Chemical speciation of trace metals in seawater: a review.

The recent development of the chemical speciation of trace metals in seawater is described. The speciation studies reveal that metal ion complexation is one of the most important processes in seawater; especially, most bioactive trace metals, such as Fe(III) and Cu, exist as complexes with ligands in dissolved organic matter. The organic ligands in seawater are characterized with metal ions selected by the HSAB concept. A strong organic ligand, which originates from marine microorganisms, is classified as a hard base including carboxylates. The free organic ligand concentrations in seawater are buffered by complexation with excess amounts of Ca and Mg in seawater. The chemical equilibrium model suggested that the concentrations of bioactive free metal ions are at an optimal level to activities of marine microorganisms. For chemical speciation, it is important to have a better understanding of the ecological roles of trace metals in seawater.     

Significance of data treatment and experimental setup on the determination of copper complexing parameters by anodic stripping voltammetry

Different procedures of voltammetric peak intensities determination, as well as various experimental setups were systematically tested on simulated and real experimental data in order to identify critical points in the determination of copper complexation parameters (ligand concentration and conditional stability constant) by anodic stripping voltammetry (ASV). Varieties of titration data sets (Cu_measuredvs. Cu_total) were fitted by models encompassing discrete sites distribution of one-class and two-class of binding ligands (by PROSECE software). Examination of different procedures for peak intensities determination applied on voltammograms with known preset values revealed that tangent fit (TF) routine should be avoided, as for both simulated and experimental titration data it produced an additional class of strong ligand (actually not present). Peak intensities determination by fitting of the whole voltammogram was found to be the most appropriate, as it provided most reliable complexation parameters.Tests performed on real seawater samples under different experimental conditions revealed that in addition to importance of proper peak intensities determination, an accumulation time (control of the sensitivity) and an equilibration time needed for complete complexation of added copper during titration (control of complexation kinetics) are the keypoints to obtain reliable results free of artefacts.The consequence of overestimation and underestimation of complexing parameters is supported and illustrated by the example of free copper concentrations (the most bioavailable/toxic specie) calculated for all studied cases. Errors up to 80% of underestimation of free copper concentration and almost two orders of magnitude overestimation of conditional stability constant were registered for the simulated case with two ligands.     

Direct Simultaneous Determination of Cu,Ni and V in Seawater Using Adsorptive Cathodic Stripping Voltammetry with Mixed Ligands

An analytical procedure is proposed for the direct simultaneous determination in a single scan of Cu, Ni and V in seawater by means of adsorptive cathodic stripping voltammetry (ACSV) with mixed ligands (DMG and catechol). Optimum conditions for the determination of these three elements were studied. Detection limits of the technique depended upon the reproducibility of the procedura blank, and were found to be 0.5 nM for Cu, 0.4 nM for Ni and 0.3 nM for V. The method is suitable for the analysis of estuarine, coastal and open‐ocean waters, and especially to study the metal contamination in areas subject to oil spill events.     

Photochemistry of organic iron(III) complexing ligands in oceanic systems

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (>99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photochemically produced radical species, such as superoxide (O2-). The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimentallobservational studies of the natural seawater ligand pool.     

ASV determination of cadmium complexation in seawater. Methodology evaluation

The methodology for using DPASV to study cadmium complexation in seawater is evaluated using EDTA as a model ligand and by analysing natural samples. The results show that the methodology gives an accurate evaluation of metal complexation when inert complexes are studied, both as regards the ligand concentration and the conditional stability constant; the error for both the parameters is lower than 10% at a ligand concentration of about 10(-8) M and a conditional stability constant of 10(9) M-1. Cadmium complexes with ligands present in natural seawater show an evident kinetic lability that may lead to underestimation of the conditional stability constant when a working electrode characterised by a very thick diffusion layer is used. The conditional stability constant in one water sample of the Adriatic coast ranged between 0.14 and 1.4 l/nmol using a rotating disk electrode at rotation rates of 300 and 6000 rpm. The results of cadmium complexation obtained for samples collected in coastal seawater show that the ligands present low specificity for the metal.     

