Determination of iron in seawater by electrothermal atomic absorption spectrometry and atomic fluorescence spectrometry: A comparative study

Two methods available for direct determination of total Fe in seawater at low concentration level have been examined: electrothermal atomization atomic absorption spectrometry (ETAAS) and electrothermal atomization laser excited atomic fluorescence spectrometry (ETA-LEAFS). In a first part, we have optimized experimental conditions of ETAAS (electrothermal program, matrix chemical modification) for the determination of Fe in seawater by minimizing the chemical interference effects and the magnitude of the simultaneous background absorption signal. By using the best experimental conditions, a detection limit of 80 ng L⁻¹ (20 μL, 3σ) for total Fe concentration was obtained by ETAAS. Using similar experimental conditions (electrothermal program, chemical modification), we have optimized experimental conditions for the determination of Fe by LEAFS. The selected experimental conditions for ETA-LEAFS: excitation wavelength (296.69 nm), noise attenuation and adequate background correction led to a detection limit (3σ) of 3 ng L⁻¹ (i.e. 54 pM) for total Fe concentration with the use a 20 μL seawater sample. For the two methods, concentration values obtained for the analysis of Fe in a NASS-5 (0.2 μg L⁻¹) seawater sample were in good agreement with the certified values.     

Interferences in the analysis of nanomolar concentrations of nitrate and phosphate in oceanic waters

This paper reports on investigations into interferences with the measurements of nanomolar nitrate + nitrite and soluble reactive phosphate (SRP) in oceanic surface seawater using a segmented continuous flow autoanalyser (SCFA) interfaced with a liquid-waveguide capillary flow-cell (LWCC). The interferences of silicate and arsenate with the analysis of SRP, the effect of sample filtration on the measurement of nanomolar nitrate + nitrite and SRP concentrations, and the stability of samples during storage are described.The investigation into the effect of arsenate (concentrations up to 100 nM) on phosphate analysis (concentrations up to 50 nM) indicated that the arsenate interference scaled linearly with phosphate concentrations, resulting in an overestimation of SRP concentrations of 4.6 ± 1.4% for an assumed arsenate concentration of 20 nM. The effect of added Si(OH)₄ was to increase SRP signals by up to 36 ± 19 nM (at 100 μM Si(OH)₄). However, at silicate concentrations below 1.5 μM, which are typically observed in oligotrophic surface ocean waters, the effect of silicate on the phosphate analysis was much smaller (≤0.78 ± 0.15 nM change in SRP). Since arsenate and silicate interferences vary between analytical approaches used for nanomolar SRP analysis, it is important that the interferences are systematically assessed in any newly developed analytical system.Filtration of surface seawater samples resulted in a decrease in concentration of 1.7-2.7 nM (±0.5 nM) SRP, and a small decrease in nitrate concentrations which was within the precision of the method (±0.6 nM). A stability study indicated that storage of very low concentration nutrient samples in the dark at 4 °C for less than 24 h resulted in no statistically significant changes in nutrient concentrations. Freezing unfiltered surface seawater samples from an oligotrophic ocean region resulted in a small but significant increase in the SRP concentration from 12.0 ± 1.3 nM (n = 3) to 14.7 ± 0.6 nM (n = 3) (Student's t-test; p = 0.021). The corresponding change in nitrate concentration was not significant (Student's t-test; p > 0.05).     

Characterization of an electrochemical mercury sensor using alternating current,cyclic, square wave and differential pulse voltammetry

Here we report the characterization of an electrochemical mercury (Hg²⁺) sensor constructed with a methylene blue (MB)-modified and thymine-containing linear DNA probe. Similar to the linear probe electrochemical DNA sensor, the resultant sensor behaved as a “signal-off” sensor in alternating current voltammetry and cyclic voltammetry. However, depending on the applied frequency or pulse width, the sensor can behave as either a “signal-off” or “signal-on” sensor in square wave voltammetry (SWV) and differential pulse voltammetry (DPV). In SWV, the sensor showed “signal-on” behavior at low frequencies and “signal-off” behavior at high frequencies. In DPV, the sensor showed “signal-off” behavior at short pulse widths and “signal-on” behavior at long pulse widths. Independent of the sensor interrogation technique, the limit of detection was found to be 10 nM, with a linear dynamic range between 10 nM and 500 nM. In addition, the sensor responded to Hg²⁺ rather rapidly; majority of the signal change occurred in <20 min. Overall, the sensor retains all the characteristics of this class of sensors; it is reagentless, reusable, sensitive, specific and selective. This study also highlights the feasibility of using a MB-modified probe for real-time sensing of Hg²⁺, which has not been previously reported. More importantly, the observed “switching” behavior in SWV and DPV is potentially generalizable and should be applicable to most sensors in this class of dynamics-based electrochemical biosensors.     

