Oysters and Noah's Ark shells are edible bivalves that live on Adriatic shore and are often used for determination of pollutants that are present in the sea because of their structure, which enables them to accumulate heavy metals from anthropogenic sources. Main advantages of analyzing shells over tissue is that it gives information about accumulated concentration of metals in bivalves. Some of the ways in which metals can be incorporated into shells, which are primarily made of aragonite and calcite, are precipitation of metals onto surface of calcium carbonate and substitution of calcium cation with other metals. Instrument that was used to analyze shells was portable XRF analyzer which has factory-calibrated modes. To validate analytical method, we also analyzed standard mixtures of CaCO3, and shell sample mixed with oxide and nitrate salts of Zn, Ni, Cu, Pb, Cd and Cr. Calculated parameters (RE, MAPE, MAE, t-test) indicate that this method is good for analysis of this type of samples. However, it has limited number of factory-calibrated modes, which showed that results need to be corrected. As a solution of this problem, we propose that manufacturers generate new calibration modes and program their instruments with option to calibrate instrument using standard samples.