A flow injection (FI) on-line precipitation manifold was coupled to an inductively coupled plasma mass spectrometer (ICP-MS) and applied to the determination of Cr, Mn, Fe, Co, Ni and Cu in certified estuarine water SLEW-2. Sodium diethyldithiocarbamate (NaDDTC) was used as group-selective precipitant. A solution of 2% NaDDTC in 1 M ammonium acetate (at pH 6.75) was mixed with samples on-line to ICP-MS. The precipitates were collected in a knotted reactor–filter system, washed with deionized distilled water, and then dissolved in 50 µL of 4 M nitric acid (containing 10 µg L⁻¹ Ga as an instrumental drift monitor) for delivery to the ICP-MS. The knotted reactor/filter system was heated (at around 70 °C) to enhance the precipitation and dissolution processes. With sample and dissolution volumes of 250 µL and 50 µL, respectively, on-line precipitation/dissolution and data collection took 209 s. An external calibration using aqueous standards in 1% HNO₃ provided results in good agreement with the certified values (with a correction for drift). Furthermore, the accuracy was preserved when the volume of standard solutions injected was different from that of the sample. Smaller volumes of standard solutions can therefore be used to minimize the total analysis time.