Selective extraction, separation and speciation of iron in different samples using 4-acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid

A method for speciation, preconcentration and separation of Fe(II) and Fe(III) in different matrices was developed using solvent extraction and flame atomic absorption spectrometry. 4-Acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid (AMPC) was used as a new complexing reagent for Fe(III). The Fe(III)-AMPC complex was extracted into methyl isobutyl ketone (MIBK) phase in the pH range 1.0-2.5, and Fe(II) ion remained in aqueous phase at all pH. The chemical composition of the Fe(III)-AMPC complex was determined by the Job's method. The optimum conditions for quantitative recovery of Fe(III) were determined as pH 1.5, shaking time of 2 min, 1.64 × 10⁻⁴ mol L⁻¹ AMPC reagent and 10 mL of MIBK. Furthermore, the influences of diverse metal ions were investigated. The level of Fe(II) was calculated by difference of total iron and Fe(III) concentrations. The detection limit based on the 3σ criterion was found to be 0.24 μg L⁻¹ for Fe(III). The recoveries were higher than 95% and relative standard deviation was less than 2.1% (N = 8). The validation of the procedure was performed by the analysis of two certified standard reference materials. The presented method was applied to the determination of Fe(II) and Fe(III) in tap water, lake water, river water, sea water, fruit juice, cola, and molasses samples with satisfactory results.     

A pH responsive electrochemical switch sensor based on Fe(notpH3) [notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)

The electrochemistry of a macrocyclic metal complex Fe(notpH₃) [notpH₆ = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylene-phosphonic acid)] reveals that the protonation/deprotonation of the non-coordinated P-OH groups in Fe(notpH₃) affects its formal potential value (E⁰′) considerably. Plotting E⁰′ as function of solution pH gives a straight line with a slope of −585 mV pH⁻¹ in the pH range of 3.4-4.0, which is about ten times larger than the theoretical value of −58 mV pH⁻¹ for a reversible proton-coupled single-electron transfer at 20 °C. A sensitive pH responsive electrochemical switch sensor is thus developed based on Fe(notpH₃) which shows an “on/off” switching at pH ∼ 4.0.     

An aptameric molecular beacon-based “Signal-on” approach for rapid determination of rHuEPO-α

The aim of this work is to describe the first example of aptameric molecular beacon (MB)-based probe for the detection of recombinant human erythropoietin (rHuEPO-α) in physiological buffer, using a novel 35 nt ssDNA aptamer (807-35 nt) originally isolated by Systematic Evolution of Ligands by Exponential enrichment (SELEX) technique in our laboratory. Both “Signal-on” and “Signal-off” MB modes were developed, respectively, in which the conformational alteration of aptamer before and after binding to rHuEPO-α can be demonstrated in terms of the correspondingly fluorescent changes. Comparing with “Signal-off” mode, “Signal-on” mode provided higher sensitivity, while with the addition of target rHuEPO-α, quenching between fluorescent 807-35 nt aptamer (F-Apt) and a short quencher-labeled complementary sequence (QDNA) was disturbed by the specific binding between rHuEPO-α and F-Apt. QDNA was thus loosened and released from F-Apt, leading to a consequently full fluorescent restoration. Systematic optimization of parameters in “Signal-on” mode were carried out, the choice of QDNA length, the hybridization site of a small supplementary DNA (SDNA) stabilizer, and the existence of Mg²⁺ cation played essential roles for the performance characterization. A convenient and sensitive determination of rHuEPO-α with a LOD of 0.4 nM was achieved.     

