CuLs were separated on a Zorbax SB-C18 column (0.5 x 150 mm, 5 µm particles), at a flow rate of 40 µL min -1 and a 30-minute gradient from 95/5% to 5/95% solvent A/B followed by a 5-minute isocratic hold at 5/95% solvent A/B (solvent A = 5 mM ammonium formate in qH2O, solvent B = 5 mM ammonium formate in MeOH). The column flow was directed into an iCAP Q MS (Thermo Scientific) fitted with a perfluoroalkoxy micronebulizer (PFA LC-2040, Elemental Scientific) and a cyclonic spray chamber cooled to 4 °C. Oxygen was introduced at 25 mL min -1 to minimize formation of reduced carbon deposits on the skimmer and sampler cones. Data was collected in kinetic energy discrimination mode with helium as the collision gas. The 63 Cu and 65 Cu ions were monitored with integration times of 0.1 and 0.05 s respectively. Analysis of the culture and medium blank extracts showed that all CuLs were produced by P. tricornutum as there were no detectable CuLs in the medium blank (Fig. ). Additionally, there were no coeluting features in the chromatograms of other bioactive metals such as iron and nickel.