Trace metal concentrations in natural freshwater samples are often below detection limits determination by inductively coupled plasma-mass spectrometry (ICP-MS). This is especially so following size fractionation of the sample. Preconcentration is therefore necessary, and sorption on a chelating resin such as Chelex-100 is often used. In order to chelate metals strongly complexed with organics, the metal-organic association has to be severed. In this study, ultra violet (UV)-irradiation was used to oxidize organic matter in a humic acid reference solution, and in a natural sample spiked with Ag, Cd, Cu and Pb, where the recoveries of the metals were calculated. Except for Ag, recoveries of Cd, Cu and Pb increased significantly up to almost 100% after irradiation. If preconcentration on Chelex-100 is used, UV-irriadiation appears to be an essential step in achieving accurate concentrations of organically chelated trace metals in freshwater.