Hydrophilic interaction liquid chromatography method for measuring the composition of aquatic humic substances

A hydrophilic interaction liquid chromatography (HILIC) method was developed to measure the composition of humic substances from river, reservoir, and treated wastewater based on their physicochemical properties. The current method fractionates the humic substances into four well-defined groups based on parallel analyses with a neutral and a cationic HILIC column, using mobile phases of varied compositions and pH. The results indicate that: (i) the proportion of carboxylic acids in the humic substances from terrestrial origins is less than half of that from treated wastewater (Jeddah, KSA), (ii) a higher content of basic compounds was observed in the humic substances from treated wastewater and Ribou Reservoir (Cholet, France) than in the sample from Loire River (France), (iii) a higher percentage of hydrophobic macromolecules were found in the humic substances from Loire River than in the other samples, and (iv) humic substances of treated wastewater contained less ionic neutral compounds (i.e., pK_a 5–9) than the waters from terrestrial origins. The physicochemical property disparity amongst the compounds in each humic substances sample was also evaluated. The humic substances from the lightly humic Loire river displayed the highest disparity, whereas the highly humic Suwannee river (Georgia, USA) showed the most homogeneous humic substances.     

Influence of molar mass distribution on the complexation of heavy metals by humic material

The association of the heavy metals zinc(II) and cadmium(II) with humic acids of different molar mass distributions has been studied by differential pulse anodic stripping voltammetry. The various humic acid samples were obtained using an experimental procedure in which an untreated Fluka humic acid sample was fractionated by pH variation. Flow field-flow fractionation was used to characterize the humic acid samples with respect to the molar distributions, which appeared to be relatively wide for all samples. The voltammetric analysis of the binding properties of the various samples for zinc(II) and cadmium(II) yielded similar values for the mean stability of the metal complex which thus appears to be independent of the molar mass distribution of the sample. Differences in the shape of the voltammetric complexation curves were the result of differences in the mean diffusion coefficients of the humics involved.     

Determination of Bismuth by Adsorptive Stripping Voltammetry Using Aluminum Oxide for Elimination of Environmental Sample Matrix Interferences

The aim of this work was to find a fast new and simple method for efficient elimination of negative influence of humic substances and synthetic surfactants in the voltammetric determination of bismuth. The negative influence of the organic matrix present in environmental water samples due to their adsorption on aluminum oxide was successfully evaluated. The organic compounds such as humic substances and surface active compounds were removed by adsorption on the surface of aluminum oxide from the water samples. The application of this method was the recovery of Bi(III) from spiked water samples.     

Magnetoactive Humic-Based Nanocomposites

Summary: We have elaborated several approaches for the preparation of magneto-active nano-hybrid composites, including i) ex situ method, which consists of the precipitation of magnetic particles from Fe(II) and Fe(III) salts followed by their incorporation into the humic acids matrix and ii) a chemical precipitation method in situ where the magnetic particles are grown within the humic acids matrix. It was proven that adsorption of humic acids on the magnetic particle surface leads to an enhanced electrostatic and steric stabilization of particles, due to the absorbed layer of highly charged macromolecular compounds. The sorbents under study were found to reveal a high sorption capacity towards uranyl ions. Using the adsorption isotherm, in the linear form of the Langmuir equation, the adsorption parameters were estimated. The results obtained show that the adsorption of uranyl ions by the magneto-active nanocomposite is enhanced in comparison with the free HA.     

Determination of Cr(VI) in the Presence of Complexing Agents and Humic Substances by Catalytic Stripping Voltammetry

The voltammetric method of Cr(VI) determination in a flow system is proposed. Determinations can be carried out in the simultaneous presence of an excess of Cr(III), complexing agents, humic substances and surfactants. The method is based on the combination of a selective accumulation of the product of Cr(VI) reduction to the metallic state and a very sensitive voltammetric method of chromium determination in the presence of DTPA and nitrates. The calibration graph is linear from 1×10^?9 to 5×10^?8 mol L^?1 for accumulation time of 30 s. The relative standard deviation is 5.2% (n=5) for Cr(VI) concentration 1×10^?8 mol L^?1. The influence of humic and fulvic acids, complexing agents and surfactants on Cr(VI) and the interfering Cr(III) signal is presented. The method was applied to Cr(VI) determination in certified reference material, soil sample, natural water sample and EDTA extracts from soil certified reference material.     

