Dynamic liquid secondary ion mass spectrometry (dyn-LSIMS) was employed to acquire continuous, on-line mass spectral data from the effluent of a reversed-phase high-performance liquid chromatograph (HPLC) used to separate a broad suite of carbon-centered radicals trapped as the O-alkylhydroxylamine adducts of an amino nitroxide that was subsequently derivatized with fluorescamine. Data obtained by the use of these combined techniques (LC/MS) can be employed to elucidate radical adduct structures; elemental compositions of the adducts can be confirmed by acquiring mass spectra at high resolution. At low resolution, introduction into the source of < 1 pmol of adduct yielded usable spectra. The first application of this technique to the identification of photochemically generated radicals in natural water samples is presented.