An algorithm to characterize cobalt ligands in GEOTRACES samples

Iron, copper, and cobalt are complexed to biologically produced ligands in seawater, and are called high priority parameters in the GEOTRACES Pacific Meridional Transect (GP15). Recent advances in coupling liquid chromatography (LC) to mass spectrometry (MS) allows metal-ligand complexes to be quantified by LC-inductively coupled plasma (ICP) MS, and characterized by LC-electrospray ionization (ESI) MS. For multi-isotope elements (Fe, Cu), the molecular mass of metal-ligand complexes are identified by a ‘bottom-up’ algorithm that searches ESIMS data, for co-eluting ion pairs (isotopologues) with a specific mass difference (1.995 D for ⁵⁶Fe and ⁵⁴Fe) and relative abundance (⁵⁶FeL : ⁵⁴FeL = 15.7). Unlike iron or copper, Co has only one isotope, which makes this isotopologue-based algorithm inapplicable to Co ligands. Here, we introduce a ‘top-down’ filter, which searches through each extracted ion chromatogram, and returns masses of all the organic compounds eluting within given retention time window. Since this time window is selected from the ICPMS data to cover retention time for cobalt-ligand complex, the algorithm output includes the mass of the complex. The sample is then analyzed using a different set of chromatographic conditions. By comparing the output from both analyses we are able to find the mass of monoisotopic complexes. The combination a ‘top down’ filter and two-dimension separation shows potential for characterizing Co ligands and other organic compounds containing monoisotopic elements such as phosphorus, iodine, and vanadium.