Study included the nine studied food groups: beverages (cola, nescafe, tea, mint, carob, ginger, roselle, caraway, and fenugreek), carbohydrate rich food (toast, shami bread, sweet potato, vino bread, macaroni with whole grain, rice, pasta, potato, local bread, and corn), milk and its product (soft cheese, skimmed milk, processed cheese, fat-free yogurt, semi-hard cheese, roomy cheese, raw milk, full-cream milk, full-cream yogurt, and local yogurt) meat and its products (luncheon meat, kufta, sausage, sussis, bastrama, veal meat, cow’s kidney, beef, sheep meat, and cow’s liver), fruits and vegetables (beet, cucumber, apple, garlic, lemon, tomato, onion, orange, carrot, and banana), protein-rich food (beans, lentils, egg, chicken, mackerel, mullet, tilapia, duck, crab, and shrimps), nuts (cashew, seed sunflower, watermelon seed, peanuts, pistachio, almond, hazelnut, walnut, and sesame seeds), fats (industrial butter, morta, industrial margarine, animal fat, coconut oil, sunflower oil, corn oil, olive oil, local margarine, and local butter), and sweetened products(gelatine, tea biscuits, jam, galaxy chocolate, raw chocolate, chocolate biscuits, sweet tahini, black honey, and bee honey). Nine food items were taken from each of the beverages, sweetened products, and nuts, while 10 food items were taken from each of the remaining groups giving the total of 87. Each food item was purchased four times from four different markets in Alexandria governorate giving a total of 348 samples. Pooling was done for each food item (four replicates) by mixing them well together to obtain random and representative sample for each. Samples were collected during the period of January–March 2018.
Samples were collected and protected from contamination or Se loss during analysis. Samples as meat, fish, and seafood were stored at − 20 °C till analysis. Yogurt, fruit vegetable, and cheese were analyzed once purchased, while in case of other food items as oil, nuts, cereal, and beverages they were stored at room temperature 27 °C until analysis.
Solid samples (cereal, legume, and nuts) were grained by grinder, while liquid samples were analyzed directly. Fresh milk and dairy products were collected in a special container to avoid contamination. Fats and connective tissue were discarded from meat and poultry, while in case of seafood only edible parts of tissue were analyzed; head, skin, viscera, scales, and tail were removed. Edible portions of vegetables and fruits were homogenized well in a porcelain mortar to obtain homogeneous samples .
All samples were prepared for selenium analysis by wet ashing procedure . Sample preparation by wet ashing was categorized according to methods of digestion. Food samples were divided into the following:
Beverages: 100 mg of each sample was added to 10 ml of HNO3 (65%), heated on hotplate at 130 °C till dryness. After cooling, 5 ml of H2O2 (40%) was added and reheated at 130 °C again till dryness. Samples were diluted with 20-ml distilled water and filtered twice through Whatman filter paper grade II to glass bottles. Milk samples: two steps were operated. First step, 2.5 ml of milk samples was added to 4 ml of nitric acid (65%), and 4 ml of hydrogen peroxide (40%) and 4 ml of distilled water were added, then heated at 200 °C till dryness. Second step, addition of 1.4 ml of HCl, then heated at 100 °C, after cooling; 2 ml of H2SO4 was added and heated at 50 °C then increase temperature to 130 °C till dryness. Samples were diluted and filtrated. Concerning dairy products: 1 g was weighted then added 10 ml of concentrated nitric acid (65%), heated at 100 °C. Then 10 ml of hydrogen peroxide (40%) was added and heated again at120 °C; finally, 2 ml of HCl was added and heated at 200 °C till dryness.
Meat and seafood:1 g of each sample type was weighted and 15 ml HNO3 (65%) was added then heated at 200 °C. After cooling, 15 ml of H2O2 was added, and then heated at 150 °C. While in case of cereal, legume, and nuts samples, 1 g was added to 20ml nitric acid (65%), 20 ml of hydrogen peroxide, and 20-ml distilled water, and then heated at 170 °C till dryness. Vegetables and fruits: 1 g of sample was added to 10 ml of the HNO3 (65%). Samples were kept overnight at the room temperature, then heated at 100 °C. After cooling, 5 ml of HCl was added and heated till dryness. Sweetened products and liquid fat samples: 5 ml of sample was added to 5.0 ml of HNO3 and heated at 100 °C till dryness. All the wet-digested samples were ready to determine Se content by Inductively Coupled Plasma-Mass Spectrometry, USA. (Varian 720-ES) .
ICP-MS determination procedure
ICP-MS measurements were performed using a VG Plasma Qua Ex-Cell (Thermo, Courtaboeuf, France). Sample solutions were pumped by a peristaltic pump from tubes arranged on a CETAC Varian 720-ES Inductively Coupled Plasma-Mass-Spectrometry USA (CETAC, Omaha, NE) .
The isotope 78Se, 82Se was selected as analytical masses in ICP-MS instrumental parameters.
Nebulizer: concentric type pumped at 0.9 ml/min. Spray chamber: Scott-type double-pass water cooled. ICP-MS standard mode for Se elements, several specific isotopes were considered and monitored according to the sensitivity of the element and/or possible isobaric and polyatomic interferences. Torch position, ion lenses, and gas output were optimized daily with the tuning solution (1 g/l) to carry out a short-term stability test on the instrument, to maximize ion signals, and to minimize interference effects from polyatomic ions and doubly charged ions. In all experiments, a relative standard deviation (RSD) of 3% was achieved. To obtain precise and accurate results, element signals were monitored by real-time display (RTD), which showed the constant sensitivity over time for the selected masses and the ratios of masses with three readings, calculated for each sample .
The diluted solution was filtered through a plastic tube designed for the autosampler of Agilent 4500 Series ICP-MS (Agilent, Palo Alto, CA, USA) that was used for the analysis of selenium content. The analysis conditions were as follows: RF power 1200 W, plasma gas (Ar) 16.0 ml/min, aux. gas (Ar) 1.0 ml/min, carrier gas (Ar) 1.14 ml/min, Barbington-type nebulizer, glass spray chamber, sampling depth 8.2 mm, Ni/Ni sampling cone/skimmer cone, and mass for selenium was m/z 82. Selenium concentrations of the samples were calculated from the regression line (r2 = 0.999) obtained using selenium standard solution (Wako Pure Chemical Industries, Osaka, Japan) .
The method was validated by analysis of selenium content of NIST Standard Reference Material 8436 durum wheat flour obtained from National Institute of Standards and Technology (Boulder, CO, USA), and the recovery tests were performed on some food groups spiked with selenium standard and the recovery value ranged (0.002–0.05 μg/g) .
Statistical analysis of the data
Data were fed to the computer and analyzed using the IBM SPSS software package version 20.0. (Armonk, NY: IBM Corp.). Qualitative data were described using number and percent. The Kolmogorov-Smirnov test was used to verify the normality of distribution. Quantitative data were described using range (minimum and maximum), mean, standard deviation, and median. Significance of the obtained results was judged at the 5% level. The used tests were Kruskal–Wallis test for abnormally distributed quantitative variables, to compare between more than two studied groups and post hoc (Dunn’s multiple comparisons test) for pairwise comparisons [19, 20].