Diatoms, a globally important group of eukaryotic phytoplankton, have many metabolic uses for Cu, including in iron acquisition and electron transport. Diatoms are known to produce ligands that complex copper, and recently it has been shown that the diatoms Thalassiosira oceanica and Phaeodactylum tricornutum can acquire Cu complexed to ethylenediaminetetraacetic acid (EDTA) through a reductive pathway, which suggests that at least some portion of organically bound Cu in seawater could be bioavailable to diatoms. Although diatoms have a metabolic requirement for Cu, they are still susceptible to cellular damage and death due to high concentrations of Cu, and some diatoms have been shown to respond to high Cu through the intracellular production of phytochelatins. Here we report the structural characterization of CuLs produced by the widely studied marine diatom P. tricornutum using the custom-built hybrid linear ion trap 21 tesla FT-ICR MS at the National High Magnetic Field Laboratory. This instrument achieves the high mass resolving power, acquisition speed, dynamic range, and mass accuracy needed for analyzing complex organic matricies, and is uniquely suited to provide structural characterization of marine CuLs.