The disclosure relates to a method for pretreating a sample for metals determination. The method includes: providing an aqueous sample mixture comprising a sample containing or suspected of containing one or more metals for detection; contacting the aqueous sample mixture with a first electrode (anode) comprising electrically conducting boron-doped diamond (BDD); electrically contacting the aqueous sample mixture with a second electrode (cathode); applying an electrical potential between the first electrode and the second electrode (i) to provide an electrical current therebetween and through the aqueous sample mixture, (ii) to generate hydroxyl ion (OH−) species at the first electrode, (iii) to oxidize and free the one or more metals for detection in the sample, thereby forming a pretreated aqueous sample comprising free metal ions in aqueous solution and corresponding to the one or more metals in the original sample; and withdrawing the pretreated aqueous sample comprising the free metal ions in aqueous solution. The pretreated aqueous sample can be analyzed for metal content using any desired conventional analysis technique.