Changes in concentrations of potentially toxic elements during storage of hard river water samples at low temperatures using different sample preservation procedures

Abstract

Purpose According to standard procedure recommended by the Water Framework Directive (WFD), dissolved concentrations of potentially toxic elements (PTEs) in river water are determined by inductively coupled plasma mass spectrometry (ICP-MS) in filtered (0.45 µm) and acidified (pH 2) samples. Properly prepared and stored composite samples can enhance the temporal representativity of monitoring without increasing analytical costs. For this purpose, the WFD recommends freezing, which can preserve the species integrity and prevent adsorption processes of PTEs. Methods Low storage temperature in hard water samples can trigger precipitation of calcium carbonate (CaCO₃) and subsequent co-precipitation of PTEs. To test and determine to what extent co-precipitation with CaCO₃ can influence the determination of PTE concentrations, composite river water samples from two case study catchments in Hungary (Zagyva and Koppány) were prepared following two different sample preservation procedures. To study the behavior of PTEs in river water during storage, in the first procedure, samples were frozen, and they were thawed, filtered, and acidified directly prior to the analysis. In the second procedure, samples were filtered on-site and acidified prior to freezing to prevent precipitation of CaCO₃ and then only thawed to carry out the chemical analyses. Concentrations of PTEs were determined by ICP-MS. Results A statistical evaluation of the results using Student’s t-test revealed significant differences between the two sample preservation procedures, suggesting that PTEs were largely co-precipitated with CaCO₃ if the samples were not acidified prior to freezing. Conclusion When establishing protocols for sample preservation procedures, the phenomenon of co-precipitation of PTEs with CaCO₃ should be considered if the samples were not acidified before freezing. Therefore, to prevent co-precipitation of PTEs with CaCO₃, samples should be filtered and acidified before freezing.