Evolution of structural features and mechanical properties during the conversion of poly[(methylamino)borazine] fibers into boron nitride fibers

Poly[(methylamino)borazine] (PolyMAB) green fibers of a mean diameter of 15 μm have been pyrolyzed under ammonia up to 1000°C and heat treated under nitrogen up to 2000°C to prepare boron nitride (BN) fibers. During the polymer-to-ceramic conversion, the mechanical properties of the green fibers increase within the 25-400°C temperature range owing to the formation of a preceramic material and remain almost constant up to 1000°C. Both the crystallinity and the mechanical properties slightly increase within the 1000-1400°C range, in association with the consolidation of the fused-B₃N₃ basal planes. A rapid increase in tensile strength (σ^R) and elastic modulus (Young's modulus E) is observed in relation with crystallization of the BN phase for fibers treated between 1400°C and 1800°C. At 2000°C, “meso-hexagonal” BN fibers of 7.5 μm in diameter are finally obtained, displaying values of σ^R=1.480 GPa and E=365 GPa. The obtention of both high mechanical properties and fine diameter for the as-prepared BN fibers is a consequence of the stretching of the green fibers on a spool which is used during their conversion into ceramic.     

Magnetic molecular wheels and grids - the need for novel concepts in “zero-dimensional” magnetism

Supramolecular chemistry has allowed the production, by self-assembly, of inorganic complexes with a [N × N] square matrix-like configuration of N² metal centers. Interest in these systems is driven by the potential applications in information technology suggested by such a “two-dimensional” (2D), addressable arrangement of metal ions. From the magnetic perspective [N × N] grids constitute molecular model systems for magnets with extended interactions on a square lattice, which have gained enormous attention in the context of high-temperature superconductors. Numerous [2 × 2] grids as well as a few [3 × 3] grids with magnetic metal ions such as Cu(II), Ni(II), Co(II), Fe(II), and Mn(II) have been created. Magnetic studies unraveled a remarkable variety in their magnetic properties, which will be reviewed in this work with emphasis on the underlying physical concepts. An intriguing issue is the connection of [2 × 2] and [3 × 3] grids with “one-dimensional” (1D) rings, as experimentally realized in the molecular wheels. For a [2 × 2] square of spin centers the distinction between a 2D grid and a 1D ring is semantic, but also a [3 × 3] grid retains 1D character: it is best viewed as an octanuclear ring with an additional ion “doped” into its center. Challenging familiar concepts from conventional magnets, the current picture of elementary excitations in antiferromagnetic rings will be discussed, as a prerequisite to understand the complex [3 × 3] grids.     

Characterisation of a hydraulic high-pressure sample introduction assisted flow injection-inductively coupled plasma time-of-flight mass spectrometry system and its application to the analysis of biological samples

A flow injection (FI) method was developed using hydraulic high-pressure nebulization as a sample introduction system, coupled to inductively coupled plasma time-of-flight mass spectrometer (ICP-TOFMS) for rapid and simultaneous determination of 19 elements. The operating conditions of the system (analyte flow rate, heating and cooling temperatures of the desolvation module, carrier gas flow rate) for the simultaneous determination of 19 analytes were optimised. The optimum parameters of the sample introduction system were found to be 1.4 ml min⁻¹ and 1.35 l min⁻¹ for the analyte solution and nebulizer flow rates, respectively. A compromised condition for heating and cooling stage temperatures of 170 and −5 °C was chosen. The detection limits were compared to those obtained by using ICP-TOFMS with alternative sample introduction techniques e.g. conventional nebulization, flow injection chemical hydride generation (FI-CHG) and the obtained results were comparable or better than those resulting from alternative sample introduction. Applying the optimised conditions the simultaneous determination of Ag, As, Ba, Cd, Co, Cu, Ga, In, Li, Mn, Mo, Pb, Sb, Se, Sn, Sr, Tl, V and Zn was carried out. Absolute detection limits (3σ) in the range of 2-750 pg and precision between 0.5 and 9.6% from five replicate measurements of 10 ng ml⁻¹ multielemental sample solutions were achieved by using a 200 μl sample loop. The developed method was applied for the analysis of certified reference materials of biological origin (TORT-2 “Lobster Hepatopancrease”, BCR-422 “Cod Muscle” and IAEA MA-B-3/TM “Fish Homogenate”), and the results showed good agreement with the certified values.     

