Gas chromatography was coupled with microplasma mass spectrometry for selective detection of organotin compounds. The microplasma ion source was a capacitively coupled radiofrequency helium plasma, which was located inside the high vacuum area of the mass spectrometer. Only 1-3 ml min^-1 of helium carrier gas from the gas chromatograph was necessary for sustaining the plasma while 0.15-1.5 ml min^-1 of hydrogen was added as reagent gas. Hydrogen was applied for prevention of carbon deposition and served to minimize the interactions between tin and the fused-silica inner surface of the microplasma ion source. Both carbon and tin were detected as positively charged atomic ions, which were expelled from the microplasma ion source and directly focused by electrostatic lenses towards the quadrupole mass analyzer. Tin exhibited high selectivity to carbon (>10⁴) and a detection limit of 3.5 pg s^-1. Copyright (C) 1999 Elsevier Science B.V.