The invention relates to a method for isolating anionic organic substances from aqueous systems using polymer nanoparticles with cationic pH-sensitive surface groups. Extraction can be carried out directly from diluted solutions, biological media (blood plasma, serum, urine, etc.) and complex buffer systems (e.g. PCR preparations contain detergents) without prior derivatization of the samples or addition of binding buffers. After separation from the surrounding medium the conjugates of the corresponding substances and polymer nanoparticles obtained in this manner can be purified through additional washing steps and effectively desalinated. Owing to the pH-sensitivity of the basic surface groups, the bound substances can be released in a targeted manner after separation by modification of the pH of the medium. By using volatile bases contamination of the released samples with ionic residues can be avoided. By adding small quantities of SDS and acetonitrile during the separation stage, the sensitivity of the method can be raised further, especially when the substance to be isolated are present in low concentrations (