Reverse flow injection analysis of nanomolar soluble reactive phosphorus in seawater with a long path length liquid waveguide capillary cell and spectrophotometric detection

A novel reverse flow injection analysis method coupled with a liquid waveguide capillary cell (LWCC) and spectrophotometric detection for the determination of nanomolar soluble reactive phosphorus in seawater was established. Reagent was injected into the sample stream and detected in a 2-m path length LWCC with detection wavelength set at 710 nm. Experimental parameters, including the reagent concentration, the injection volume, the flow rate and the length of the mixing coil, were optimized based on univariate experimental design. The interference of silicate and arsenate were also investigated. Under optimized conditions, the linearity and the detection limit of the proposed method were found to be 0-165.0 nM and 0.5 nM, which was estimated to be three times the standard deviation of the measurement blanks (n = 9). The relative standard deviations for the determination of 24.7 and 82.5 nM samples were 1.54% and 1.86% (n = 9), respectively. Three seawater samples were analyzed with recoveries ranging from 87.8% to 101.8%. Using the Student's t-test at the 95% confidence level, the results of the proposed method and a segmented flow analyzer reference method for determination of the two samples showed no significant difference. The proposed method had the advantages of being less reagent consuming, more sensitive and with higher throughput (15 h⁻¹).     

Morphology of polystyrene-block-poly(styrene-co-acrylonitrile) and polystyrene-block-poly(styrene-co-acrylonitrile-co-5-vinyltetrazole) diblock copolymers prepared by nitroxide-mediated radical polymerization and “click” chemistry

Well-defined polystyrene-block-poly(styrene-co-acrylonitrile) PS-block-P(S-co-AN) and poly(styrene-co-acrylonitrile-co-5-vinyltetrazole) PS-block-P(S-co-AN-co-5VT) block copolymers with various content of acrylonitrile units in the statistical block were synthesized by nitroxide mediated radical polymerization (NMRP) and post-functionalized using efficient “click” chemistry process. In the second step, acrylonitrile units were successfully modified using 1,3-dipolar cycloaddition (“click” chemistry) type polymer analogue reaction. The original pristine diblock copolymers can be molecularly dissolved in THF and dioxane while the “tetrazolated” versions aggregate to clusters as determined by dynamic light scattering (DLS). Small-angle X-ray scattering (SAXS) and Transmission Electron Microscopy (TEM) revealed ordered lamellar morphology with interlamellar spacing d = 60 nm increasing to d = 80 nm for “tetrazolated” diblock copolymers. The morphological features of diblock copolymer thin layers observed by Atomic Force Microscopy (AFM) depend on the tunable content of both acrylonitrile and 5-vinyltetrazole units and on the quality (polarity) of the solvents used.     

New method for simultaneous determination of Fe(II) and Fe(III) in water using flow injection technique

The method exploits the possibilities of flow injection gradient titration in a system of reversed flow with spectrophotometric detection. In the developed approach a small amount of titrant (EDTA) is injected into a stream of sample containing a mixture of indicators (sulfosalicylic acid and 1,10-phenanthroline). In acid environment sulfosalicylic acid forms a complex with Fe(III), whereas 1,10-phenanthroline forms a complex with Fe(II). Measurements are performed at wavelength λ = 530 nm when radiation is absorbed by both complexes. After injection EDTA replaces sulfosalicylic acid and forms with Fe(III) more stable colourless complex. As a result, a characteristic “cut off” peak is registered with a width corresponding to the Fe(III) concentration and with a height corresponding to the Fe(II) concentration. Calibration was performed by titration of four two-component standard solutions of the Fe(II)/Fe(III) concentrations established in accordance with 2² factorial plan. The method was tested with the use of synthetic samples and then it was applied to the analysis of water samples taken from artesian wells. Under optimized experimental conditions Fe(II) and Fe(III) were determined with precision less than 0.8 and 2.5% (RSD) and accuracy less than 3.2 and 5.1% (relative error) within the concentration ranges of 0.1-3.0 and 0.9-3.5 mg L⁻¹ of both analytes, respectively.     

