Sample Description
The fruits of wild-growing blueberries were collected during September of 2016, in three different locations in central Poland (the Mazovia region). Sample W1 was from a 12-year-old pine forest (N 52° 41′, E 21° 29′), sample W2 from a 25-year old pine forest (N 52° 49′, E 21° 45′), and sample W3 from a 77-year-old pine wood (N 52° 01′, E 21° 06′). The berries were harvested optimally ripe based on color, flavor, and structure.
Additionally, one sample (W4) was obtained from the local marketplace. One sample of cultivated American blueberries was purchased from a local market (sample C1), and the second one (sample C2) was collected from the garden plot outside Warsaw city (Poland).
Once the samples were collected, they were frozen and stored in a freezer at −20 °C. For analysis, the samples were thawed at refrigerator temperature (∼4 °C) and homogenized by blender.
Preparation of Samples
For determination of metal contents, the samples were digested using the mixture of concentrated acids HNO3 and HClO4 (4:1 v/v). 0.5 g of fruit sample was placed into a 100-mL flask, and 10 mL of acid mixture was added to it. This mixture was heated for 30 min at 50 °C on the hot plate. Then, the temperature was slowly raised to 160–170 °C, and digestion was performed at the beginning for 120 min and finally at 200 °C for 60 min. The digested sample solution was allowed to cool down and transferred into a 50-mL volumetric flask diluted by adding distilled water. As the level of Cd and Cr in the analyzed samples was very low, additional digestion procedure was performed using higher amount of analyzed fruits (5 g).
Digestion of a reagent blank was performed in parallel.
Drying of fruits was performed in a laboratory dryer at 60 °C for 24 h.
For extraction of metals from fresh or dried blueberries, 20 mL of freshly boiled (∼95 °C) distilled water was added to 0.7 g of appropriate fruits. After 20 min of brewing and subsequent cooling, the samples were centrifuged and transferred to volumetric flasks and diluted to a final volume of 25.0 mL. The extracts were prepared directly before analysis, and all determinations were carried out in triplicate.
Element Analysis
The concentration of elements (Al, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, and Zn) were determined by inductively coupled plasma (ICP OES) using a Thermo Scientific spectrometer, model iCAP 6000. The operating parameters set to the spectrometer were those recommended by its manufacturer, i.e., the forward power of 1.15 kW, the Ar auxiliary gas flow rate of 0.2 L/min, the Ar nebulizing gas flow rate of 0.42 L/min, and coolant gas flow rate of 12 L/min. The intensity readings were repeated three times using the integration time of 1 s. The most prominent lines of the studied metals were selected, i.e., Al 396.1 nm, Ca 317.9 nm, Cd 214.4 nm, Cr 267.7 nm, Cu 224.7 nm, Fe 259.9 nm, K 766.4 nm, Mg 285.2 nm, Mn 260.5 nm, Na 589.5 nm, Ni 231.6 nm, Pb 220.3 nm, and Zn 206.2 nm.