[en] A non-ionic polymeric sorbent DAX-8 (XAD-8) was applied to the isolation of humic and fulvic acids from groundwater. This procedure was able to isolate approximately 40% of the DOC as humic solutes. For the investigation of the structure and molecular size distribution of the isolated humic solutes, the hyphenated SEC-ESI-MS system with a quadrupole mass spectrometer and SEC-UV and fluorescence detectors was utilized. For the higher-molecular-weight humic acids, the ESI-MS loses sensitivity compared with the parallel UV detection, because of the difficulty in getting the ionized humic compounds to fly efficiently through the mass spectrometer. (orig.)