The use of Nafion-coated thin mercury film electrodes for the determination of the dissolved copper speciation in estuarine water

Differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE) was implemented to determine the dissolved copper speciation in saline estuarine waters containing high concentrations of dissolved organic matter (DOM). The study used model ligands and estuarine water from San Francisco Bay, California, USA to demonstrate that the NCTMFE is more effective at distinguishing between electrochemically inert and labile copper species when compared to the conventional thin mercury film electrode (TMFE). Copper titration results verify that the NCTMFE better deals with high concentrations of DOM by creating a size-exclusion barrier that prevents DOM from interacting with the mercury electrode when performing copper speciation measurements. Pseudovoltammograms were used to illustrate that copper complexes found in natural waters were more apt to be electrochemically inert at the NCTMFE relative to the TMFE when subjected to high negative overpotentials. Copper speciation results using the NCTMFE from samples collected in San Francisco Bay estimated that >99.9% of all copper was bound to strong copper-binding ligands. These L₁-class ligands exceeded the concentration of total dissolved copper in all samples tested and control the equilibrium of ambient [Cu²⁺] in the San Francisco Bay estuary.     

A new method for precise determination of iron,zinc and cadmium stable isotope ratios in seawater by double-spike mass spectrometry

The study of Fe, Zn and Cd stable isotopes (δ⁵⁶Fe, δ⁶⁶Zn and δ¹¹⁴Cd) in seawater is a new field, which promises to elucidate the marine cycling of these bioactive trace metals. However, the analytical challenges posed by the low concentration of these metals in seawater has meant that previous studies have typically required large sample volumes, highly limiting data collection in the oceans. Here, we present the first simultaneous method for the determination of these three isotope systems in seawater, using Nobias PA-1 chelating resin to extract metals from seawater, purification by anion exchange chromatography, and analysis by double spike MC-ICPMS. This method is designed for use on only a single litre of seawater and has blanks of 0.3, 0.06 and <0.03 ng for Fe, Zn and Cd respectively, representing a 1–20 fold reduction in sample size and a 4–130 decrease in blank compared to previously reported methods. The procedure yields data with high precision for all three elements (typically 0.02–0.2‰; 1σ internal precision), allowing us to distinguish natural variability in the oceans, which spans 1–3‰ for all three isotope systems. Simultaneous extraction and purification of three metals makes this method ideal for high-resolution, large-scale endeavours such as the GEOTRACES program.     

Determination of iron and copper in seawater at pH 1.7 with a new commercially available chelating resin, NTA Superflow

The use of a commercially available chelating resin with NTA-type functional groups for concentration of trace metals from seawater is described. Trace metal recoveries from this NTA Superflow chelating resin are pH dependent. At a pH of ≤2 only iron(III) and copper are quantitatively recovered from the resin. Iron(II) cannot be quantitatively recovered from this resin below a pH of 5. However, oxidation of acidified seawater samples (pH 1.7) with H₂O₂ prior to loading onto the resin has been demonstrated to allow quantitative recovery of total dissolved iron. Deferrioxamine and Rhodoturlic Acid, two commercially available siderophores were used to investigate the effect of strong Fe(III)-binding organic ligands on the ability to retain iron at different pH values. Acidification of seawater samples to pH 1.7 dissociates the iron complexed to these organic ligands, thereby allowing total dissolved iron and copper to be determined. Acidified samples from Monterey Bay were analyzed by a flow injection method coupled to ICP-SFMS detection using the NTA Superflow resin in the pre-concentration step. Results from this study show that when seawater samples are stored acidified (pH 1.7) over time, a portion of iron(III) is reduced to iron(II), thus necessitating the use of H₂O₂ to reoxidize the Fe(II) to Fe(III) prior to analysis. Total dissolved concentrations of iron and copper can be directly obtained on seawater samples at pH 1.7 with this method, eliminating the need to buffer the sample to a higher pH prior to column loading. This resin has the potential to be used in shipboard or in situ flow injection methods.     

Ligand‐Free,Cu‐ and Fe‐Catalyzed Selective Ring‐Opening Arylations of Benzoxazoles with Aryl Iodides

Cu‐ or Fe‐based catalyst systems have been reported to selectively catalyze the N,N‐diarylation or N‐monoarylation of benzoxazoles ring‐opening with aryl iodides in the absence of additional added ligand in polyethylene glycol under an inert atmosphere. Two types of coupling products (triphenylamines and diphenylamines) have been examined and the reaction routes can be simply controlled by changing the metal salts (Cu or Fe) as catalyst. A range of substrates have been investigated for the diverse reactions, and the corresponding arylation products were achieved in good to high yields. This selective, low‐cost, and environmentally friendly protocol displays great potential for replacing existing methodologies as well as extending the synthetic applications of benzoxazoles.     

