Investigating the potential of solid-phase extraction and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) for the isolation and identification of dissolved metal-organic complexes from natural waters

In aquatic environments, iron and copper often occur at low concentrations, and are bound to dissolved organic matter (DOM). The detection and chemical characterization of such iron- and copper-complexes are only slowly progressing due to the high diversity of natural DOM. Recent developments of sample preparation and measurement techniques have opened up new opportunities to selectively isolate and identify metal-organic compound groups from natural water samples. In this study, the isolation of the model complex desferrioxamine-iron (DFO-Fe) and naturally occurring iron- and copper-organic complexes from a groundwater and a seawater sample was investigated with a variety of solid-phase extraction (SPE) sorbents (polar to nonpolar) and extraction/elution (low and neutral pH) setups. Natural DOM, the model complex desferrioxamine-iron (DFO-Fe), and natural Cu-binding ligands were recovered in a range of 60-100% at low pH by the majority of the sorbents. In contrast, extraction efficiencies for solid-phase extractable iron-DOM and copper-DOM compounds (SPE-Fe-DOM and SPE-Cu-DOM) were overall lower and highly influenced by pH settings, sample type, and sorbent type. Especially, the SPE techniques preferentially isolated SPE-metal-DOM with neutral/weak acidic to basic characteristics, fractions which made up a relatively small amount (~ 10-15%) of the total natural SPE-DOM from the two water samples. Furthermore, our data indicate that solid-phase extractable small Fe-DOM colloids and acid-stable Fe- and Cu-organic compounds occur in natural DOM. A series of desferrioxamine-copper (DFO-Cu) and desferrioxamine-iron (DFO-Fe) calibration solutions were measured by ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), and both complexes were detectable in positive ion mode at concentrations as low as 150 nM (DFO-Cu) and 2 nM (DFO-Fe). DFO-Fe and DFO-Cu in the nM range were also detectable when added to a groundwater DOM sample to evaluate matrix effects by the natural DOM background. Our study demonstrates that SPE combined with high-resolution FT-ICR-MS is currently the most promising technique to isolate and unambiguously identify dissolved metal-organic complexes in highly diverse natural water samples.