Traces of Cu, Ni, Sb, Co, Ag, Cd, Mo, In and Pb in biological materials and natural water samples were determined by axial inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) with ultrasonic nebulization after flow injection (FI) on-line sorption/preconcentration in a knotted reactor (KR). The ability to collect complete mass spectra at a high frequency (more than 20 000 complete mass spectra per second) makes ICP-TOFMS nearly ideal for the detection of FI (transient) signals. The analytes were complexed with ammonium pyrrolidinedithiocarbamate (APDC) from acidic solutions in the flow system and sorbed onto the KR. The relatively small volume of methanol used as eluent, along with the employment of an ultrasonic nebulizer (USN) with membrane desolvation, minimizes the problems produced by the introduction of organic solvents into the plasma. Using a preconcentration time of 120 s and a sample flow rate of 2.8 ml min⁻¹ an enhancement factor of 5 to 70 was achieved. The detection limits (3σ) varied from 0.5 ng l⁻¹ for Sb to 26 ng l⁻¹ for Pb. The precision (RSD) for 11 replicate determinations of 0.2 µg l⁻¹ of each analyte was better than 5%. The accuracy of the method was demonstrated by the analysis of four certified reference materials: a synthetic freshwater (NIST SRM 1643d), a natural filtered coastal water (NRCC CASS-3) and two biological materials (BCR CRM 422 Cod muscle and BCR CRM 278R Mussel tissue) and by recovery measurements on spiked samples.