Wavelet neural networks to resolve the overlapping signal in the voltammetric determination of phenolic compounds

Three phenolic compounds, i.e. phenol, catechol and 4-acetamidophenol, were simultaneously determined by voltammetric detection of its oxidation reaction at the surface of an epoxy-graphite transducer. Because of strong signal overlapping, Wavelet Neural Networks (WNN) were used in data treatment, in a combination of chemometrics and electrochemical sensors, already known as the electronic tongue concept. To facilitate calibration, a set of samples (concentration of each phenol ranging from 0.25 to 2.5mM) was prepared automatically by employing a Sequential Injection System. Phenolic compounds could be resolved with good prediction ability, showing correlation coefficients greater than 0.929 when the obtained values were compared with those expected for a set of samples not employed for training.     

MCM-41 mesoporous material modified carbon paste electrode for the determination of cardiac troponin I by anodic stripping voltammetry

An anodic stripping voltammetric method for the determination of cardiac troponin I (cTnI) at a MCM-41 mesoporous material modified carbon paste electrode (MCM-MCPE) was investigated. The test was based on the dual monoclonal antibody “sandwich” principle using colloidal gold as a labeled substrate. Four main steps were carried out to obtain the analytical signal, i.e. electrode preparation, immunoreaction, silver enhancement, and anodic stripping voltammetric detection. The anodic stripping peak current increased linearly with the concentration of cTnI over the range of 0.8-5.0 ng/ml. A detection limit of 0.5 ng/ml was obtained. The established method was applied to detect cTnI in acute myocardial infarction (AMI) samples using routine enzyme-linked immunoadsorbent assay (ELISA) for comparison analysis, and good results were obtained.     

Voltammetric determination of iodide by use of an interdigitated microelectrode array

An interdigitated microelectrode array (IDA) sensor has been applied to the determination of iodide in mineral water. It is based on reversible charge transfer in the redox system I2/2I- at a platinum microelectrode. The analytical signal from the IDA system was obtained by use of a bipotentiostat in dual mode. One segment of the IDA (generator) was polarized to the limiting current for oxidation of iodide to iodine in 0.1 mol L(-1) HClO4. The second segment (collector) was fixed at a potential value corresponding to the limiting current of iodine reduction. The geometrical arrangement of the IDA enables this transfer with high efficiency. Because the diffusion layer of both segments overlaps the iodide produced on the collector, the iodide diffuses back to the generator where it is reoxidized. Therefore, redox cycling will enhance the voltammetric signal of the IDA. The signal obtained with a vertically separated IDA was 20 times higher than that in the single mode. Because multiplication of the signal reduces the detection and determination limits, direct voltammetric determination of iodide in mineral water is possible.     

New liquid chromatography columns for highlighting the interaction of ligand candidates with humic acid

Humic acid was the main compound in soil and reduced the availability of some organic compounds in soils. In this work, humic acid was immobilized for the first time on a homemade neutravidin poly(glycidyl methacrylate-co-ethylene dimethacrylate) capillary column with a 20 μm i.d. for the screening of potential ligands to humic acid and the evaluation of their molecular recognition mechanism. This homemade humic acid column enabling it to work at very low backpressure (0.60 MPa at 20 nl/min flow rate), had a long lifetime, excellent repeatability, and negligible non-specific binding sites. The performance of this affinity humic acid column was demonstrated by the evaluation of recognition assay for a series of known ligands of humic acid (a series of rodenticide molecules) which is the heart of the fragment-based drug design. In addition, this column was used successfully for highlighting the binding mechanism to humic acid of the severe acute respiratory syndrome coronavirus-2-spike protein. As well this new humic acid miniaturized liquid chromatography column developed in this work could be used in the feature for another solute molecule-humic acid binding studies or for a separative mode.     

Comparison of voltammetric detection assisted by multivariate curve resolution with amperometric detection in liquid chromatographic analysis of cysteine-containing compounds

A voltammetric detection mode (VD) in conjunction with multivariate curve resolution with alternating least squares (MCR-ALS) method is applied to the analysis of cysteine-containing compounds and compared with a well established amperometric detection (AD) mode in a thin-layer dual Hg/Au cell. VD-MCR-ALS provides an increase in selectivity for cases where satisfactory separation of electroactive compounds is not allowed. However, concentrations needed for a good quantification in VD are higher than in AD due to much large contribution of background in VD.     