Improved method for shipboard determination of iron in seawater by flow injection analysis

A flow injection analysis (FIA) catalytic spectrophotometric method for the determination of dissolved iron in seawater was further developed to yield a more sensitive assay with a low detection limit. The method employs an initial sample acidification step followed by an iron pre-concentration step involving an in-line 8-hydroxy-quinoline (8-HQ) metal-chelating resin column. The copper capacity and elution efficiency, as well as the iron FIA performance of three trace-metal clean resins were compared, resulting in the selection of a clean silica gel support for the 8-HQ ligand. The concentrated sample is eluted from the resin with an acidic carrier and mixed with reagents, initiating an iron-catalyzed, color-forming reaction. Increasing the reaction temperature from 18 to 30 °C doubled the sensitivity; reaction temperature control was necessary to obtain good reproducibility in the field. Reagent blanks were as low as 0.05 nM and a detection limit of 0.016 nM was obtained from three times the S.D. of a 0.06 nM seawater sample repeated six times. A 0.06 nM detection limit was calculated from shipboard experiments where total dissolved iron was determined for 10 different samples from the same station. The instrumental sensitivity and precision evolved to the point where the blank associated with the technique is the major factor influencing its detection limit.     

“Oxidative etching-aggregation” of silver nanoparticles by melamine and electron acceptors: An innovative route toward ultrasensitive and versatile functional colorimetric sensors

An innovative and versatile functional colorimetric sensor for melamine (MA) and H₂O₂ was developed with simplicity, excellent selectivity and ultrasensitivity. The detection mechanism was based on the “oxidative etching-aggregation” of silver nanoparticles (AgNPs) by the cooperation effect of MA and electron acceptors such as H₂O₂, ozone or Fe(NO₃)₃. The detection limits of this method for MA could reach as low as 0.08 nM, 0.16 nM and 3 nM when H₂O₂, ozone or Fe(NO₃)₃ was used as an electron acceptor, respectively. When using H₂O₂ as a typical electron acceptor, the method enabled the detection of H₂O₂ with a detection limit of 0.2 nM. This proposed method offered a new way to design MA and H₂O₂ sensors and might be easily extended to detect other nucleophilic reagents and electron acceptors based on colorimetric sensors.     

The production and verification of pristine semi-fluorinated thiol monolayers on gold

In this work the interaction between human serum albumin (HSA) and a monofluorinated phospholipid, 1-palmitoyl-2-[16-fluoropalmitoyl-phosphatidylcholine] (F-DPPC), was studied by using Langmuir monolayer and Brewster angle microscopy (BAM) techniques. Different amounts of F-DPPC were spread on a previously formed HSA monolayer located at the air/water interface at 25 °C and the mixed monolayers thus obtained showed the existence of a liquid expanded-liquid condensed (LE-LC) phase transition (at 14 mN/m), attributed to the pure F-DPPC monolayer, coexisting with a second transition (at 22-24 mN/m) corresponding to the protein conformational change from an unfolded state to another in “loops” configuration. Relative thickness measurements recorded during the compression of the mixed monolayers showed the existence of an “exclusion” surface pressure (π_exc), above which the protein is squeezed out the interface, but not totally. BAM images reveal that some protein molecules in a packed “loops” configuration remain at the interface at surface pressures higher than the “exclusion” surface pressure. The application of the Defay-Crisp phase rule to the phase diagram of the F-DPPC/HSA system can explain the existence of certain regions of surface pressure in which the mixed monolayer components are miscible, as well as those others that they are immiscible.     

Adaptation of a load-inject valve for a flow injection chemiluminescence system enabling dual-reagent injection enhances understanding of environmental Fenton chemistry

Environmental Fenton chemistry has been poorly constrained within the marine environment at a multi-component level. A simple, unique, reconfiguration of a flow-injection analytical system combined with luminol chemiluminescence allows quasi-simultaneously the measurement, using a single load-inject valve and a single photon multiplier tube, of reduced iron, Fe(II), and hydrogen peroxide. The system enables rapid, every 22 s, measurements with good accuracy at environmentally relevant concentrations, less than 5% relative standard deviations on both a 5 nM Fe(II) standard and a 60 nM hydrogen peroxide standard. Limits of detection were as low as 40 pM Fe(II) and 100 pM hydrogen peroxide. The system showed excellent capability by measuring from within an organic rich seawater the photochemically induced production of Fe(II) and hydrogen peroxide and their subsequent cycling and Fenton like interactions.     

