solvents and reagents used with the sample matrix. No analytic peak was detected in laboratory reagent blank. An aliquot of fortified samples matrix were prepared to which known quantities of the pesticides were added in the laboratory in ppb range. This laboratory fortified matrix was analyzed exactly like the sample. Extraction and clean up were done as mentioned and the recoveries from untreated control samples of dry fish fortified with the analyzed compounds.