Voltammetric determination of benazepril and ramipril in dosage forms and biological fluids through nitrosation

A simple and highly sensitive voltammetric method was developed for the determination of benazepril (I) and ramipril (II). The compounds were treated with nitrous acid, and the cathodic current produced by the resulting nitroso derivatives was measured. The voltammetric behavior was studied by adopting direct current (DCt), differential pulse (DPP), and alternating current (ACt) polarography. Both compounds produced well-defined, diffusion-controlled cathodic waves over the whole pH range in Britton-Robinson buffers (BRb). At pH 3 and 5, the values of diffusion-current constants (Id), were 5.90 +/- 0.40 and 6.66 +/- 0.61 for I and II, respectively. The current concentration plots for I were rectilinear over the range of 1.5-40 and 0.1-30 microg/mL in the DCt and DPP modes, respectively; for II, the range was 2-30 and 0.1-20 microg/mL in the DCt and DPP modes, respectively. The minimum detectabilities (S/N = 2) were 0.015 microg/mL (about 3.25 x 10(-8)M) and 0.012 microg/mL (about 2.88 x 10(-8)M) for I and II, respectively, adopting the DPP mode. Results obtained for the proposed method when applied to the determination of both compounds in dosage forms were in good agreement with those obtained using reference methods. Hydrochlorthiazide, which is frequently co-formulated with these drugs, did not interfere with the assay. The method was also applied to the determination of benazepril in spiked human urine and plasma. The percentage recoveries adopting the DPP mode were 96.2 +/- 1.21 and 95.7 +/- 1.61, respectively.     

Highly Stable Voltammetric Detection of Nitroaromatic Explosives in the Presence of Organic Surfactants at a Polyphenol‐Coated Carbon Electrode

A polyphenol‐coated screen‐printed carbon electrode is used for highly sensitive voltammetric measurements of the 2,4,6‐trinitrotoluene (TNT) explosive in the presence of surface‐active substances. The permselective/protective polyphenol coating offers excellent resistance to surfactant fouling, while allowing facile transport of the target TNT. High levels of gelatin, humic acid and sodium dodecyl sulfate (SDS) (up to 50 mg/L) have negligible effects upon the square‐wave voltammetric TNT response. The TNT peak current and potential remain nearly the same in the presence of these organic macromolecules, as compared to substantial peak suppressions and shifts at the bare electrode. Control of the electropolymerization time was used for achieving the desired exclusion of interfering surface‐active macromolecules while allowing transport of the target TNT. The response for ppm levels of TNT is highly linear and stable for prolonged operations in the presence of surface‐active substances. By meeting the high sensitivity, selectivity, stability, portability and low‐cost demands, such voltammetric sensing holds great promise for field‐based voltammetric monitoring of nitroaromatic explosive compounds.     

Functionalization of flat Si surfaces with inorganic compounds—Towards molecular CMOS hybrid devices

Covalent modification of flat silicon surfaces is a key step in integrating CMOS technology and molecular electronics that may lead to novel hybrid microelectronic devices. While much of the research has been focused on the functionalization of Si by organic compounds, interest in the functionalization with metal-containing species has intensified in recent years because of the unique attributes of inorganic species including rich redox characteristics and high ground state spins. Described in this short review are (i) synthetic approaches to immobilize inorganic compounds; (ii) structural, spectroscopic and voltammetric techniques for characterization of molecular layers; and (iii) preliminary device fabrication.     

Electrochemical DNA biosensor for analysis of wastewater samples

The application of a disposable electrochemical DNA biosensor to wastewater samples is reported. The DNA biosensor is assembled by immobilising double-stranded calf thymus DNA on the surface of a disposable, carbon screen-printed electrode (SPE). The oxidation signal of the guanine base, obtained by a square wave voltammetric scan, is used as analytical signal. The presence of compounds with affinity for DNA is measured by their effect on the guanine oxidation. The comparison of the results with a toxicity test based on bioluminescent bacteria has confirmed the applicability of the method to real samples.     

Voltammetric/amperometric detection for liquid chromatography

A voltammetric/amperometric detector based on a dual-electrode electrochemical detector is described for liquid chromatography. The detector combines the advantages of both voltammetric and amperometric detection. A three-dimensional data array of current response as a function of both time (chromatographic domain) and potential (electrochemical domain) is obtained. From the chromatographic point of view, this allows post-experimental choice of the optimal detection potential. Different detection potentials can even be chosen for each chromatographic peak. Having the voltammetric data as well as the chromatographic data provides ready identification of chromatographically unresolved compounds and the ability to resolve such co-eluting compounds voltammetrically. The voltammetric data also provide a second method of peak identification for greater certainty in peak assignments. Voltammetric detection limits of less than 10 pmol of material injected on the column were achieved with this detection method. From the electrochemical perspective, voltammetric/amperometric detection provides a technique for obtaining hydrodynamic voltammograms with small amounts or small volumes of sample. Voltammograms can also be obtained for the individual components of complex mixtures without the need for isolation steps.     