Toward optimized high-relaxivity MRI agents: thermodynamic selectivity of hydroxypyridonate/catecholate ligands

The thermodynamic selectivity for Gd(3+) relative to Ca(2+), Zn(2+), and Fe(3+) of two ligands of potential interest as magnetic resonance imaging (MRI) contrast agents has been determined by NMR spectroscopy and potentiometric and spectrophotometric titration. The two hexadentate ligands TREN-6-Me-3,2-HOPO (H(3)L2) and TREN-bisHOPO-TAM-EA (H(4)L3) incorporate 2,3-dihydroxypyridonate and 2,3-dihydroxyterephthalamide moieties. They were chosen to span a range of basicity while maintaining a structural motif similar to that of the parent ligand, TREN-1-Me-3,2-HOPO (H(3)L1), in order to investigate the effect of the ligand basicity on its selectivity. The 1:1 stability constants (beta(110)) at 25 degrees C and 0.1 M KCl are as follows. L2: Gd(3+), 20.3; Ca(2+), 7.4; Zn(2+), 11.9; Fe(3+), 27.9. L3: Gd(3+), 24.3; Ca(2+), 5.2; Zn(2+), 14.6; Fe(3+), 35.1. At physiological pH, the selectivity of the ligand for Gd(3+) over Ca(2+) increases with the basicity of the ligand and decreases for Gd(3+) over Fe(3+). These trends are consistent with the relative acidities of the various metal ions;- more basic ligands favor harder metals with a higher charge-to-radius ratio. The stabilities of the Zn(2+) complexes do not correlate with basicity and are thought to be more influenced by geometric factors. The selectivities of these ligands are superior to those of the octadentate poly(aminocarboxylate) ligands that are currently used as MRI contrast agents in diagnostic medicine.     

Direct simultaneous determination of Co, Cu, Fe, Ni and V in pore waters by means of adsorptive cathodic stripping voltammetry with mixed ligands

An analytical procedure is proposed for the direct simultaneous determination in a single scan of Co, Cu, Fe, Ni and V in sediment pore waters by means of adsorptive cathodic stripping voltammetry (ACSV) with mixed ligands (DMG and catechol). Optimum conditions for the determination of these five elements were studied. Detection limits of the technique depended upon the reproducibility of the procedure blank, and were found to be 0.04 nM Co, 0.09 nM Cu, 1.29 nM Fe, 0.46 nM Ni and 2,52 nM V making the method suitable for the direct simultaneous determination of these five metals in pore waters, estuarine waters and probably coastal waters.     

Two-dimensional turbulent flow chromatography coupled on-line to liquid chromatography–mass spectrometry for solution-based ligand screening against multiple proteins

We present herein a novel bioseparation/chemical analysis strategy for protein–ligand screening and affinity ranking in compound mixtures, designed to increase screening rates and improve sensitivity and ruggedness in performance. The strategy is carried out by combining on-line two-dimensional turbulent flow chromatography (2D-TFC) with liquid chromatography–mass spectrometry (LC–MS), and accomplished through the following steps: (1) a reversed-phase TFC stage to separate the protein/ligand complex from the unbound free molecules, (2) an on-line dissociation process to release the bound ligands from the complexes, and (3) a second mixed-mode cation-exchange/reversed-phase TFC stage to trap the bound ligands and to remove the proteins and salts, followed by LC–MS analysis for identification and determination of the binding affinities. The technique can implement an ultra-fast isolation of protein/ligand complex with the retention time of a complex peak in about 5 s, and on-line prepare the “clean” sample to be directly compatible with the LC–MS analysis. The improvement in performance of this 2D-TFC/LC–MS approach over the conventional approach has been demonstrated by determining affinity-selected ligands of the target proteins acetylcholinesterase and butyrylcholinesterase from a small library with known binding affinities and a steroidal alkaloid library composed of structurally similar compounds. Our results show that 2D-TFC/LC–MS is a generic and efficient tool for high-throughput screening of ligands with low-to-high binding affinities, and structure-activity relationship evaluation.     