A label-free and signal-on supersandwich fluorescent platform for Hg sensing

A label-free supersandwich fluorescent assay was demonstrated for the first time by taking Hg²⁺ as a detection candidate. The principle of the proposed supersandwich fluorescent platform is based on the formation of supersandwich structure by T-Hg²⁺-T coordination and the fluorescence enhancement of the intercalated Genefinder (GF) in double strand DNA (dsDNA). Such supersandwich fluorescent DNA sensor exhibits a linear range of 10–300 nM for the detection of Hg²⁺, with a detection limit of 2.5 nM on the basis of the 3σ/slope (σ represents the standard deviation of the blank samples), which is well below the permit of the U.S. Environmental Protection Agency (<10 nM). The detection can be fulfilled in less than 10 min. The proposed mix-and-detect fluorescent platform exhibits excellent sensitivity, selectivity, and convenient manipulation. The assay was successfully used to detect Hg²⁺ in the lake water samples, which suggested its potential in practical samples.     

A simple and green analytical method for determination of iron based on micro flow analysis

A simple, inexpensive and reagent-less colorimetric micro flow analysis (μFA) system was implemented in a polymethyl methacrylate (PMMA) micro fluidic manifold. A T-shaped micro channel on a PMMA chip was fabricated by laser ablation and topped with molded polydimethylsiloxane (PDMS). The fabricated μFA system was integrated with the optical components as detector and applied to the determination of iron in water samples. It is based on the measurement of Fe(III)-nitroso-R salt complex at 720 nm formed by the reaction between Fe(III) and nitroso-R salt in an acetate buffer solution pH 5. The proposed μFA consumed very small amount of reagent and sample, it released waste of less than 2.0 mL h⁻¹. The relative standard deviation (R.S.D.) was less than 2% (n = 11) with the recovery of 98.7 ± 0.12 (n = 5). The linear range for the determination of iron in water samples was over the range of 0.05-4.0 μg mL⁻¹ with a correlation coefficient (r²) of 0.9994. The limit of detection (3σ) and limit of quantitation (10σ) were 0.021 μg mL⁻¹ and 0.081 μg mL⁻¹, respectively with a sample throughput of 40 h⁻¹.     

Catalytic adsorptive stripping determination of trace chromium (VI) at the bismuth film electrode

A sensitive adsorptive stripping voltammetric protocol at a bismuth-coated glassy-carbon electrode for trace measurements of chromium (VI) in the presence of diethylenetriammine pentaacetic acid (DTPA) is described. The new protocol is based on accumulation of the Cr-DTPA complex at a preplated bismuth film electrode held at −0.80 V, followed by a negatively-going square-wave voltammetric waveform. Factors influencing the stripping performance including the film preparation, solution pH, DTPA and nitrate concentrations, deposition potential and deposition time, have been optimized. The resulting performance compares well with that observed for analogous measurements at mercury film electrodes. A preconcentration time of 7 min results in a detection limit of 0.3 nM Cr(VI) and after 2 min a relative standard deviation at 20 nM of 5.1% (n = 25). Applicability to river water samples is demonstrated. The attractive behavior of the new “mercury-free” chromium sensor holds great promise for on-site environmental and industrial monitoring of chromium (VI). Preliminary data in this direction using bismuth-coated screen-printed electrodes are encouraging.     

Luminol chemiluminescense reaction: a new method for monitoring nitric oxide in vivo

A new method employing a combination of microdialysis sampling and chemiluminescence (CL) detection was developed to monitor nitric oxide (NO) in vivo. A special probe was designed with an interference-free membrane to achieve very high selectivity for NO. High sensitivity was achieved by optimizing the working system and improving the NO sampling time. This system was used in vivo to monitor blood and brain tissue in rats and rabbits. The system is sensitive enough to detect variations of NO formation under different physiological states. The linear valid range of NO determination is 5 nM to 1 μM, with a 3σ detection limit of 1 nM; real NO concentrations in test animals used in this work were found to be in the range of 1-5 nM or even less. Finally, the effects of body temperature, NO donors, Viagra, NO activators, NO cofactors and NO interference (such as O₂) were investigated carefully in different physiological situations.