The total metal concentration in the digested samples of berries was determined by ICP OES method. In order to validate the analytical protocol, linearity of the calibration curve range for each element generated by injection of standard solutions as well as limit of quantification (LOQ) were evaluated. The validation data of the analytical methodology are summarized in Table 1.
Recovery experiments were performed for all the elements at their concentration ranges similarly found in berry fruits. The observed recoveries (data not shown) ranged between 87 and 109%, with the relative standard deviations lower than 10% in all cases. These results confirmed that no significant metal losses occurred during the digestion procedure. To verify the applicability of the proposed method, standard reference materials Mixed Polish Herbs (INGT-MPH-2) and Leaves of Poplar (NCS DC 73350) were also utilized. Digestion of these materials was performed with the same decomposition procedure used for the blueberry samples. The obtained experimental results were compared with those provided by the manufacturers and are presented in Table 2.
The results concerning the analyzed elements in different samples of wild and cultivated blueberry samples are presented in Table 3. All the results, expressed in milligram per kilogram on wet weight basis, are characterized by mean value with the corresponding standard deviation. Generally, in fruits collected in the forest (wild blueberries), higher levels of macroelements such as Ca, Na, and Mg (metals of natural soil origin) were observed in comparison with cultivated highbush blueberries. These minerals are dietary requirements in human nutrition and have various physiological effects. The values for Ca and Mg are at least 10-fold lower than those of potassium. This fact is common for almost all types of plants because potassium is more mobile in the xylem and phloem (vascular tissues that perform transportation of food and water in a plant) than calcium or magnesium and this element tends to be more concentrated in various parts of the plant . Among microelements, higher contents of Mn and Zn were detected in naturally growing plants. Manganese is an essential element and is bound to a number of essential enzymes, for example, the activity of superoxide dismutase is suppressed by low Mn status . Similar levels of Cu, Cr, Fe, and Ni were detected in both wild-growing and cultivated plants.
Significantly higher content of cadmium was detected in fruits of farm-raised in comparison to wild cultivars. It can be explained by the much higher Cd content in the soil sample collected from the same place (0.398 mg/kg) in comparison with 0.041–0.087 mg/kg determined in soils, where W1–W4 samples were grown. According to The Commission of the European Communities , the maximum allowed levels for Cd and Pb are 0.05 and 0.2 mg/kg, respectively, which are higher than the values detected in this study. Hence, none of these berries would represent a risk to human health from this point of view. Vollmannova et al.  found higher Cu, Zn, and Pb and similar Cd contents in wild blueberries collected in Slovakia in comparison with highbush blueberry cultivar; however, the soil content where the fruits were grown was not examined.
In the present study, the Fe concentration in cultivated blueberries is higher than that in other reports [12, 16], probably due to soil conditions. The sample was collected from the garden plot where acidic soil with high content of Fe (4260 mg/kg) as well as Al (3000 mg/kg) dominates. A similar behavior for iron in different berry fruits cultivated in Brazil has been reported . That fact can justify our results.
Considering the content of individual elements, statistically significant differences were observed among the wild-growing blueberries collected in different locations (samples W1–W4). These variations were probably due to different growing conditions, soil load, and soil properties as well as periodic changes in pollution of soil, water, and air . Although, we selected a limited area for sample collection in order to have uniform climatic conditions. The results of one-way ANOVA followed by Tukey’s test indicated that the contents of Cd, Cr, Cu, and Mn vary significantly (p < 0.05) among all four studied samples.
Blueberries, in fresh or dry form, can be also used for preparation of hot or cold beverages such as tea, juice, or syrup. Thus, it was interesting to check the efficiency of extraction of metal ions by hot water from the fresh as well as from the dried blueberry fruits under study. Samples W1 and C2 (wild and cultivated berries) were chosen for these experiments. Drying of these fruits was performed at 60 °C for 24 h, and acidic digestion was then performed with the same procedure as for fresh fruits. Extraction of metals was performed using 20 mL of freshly boiled distilled water for 20 min. Concentration of metals in the obtained infusions was determined by ICP OES method. The obtained results are presented in Fig. 1. The metal extraction efficiency varied widely for the different blueberries (wild or cultivated) as well as their form (fresh or dried). Generally, the percent of extraction exceeded 80% for K, Cr, and Cu (only for fresh fruits) from all types of fruits. Also the efficiency of extraction for Mn from both wild blueberries and Zn from fresh fruits are very high. The low extractability of Fe was also reported from different kinds of tea . However, the bioaccessibility of this element is greater than that in the other berry fruits . Interesting, the leaching of toxic elements such as Pb and Cd from fresh and dried wild blueberries was very little.