Intake of major food items and intakes of individual food items were used to characterize the food consumption pattern for adults in Bangladesh.18 The five most consumed types of grains and pulses, i.e., rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), lentil (Lens culinaris), and black gram (Vigna mungo), were collected from an urban area situated at the northern part in Bangladesh. Wheat, maize, and pulses were collected during February−March 2012, and rice samples were collected during August−September, 2013. In the process of sample transport and storage, we ensured that the samples did not make contact with metal instruments in order to avoid cross-contamination. From each sampling site a composite sample for each food item was prepared and homogenized using a food processor. After collection from the field, about 200 g of fresh food samples were put in the rotary type food processor. The pretreated samples were first mixed and homogenized and then oven-dried at 105 °C for 24 h to remove the moisture content. The dried samples were then stored at −20 °C in the laboratory of the Institute of Nutrition and Food Science (INFS), University of Dhaka, Bangladesh. About 48 composite soil samples were also collected from the same fields of collected food samples. At each sampling station, soil sample (up to 10 cm depth) was collected in the form of subsamples, which were thoroughly mixed to form a composite sample. Soil samples were air-dried at room temperature for 2 weeks, then ground, and homogenized. The dried soil samples were crumbled and pulverized with a porcelain mortar and pestle, sieved through a 2 mm nylon sieve, and stored in an airtight clean zipper bag in freezer conditions until chemical analysis was carried out.
Measurement of Soil Properties. The physicochemical parameters of soil such as pH, EC, % C, and texture were measured. The pH of sediments was measured in 1:2.5 soil to water ratio. The soil/ deionized water mixture was stirred with a clean glass rod, and the slurry was allowed to equilibrate for 30 min.19 The pH values were recorded using a Horiba U-23 instrument with the calibration of pH 4.0, pH 7.0, and pH 9.0 standards. For electrical conductivity (EC) determination, 5.0 g of sediment was taken in 50 mL polypropylene tubes. Then, 30 mL of distilled water was added to the tube. The lid was closed properly and was shaken for 5 min. After that, EC was measured using an EC meter (Horiba D-52).20 Percent organic carbon of soil was measured using an elemental analyzer (model type: Vario EL III, Elenemtar, Germany). The catalytic combustion was carried out at a permanent temperature of up to 1200 °C. The element concentration from the detector signal and the sample weight based on stored calibration curves were measured. Particle size distribution was determined using the hydrometer method.21 The soils were classified using the United States Department of Agriculture (USDA) classification system (gravel [>2 mm], sand [2−0.05 mm], silt [0.05−0.002 mm], and clay [<0.002 mm]).
Chemicals and Analytical Methods. A microwave digestion system (Berghof, Germany) was used in the preparation of samples to analyze various metals. The microwave digestion system has been designed to perform chemical digestion procedures under extreme pressure and temperature conditions in chemical laboratories. Digestion reagents that were used included 5 mL of 69% HNO3 acid (Kanto Chemical Co. Japan) and 2 mL of 30% H2O2 (Wako Chemical Co. Japan). The weighed samples of 0.3 g were then placed into the digestion reagent in a Teflon vessel (DAP-60K type). After digestion, samples were then transferred into a Teflon beaker and total volume was made up to 25 mL with Milli-Q water (Elix UV5 and Milli-Q, Millipore, USA). The digest solution was then filtered (DISMIC 25HP PTFE syringe filter (pore size = 0.45 mm, Toyo Roshi Kaisha, Ltd., Japan) and stored in a screw cap plastic tube (Nalgene, New York).
Quality Control. All test batches were evaluated using an internal quality approach and validated if they satisfied the defined internal quality controls (IQCs). For each experiment, a run included blank, certified reference materials (CRM) as internal standard in samples, and samples analyzed in triplicate to eliminate any batch-specific error. Standard stock solutions containing 10 μg/L of each element (Cd, As, Pb, Cr, Ni, Zn, Se, Cu, Mo, Mn, Sb, Ba, V, and Ag) and internal standard solutions containing 1.0 mg/L of indium (In), yttrium (Y), beryllium (Be), telium (Te), cobalt (Co), and titanium (Ti) were purchased from Spex CertiPrep USA. Multielement standard solution was used to prepare a standard curve. Before starting the sequence, relative standard deviation (RSD < 5%) was checked by using tuning solution purchased from Agilent Company. Metals in samples were analyzed using an inductively coupled plasma mass spectrometer (ICP-MS).