2.1 Sampling Area Seventeen different species of marine fish samples (n = 25) were purchased randomly from three different local markets of Chittagong area near the coastal site of Bay of Bengal. Fish samples were labelled and preserved using ice and transported to laboratory. All the samples were kept at -20 °C until analysis. 2.2 Certified Chemical Reagents and Solvents Organochlorine compounds (4,4´-DDT, 2,4´-DDT, 4,4´-DDE and 4,4´-DDD) of 99% purity were purchased from Dr. Ehrenstorfer, Germany. The following analytical grade chemicals, reagents and solvents were used in this research work: anhydrous sodium sulphate (Scharlab S. L., 08181 Sentmenat Spain), silica sand (Kanto Chemical Co. Inc.), ethyl acetate (RCI Labscan Limited, USA), n-hexane (RCI Labscan Limited, USA), concentrated H2SO4 (Merck, Darmstadt, Germany), acetone (Sigma-Aldrich, France), silica gel (Merck KGaA, 64271 Dramstadt, Germany), NaCl and H2SO4 (98%) were purchased from Merck, Germany. 2.3 Extractions and Clean-up Procedures of Fish Samples Fish samples were extracted by two separate methods, i.e., SPD (solid phase dispersion) and QuEChERS (quick, easy, cheap, effective, rugged and safe). In SPD method, the muscle tissues of the fish samples were blended homogeneously. Blended sample (10.0 g) was taken into a pre-cleaned and dried mortar with silica sand (10.0 g) and anhydrous sodium sulphate (30.0 g). This powder (sample, sand and sodium sulphate) was taken in a 250 mL ground joint conical flask and extracted by shaking for 3 minutes successively with 60, 20, 20 mL ethyl acetate. The extracts were combined and filtered through anhydrous sodium sulphate. The filtrated extract was evaporated to dryness and the weight of the fatwas recorded. The solvent was exchanged from ethyl acetate to n-hexane by evaporation and the volume of the extract was adjusted up to 4 mL. From this 2 mL concentrated extract was transferred quantitatively in a graduated test tube. Clean-up was carried out through silica gel: saturated sulphuric acid (2:1 g/g) column. The extract was then concentrated to 1 mL through the nitrogen gas evaporator. Then it was taken into a GC vial for injection to the GC-ECD. In QuEChERS method, 10 g homogenized blended marine fish sample was taken in a 50 mL tefl on centrifuge tube. And 20 mL ethyl acetate was added to it and shaken by hand for 1 minute and vortexed for about 2 minutes. Anhydrous magnesium sulphate (6.0 g) and sodium chloride (1.5 g) were added and the mixture was again vortexed for 1 minute followed by centrifugation (4,000 rpm for 5 minutes). The supernatant was taken into a clean, dried and pre-weighed round bottom flask and the solvent was evaporated (below 40 °C) to dryness by rotary vacuum evaporator. Then, the weight of total fat content was recorded. The solvent was exchanged from ethyl acetate to n-hexane by evaporation and the volume of the extract was adjusted up to 4 mL. From this 2 mL concentrated extract was transferred quantitatively in a graduated test tube. Then, 2 mL saturated H2SO4 was added and shaken for 1 minute. Then the mixture was centrifuged for 10 minutes. The supernatant was taken using a pasture pipette, kept into GC vial and analyzed by GC-ECD. 2.4 LOD, LOQ and Recovery Experiment At first, 100 mg/L primary standard solution was prepared by diluting proper amount of DDTs (99% purity) in n-hexane and serially diluted to the different concentrated solution like 0.2, 0.1, 0.05, 0.02, 0.01, 0.005, 0.002, 0.001 and 0.0005 mg/kg. The limit of detection (LOD) was determined by injecting serially diluted mixtures of standard DDTs solution in GC-ECD. For LOD, the peak area of each standard was considered 3 times higher than the baseline noise, i.e., signal to noise ratio was 3:1. LODs for DDT, DDE and DDD were found to be 0.10 ng/g. The LOQ is the minimum concentration that can be quantified at a specified level of precision or accuracy (or both). For LOQ, the peak area of each standard was considered 10 times higher than the baseline noise, i.e., signal to noise ratio was 10:1 and LOQ for fish samples DDTs residue analysis 0.30 ng/g.