Sample preparation
Only edible parts of each fruit and vegetable type were used for the analysis. Any rotted or damaged parts were removed before preparation as a simulation to the real food preparation for human consumption. Fresh sample was weighed and thencut into small portionsfor drying in an oven at 105 °C. Completely dry sample was achieved within 72–96 h. Complete dryness was checked visually. Dry sample was ground in a householdcoffeegrinderandsievedina0.8-mmmesh.Itwas then stored in a pre-labeled polyethylene bag for digestion. A wet digestion method, which was described by Altundag and Tuzen (2011), was modified and applied for all fruits and vegetables with the exception of spinach. A dry ashing digestion method described by Karla (1998) was used for spinach samples. In the wet digestion method, about 5 g of the dried andgroundsamplewas placedina glass digestiontube. Avolumeof9mLofnitricacid(70%)wasaddedtothe sample and left for about 4 h at room temperature. Further 9 mL of nitric acid was added, and sample was left to digest overnight at room temperature. A third portionof9mLofnitricacidwasadded,andtheheating program was started in the digestion block with a temperature of 45 °C for 45 min then raised to 90 °C for more 45 min. After which, the sample was left to cool
Fig. 1 Study area and sampling locations in Aseer Region
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down to room temperature. Five milliliters of hydrogen peroxide was thenadded,and temperature was raised to 120 °C for 2 h. The steps of cooling, adding hydrogen peroxide, and heating at 120 °C for 2 h were repeated two times to completely digest samples. The digest was then filtered through Whatman® no. 42 filter paper into a50-mLvolumetricflask,andthevolumewascompleted to the mark with deionized water. This solution was used directly for AAS measurement or in some cases further dilution was conducted. In the dry ashing method, 5 g of dried and ground spinachleaveswasplacedina50-mLporcelaincrucible and ashed in a programmable muffle furnace. Temperature was raised gradually in a rate of 275 °C/h to reach 550 °C. Sample was kept ashing at this temperature for 16 h before cooling down. When cool, drops of deionized water followed by 10 mL of 50 % (v/v) nitric acid were added slowly. The crucible was then placed on a hot plate to dryness and then placed again in the muffle furnace for ashing at 550 °C for 2 h. This process was repeated from three to four times till getting a white or gray ash. The ash was then dissolved in 10 mL of 20 % (v/v) nitric acid on a hot plate at about 100 °C. The solution was left to cool down and then filtered through Whatman® no. 42 filter paper into a 50-mL volumetric flask. The volume was then completed to the mark with deionized water. This solution was used directly for AAS measurement or in some cases further dilution was conducted.
Atomic absorption spectrophotometry measurements