Trace analysis of acids and bases by conductometric titration with multiparametric non-linear regression

A chemometric method for analysis of conductometric titration data was introduced to extend its applicability to lower concentrations and more complex acid-base systems. Auxiliary pH measurements were made during the titration to assist the calculation of the distribution of protonable species on base of known or guessed equilibrium constants. Conductivity values of each ionized or ionizable species possibly present in the sample were introduced in a general equation where the only unknown parameters were the total concentrations of (conjugated) bases and of strong electrolytes not involved in acid-base equilibria. All these concentrations were adjusted by a multiparametric nonlinear regression (NLR) method, based on the Levenberg-Marquardt algorithm. This first conductometric titration method with NLR analysis (CT-NLR) was successfully applied to simulated conductometric titration data and to synthetic samples with multiple components at concentrations as low as those found in rainwater (∼10 μmol L⁻¹). It was possible to resolve and quantify mixtures containing a strong acid, formic acid, acetic acid, ammonium ion, bicarbonate and inert electrolyte with accuracy of 5% or better.     

Derivatives of phosphate Schiff base transition metal complexes: synthesis, studies and biological activity

We report the synthesis and structural characterization of series of tetra- and hexacoordinate metal chelate complexes of phosphate Schiff base ligands having the general composition LMX(n).H(2)O and L(2)MX(n) (L=phosphate Schiff base ligand; M=Ag(+), Mn(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), or Fe(3+) and X=NO(3)(-), Br(-) or Cl(-)). The structure of the prepared compounds was investigated using elemental analysis, IR, 1H and 31P NMR, UV-vis, mass spectra, solid reflectance, magnetic susceptibility and conductance measurements as well as conductometric titration. In all the complexes studied, the ligands act as a chelate ligand with coordination involving the phosphate-O-atom and the azomethine-N-atom. IR, solid reflectance spectra and magnetic moment measurement are used to infer the structure and to illustrate the coordination capacity of ligand. IR spectra show the presence of coordinated nitrate and water molecule, the magnetic moments of all complexes show normal magnetic behavior and the electronic spectra of the metal complexes indicate a tetra- and octahedral structure for Mn(2+), octahedral structure of Fe(3+) and both square-planar and distorted octahedral structure for Cu(2+) complexes. Antimicrobial activity of the ligands and their complexes were tested using the disc diffusion method and the chosen strains include Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Microsporum canis, Trichophyton mentagrophyte and Trichophyton rubrum. Some known antibiotics are included for the sake of comparison and the chosen antibiotic are Amikacin, Doxycllin, Augmantin, Sulperazon, Unasyn, Septrin, Cefobid, Ampicillin, Nitrofurantion, Traivid and Erythromycin.     

Effect of the deposition potential on the voltammetric determination of complexing ligand concentrations in sea-water.

The effect of the deposition potential on the determination of complexing ligand concentrations in natural waters was investigated by titration with Cu from sea-water samples originating from the North Sea and monitoring the labile Cu concentration after deposition at two potentials, viz., -0.05 and -0.7 V. The amount of deposited metal was measured by cathodic stripping voltammetry with ligand competition, using tropolone as the added ligand. It was found that the complexing ligand concentration detected decreased by more than 60% when the more negative deposition potential was applied. The data indicate that a significant fraction of the organic complexes of Cu in natural waters is electrochemically labile at negative deposition potentials and is not determined by anodic stripping voltammetry. The concentrations and conditional stability constants of the complexing ligands in the North Sea are similar to those detected previously in the Irish Sea.     

Intermolecular interaction studies using small volumes

We present the use of 1-mm room-temperature probe technology to perform intermolecular interaction studies using chemical shift perturbation methods and saturation transfer difference (STD) spectroscopy using small sample volumes. The use of a small sample volume (5-10 μl) allows for an alternative titration protocol where individual samples are prepared for each titration point, rather than the usual protocol used for a 5-mm probe setup where the ligand is added consecutively to the solution containing the protein or host of interest. This allows for considerable economy in the consumption and cost of the protein and ligand amounts required for interaction studies. For titration experiments, the use of the 1-mm setup consumes less than 10% of the ligand amount required using a 5-mm setup. This is especially significant when complex ligands that are only available in limited quantities, typically because they are obtained from natural sources or through elaborate synthesis efforts, need to be investigated. While the use of smaller volumes does increase the measuring time, we demonstrate that the use of commercial small volume probes allows the study of interactions that would otherwise be impossible to address by NMR.     

