Abstract
In recent years, electrospray-ionization mass spectrometry (ESI-MS) has been increasingly used to complement the bulk determination of metal-ligand equilibria, for example via competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). However, ESI-MS speciation analyses may be impacted by instrumental artefacts such as reduction reactions, fragmentation, and adduct formation at the ESI source, changes in the ionization efficiencies of the detected species in relation to sample matrix, and peak overlaps in response to increasing sample complexity. In our study, equilibria of the known artificial ligands citrate, ethylenediaminetetraacetic acid (EDTA), 1-nitroso-2-naphthol (NN), and salicylaldoxime (SA) with iron (Fe) and copper (Cu) were investigated by ultra-high resolution ESI-MS, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), under a variety of sample matrix and ionization settings. The acquired mass spectra were compared with metal-ligand equilibrium data from the literature as well as an adapted speciation model. Overall, the mass spectra produced representative species mentioned in previous reports and predicted by the speciation calculations, such as Fe(Cit), Cu(Cit)2, Fe(EDTA), Cu(EDTA), Fe(NN)3, and Cu(SA)2. The analyses furthermore revealed new species which had been hypothesized but not measured directly using other methods, for example ternary complexes of citrate with Fe and Cu, Cu(SA) monomers, and the dimer Fe(SA)2. Finally, parallel measurements of a Cu+SA calibration series and a Cu+SA+EDTA competition series indicated that FT-ICR-MS can produce linear responses and low detection limits analogous to those of ACSV. We propose that ultra-high resolution FT-ICR-MS can be used as a representative tool to study interactions of trace metals with artificial as well as natural, unknown ligands at the molecular level.
Highlights
Electrospray-ionization mass spectrometry (ESI-MS) has become a promising new technique for the description of metal-ligand equilibria using known ligands
In contrast to the numerous adduct-containing compounds detected via FT-ICR-MS, PHREEQC results had predicted that > 99% of Na+, K+, and Cl− occurred as free ions in aqueous solution, while unbound ligand species differed only in their degree of protonation by free H+ ions
We suggest that ionized ligands, such as citrate and ethylenediaminetetraacetic acid (EDTA), produce better results with aqueous ammonia as added base, while less polar ligands such as NN and SA may profit from an ionizationenhancing base like sodium hydroxide
Summary
Electrospray-ionization mass spectrometry (ESI-MS) has become a promising new technique for the description of metal-ligand equilibria using known ligands. The number of studies in the field is scarce, they have shown that ESI-MS is currently the only technique with the potential to study a large number of metalorganic complexes simultaneously at the molecular level: ESIMS and in particular ESI-FT-ICR-MS (Electrospray-ionization Fourier-transform ion cyclotron resonance mass spectrometry) are capable of resolving the high complexity of DOM, where thousands of unique compounds occur at pico- to nanomolar concentrations (Repeta, 2015) As such, they complement one of the most wide-spread methods currently applied to characterize the bulk metal-binding capacity of natural DOM, competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV, Buck and Bruland, 2005; Buck et al, 2007). ESI-MS, and in particular high-resolution ESI-FTICR-MS, holds great potential to extend this field of research (Waska et al, 2015)
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