Voltammetric study of the nature of adsorbed residues arising from irreversible adsorption of acetaldehyde and ethanol on Pt(111) in acid media: first oxidation peak

Irreversible adsorption of ethanol and acetaldehyde on Pt(111) in acid medium results in different adsorbed compounds. This paper describes a study of the nature of the adsorbed species, giving an anodic voltammetric peak at 0.56 V (vs. RHE). These species could be associated with one carbon atom species, because (i) the existence of adsorbed CO (in the case of ethanol and acetaldehyde) implies the rupture of the C---C bond, and (ii) the electrochemical reduction of formaldehyde at potentials lower than or equal to 0.1 V gives the peak at 0.56 V. This reduction must give one carbon atom species.     

Utilisation of polypyrrole modified electrode for the determination of pesticides

Cyclic voltammetric studies of isoproturon and carbendazim using polypyrrole modified glassy carbon electrode were carried out. The electrode and reaction conditions, which yielded maximum current signal, were selected for the development of stripping voltammetric procedure for the determination of the pesticides. The oxidation peak around 1.3?V obtained for isoproturon and carbendazim while employing polypyrrole modified electrode showed maximum current response. This peak was chosen for stripping analysis using square wave mode. The experimental parameters were optimized and the calibration plot was obtained. The LOD was 0.5?ng?mL^?1 for isoproturon and 5?ng?mL^?1 for carbendazim. The relative standard deviation for 5 identical measurements was 2.81% and 3.33% for isoproturon and carbendazim respectively. The applicability of the method was verified by determining the pesticides in spiked soil and water samples.     

Hyphenated techniques of thermal analysis for characterisation of soil humic substances

Our aim was to investigate the thermal behaviour of humic substances extracted from temperate and tropical soils by means of hyphenated techniques of thermal analysis (e. g. simultaneous thermal analysis DSC/TG coupled with mass spectrometry, MS, for the analysis of evolved gas, EGA) in order (i) to verify whether the chemical composition of isolated humic substances also reflected the differences in microbial parameters previously measured in related soil samples and (ii) to identify suitable indices of thermal stability. Our results show that the investigation of humic substances by thermal methods can provide information on soil organic matter dynamics. Differences in thermal behaviour between the two groups of soils were found. The indices of thermal stability here proposed, IR (index of thermal recalcitrance), and ID (index of thermal decomposability) clearly showed that in humic substances from tropical soils the thermally recalcitrant organic fraction dominated, whilst in temperate humic substances the opposite held. This agrees with previous results on the microbial dynamics and organic matter turnover of the respective soils and indicates that these indices of thermal stability could represent a useful tool in soil environmental quality investigations.     

Spectroscopic studies on the spontaneous assembly of phenosafranin on glycosaminoglycans templates

Spectroscopic studies showed that binding of phenosafranin (PSF) molecules to glycosaminoglycans (GAGs) resulted in the following observations: (i) appearance of a 52.6 nm hypsochromic shift of the visible absorption band; (ii) static quenching of fluorescence from PSF; (iii) induction of strong circular dichroism (CD) signal of PSF. Stoichiometry of the PSF-GAGs complex was determined by spectrophotometric titration, spectrofluorimetric titration and MacIntosh extraction method. These studies demonstrated the formation of the extended helical PSF array aligned on the helical backbone of GAGs templates by electrostatic force, and the dimeric binding mode of PSF to each anionic site was proposed. The comparative studies between PSF-heparin (Hep) and PSF-chondroitin 4-sulfate (CS) complexes revealed that: (i) stoichiometry of PSF-Hep complex was 1.8 times of PSF-CS complex; (ii) Hep was more effective than CS (1.8 times) in decreasing the absorbance of PSF; and (iii) Stern-Volmer constants of the Hep-PSF system were greater than that of the CS-PSF system. These differences were attributed to the different charge density on the Hep and CS molecules, which in turn suggested that the electrostatic force was dominant in the interaction between PSF and GAGs.     

Measurement of Total Antioxidant Capacity by Electrogenerated Iodine at Disposable Screen Printed Electrodes

Total antioxidant capacity is an important parameter for the evaluation of the oxidative status in different kinds of biological samples such as plant extracts, or in food industry. We report a fast, easy, portable, cost‐effective electroanalytical method to measure total antioxidant capacity, based on the reaction of natural antioxidants with electrogenerated iodine using disposable platinum screen‐printed electrodes. This reaction can be measured by the increment of the electrochemical current signal of iodide oxidation to iodine during a voltammetric cycle. Iodine reacts with reducing compounds such as glutathione, ascorbate, gallic acid and NADH without interference of the corresponding oxidized counter‐parts. The addition of ascorbate oxidase also allows the concentration of ascorbate to be determined. The method was tested with real samples of plant extracts and the results correlated well with those obtained with a standard spectrophotometric method.