Trace Cu(2+) was detected with high selectivity using specific complexation with bathocuproinesulfonate (BCS) through flow-injection electrospray ionization mass spectrometry (FI-ESI-MS), which separates Cu(2+) from coexisting metal ions by forming a Cu-BCS complex with a high mass number. Here, only [Cu(I)(BCS)2](3-) was obtained with a high ion count. Its calibration curve was linear from 1.0 × 10(-8) to 1.0 × 10(-5) M. This method was applied to determine the Cu-complexing capacities of humic acid solution and river water samples by adding traces of Cu(2+).