Effect of nitrate on the determination of iron concentration in phytoplankton culture medium by liquid scintillation counting (LSC) method using 55Fe as radioisotope tracer

Liquid scintillation counting (LSC) method using a radioisotope tracer has several advantages such as simple procedure, high sensitivity and low detection limit, and has been used for the determination Fe concentrations in water samples. Several factors such as nitrate concentration, pH, chelating ligand affect the efficiency of this method in the determination of iron (Fe) in waters. In this study, the effect of nitrate in phytoplankton culture medium on the determination of Fe concentration by LSC method using ⁵⁵Fe radioisotope tracer was evaluated. The measured Fe concentrations in the medium were lower than its added concentration (1.5 μM) when liquid samples contain nitrate. Fe concentrations decreased exponentially as nitrate concentrations increased up to 2.64 mM, reaching a constant value of 1.31 μM Fe at nitrate concentrations higher than 2.64 mM. A correction factor (f = 1.14) was calculated from the decrease rate of Fe concentrations at different nitrate concentrations in the phytoplankton culture medium. This correction factor can be used to correct the measurement values of Fe concentrations in phytoplankton culture medium obtained from LSC method. Our results showed that up to 94 % of the added Fe can be determined by LSC using ⁵⁵Fe radioisotope tracer. The remaining 6 % was probably bound to the walls of the culturing vessel. This method is also applicable for the measurement of Fe size-fractionation in phytoplankton culture medium.     

Anionic ligand effect on the nature of epoxidizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X(-) = anionic ligand) and iodosylbenzene (PhIO) were 14 and 0.9 when the X(-) of Fe(TPP)X was Cl(-) and CF(3)SO(3)(-), respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion and X(-) = anionic ligand], and various terminal oxidants such as PhIO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H(2)O(2). While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-), the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-). When the anionic ligand was NO(3)(-), the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(III) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-), whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-).     

Hydrophobic Effects of o-Phenanthroline and 2,2′-Bipyridine on Adsorption of Metal(II) Ions onto Silica Gel Surface

The effects of o-phenanthroline and 2,2′-bipyridine on the adsorption of metal(II) (Fe, Co, Ni and Cu) ions onto silica gel surface have been studied. The adsorption is expressed in terms of the measured concentrations of both metal and ligand at equilibrium. Each adsorption of the four metal ions is increased with the presence of the ligands. In addition, adsorption increases slowly with pH at low pH values and then increases rapidly up to near the pK_a value of silica gel (≈6.5). The adsorption of each metal ion at low pH is increased with increased ligand concentration. However, at high pH the adsorptions of Fe(II) and Cu(II) are decreased with increased ligand concentration whereas the adsorptions of Co(II) and Ni(II) are always increased. At low pH values the ligand to metal ratio adsorbed on the silica gel surface is ca. 3:1 while at high pH values it is 1:1, 2:1, and 3:1, corresponding to the initial ligand to metal ion concentration ratio. The addition of ethanol to the phenanthroline-SiO₂ solution results in a decrease in the adsorption of phenanthroline. The effect of ethanol is also observed in the Fe(II)-phenanthroline-SiO₂ system. The behavior of the adsorption is interpreted qualitatively by hydrophobic expulsion, the formation of surface complexes, and electrostatic interaction. It is concluded that hydrophobic expulsion plays an important role in the adsorption of metal ions in the presence of hydrophobic ligands on silica gel surface.     

Coordination properties of some nickel(II) monophosphono dipeptides species in aqueous solution: The role of phosphonic oxygen in complex stabilization and ligand-fields analysis

Equilibrium studies on the nickel(II) complexes of oxygen and nitrogen donor ligand (monophosphono dipeptides: 1- (N-l-leucylamino)methylphosphonic acid – Leu-Gly(P), and a thioether sulfur donor ligands (monophosphono dipeptides: 1-N-(glycyloamino)-2-(S-benzylthio)ethanephosphonic acid – Gly-(Bz)Cys(P), d(−) and Gly-(Bz)Cys(P), d(+) were performed by potentiometric titration and NIR–Vis spectroscopy. Additionally, the ligand-field parameters (CFM/AOM) were estimated and discussed in the tetragonal distortion framework. The lowest tetragonal distortion was observed in the case of the [NiHL] species, whereas the strongest in the case of [NiL₂] species. In the latter species the ligand-field analysis confirms the coordination of the deprotonated phosphonic groups to the metal ions in both axial positions. In the case of nickel(II) complexes with Gly-(Bz)Cys(P), d(−) or Gly-(Bz)Cys(P), d(+) additional interaction between sulfur donor atom and nickel(II) was suggested.