Molecular characterization of non-porphyrin trace-metal compounds of geochemical and process significance using high-performance liquid chromatography in combination with element-selective detection

A high-performance liquid chromatograph coupled to element-selective detectors, such as graphite furnace atomic absorption and inductively coupled plasma emission spectrometers, is a powerful analytical instrument that facilitates the molecular characterization of trace-metal compounds found in complex matrices. This combination can also be used effectively in conjunction with off-line spectroscopic and derivatization techniques to elucidate molecular structures. The separation and element-selective detection couple can also be extremely helpful in monitoring the fate of trace-metal compounds that significantly affect petroleum processes.     

Stripping voltammetry for the determination of trace metal speciation and in-situ measurements of trace metal distributions in marine waters

Progress in marine chemistry has been driven by improved sampling and sample handling techniques, and developments in analytical chemistry. Consequently, during the last 20 years our understanding of marine trace metal biogeochemistry has improved a great deal. Stripping voltammetric techniques (anodic stripping voltammetry and adsorptive cathodic stripping voltammetry) have made an important contribution to this understanding. The selectivity and extremely low detection limits have made stripping voltammetry a widely used technique for trace metal speciation and trace metal distribution measurements in seawater. Stripping voltammetry is very suitable for ship-board and in-situ applications because of the portability, low cost and capability for automation of the voltammetric instrumentation. Future developments in stripping voltammetry can be expected in the field of stand-alone submersible voltammetric analysers, capable of continuous trace metal measurements. Future applications of stripping voltammetry can be found in the interactions between trace metal speciation and growth and the functioning of organisms in pristine and metal polluted marine waters.     

Instrumental configurations for the determination of sub-micromolar concentrations of electroactive species with carbon,gold and platinum microdisk electrodes in static and flow-through cells

Microelectrodes should provide a greater analytical sensitivity than electrodes of conventional size. However, the detection of micromolar or lower concentrations with microdisk electrodes requires measurement of femtoamp currents, which is outside the range of most commercially available instrumentation. The novel use of a picoammeter or femto-ammeter as a current amplifier permits commercial instrumentation to be used with microdisk electrodes. Such instrumentation incorporating a picoammeter or femtoammeter is limited by the relatively slow rise time; this places restrictions on scan rates in all voltammetric techniques and on pulse widths in transient techniques such as differential pulse and square wave voltammetry. Because of the small currents, the ohmic (iR) drop is very small and polarization of the reference electrode is unimportant; thus a two-electrode format without a potentiostat can be used. Consequently, a microprocessor-based function generator and data storage system, in conjunction with a pico- or femto-ammeter, is satisfactory in providing inexpensive, versatile and very sensitive instrumentation for voltammetric detection with microdisk electrodes. Convenient methods for fabricating platinum, gold and carbon microdisk electrodes for use in stationary and flowing solution configurations are also presented.     

Recent developments in electrochemical flow detections--a review part II. Liquid chromatography

This article is a review of the progress in application of electrochemical detections in liquid chromatography in recent 15–20 years. Based on 238 references, mostly to original research papers, it presents applications of amperometric and voltammetric detections, as well as coulometric, conductimetric and potentiometric ones. In case of those which have reached already the stage of routinely employed detections with commercially available instrumentation (amperometry, coulometry, conductometry) especially novel and original applications are presented. In case of voltammetric and potentiometric detections a ways of their improvements are showed, directed towards obtaining competitive results with other detection methods.     

Modulated polarographic and voltammetric techniques in the study of natural water chemistry

Modulated polarographic and voltammetric techniques are of particular importance in natural water chemistry because of their sensitivity not only to very low concentrations of electroactive components but also to their chemical form. Direct polarographic techniques are most useful for the analysis of non-metallic components at low concentrations since metal levels are only rarely high enough for the analysis of untreated samples. Preconcentration by chemical or electrochemical techniques have both been employed. Potentially the most productive field of application of polarographic and voltammetric methods is in determining the chemical speciation of electroactive components in natural waters. Some clarification is required of the chemical and biological significance of operational classifications currently employed. Intermetallic interferences and the influence of surface films on electrode behaviour need to be more thoroughly investigated before analyses or speciation studies on untreated samples can be routinely undertaken. Chemical and electrochemical understanding rather than increased sophistication in the instrumentation is required at this stage if full advantage is to be taken of the capabilities of modulated polarographic and voltammetric methods in natural water chemistry.     

Electrochemical Detection of Nitrite Based on Reaction with 2,3‐Diaminonaphthalene

Abstract

A voltammetric method for the determination of trace amounts of nitrite is proposed. In acidic solution, 2,3‐diaminonaphtalene (DAN) reacts with nitrite to form the 1‐N‐naphtotriazole. At the end of this reaction, the unreacted DAN was measured at a carbon paste electrode by using differential pulse voltammetry. Different parameters, such as pH of the medium, effect of the reaction time, and the effect of DAN concentration on the percentage of signal decrease, have been optimised. Detection limit of 0.2 µmol L^?1 was achieved. Results were improved by using bath injection analysis technique which permit 120 analysis per hour. Nitrites were determined in the real samples. The results have been compared with those obtained by spectrophotometric method based on the Griess reaction, and they are satisfactory. The method developed can be applied for rapid detection and monitoring trace of nitrite for field measurements because the instrumentation required is portable.     

Development of a microprocessor-based electrochemical instrument interfaced to a microcomputer system for differential-pulse stripping voltammetry in different time domains

An inexpensive, compact microprocessor-based instrument for polarographic and voltammetric analysis has been developed for use in chemically hazardous laboratories, radiation laboratories or clean laboratories. This low-cost unit controls the experiment, collects the data and can be regarded as expendable. The microprocessor-based instrumentation is interface to a microcomputer system, which is external to the laboratory and has all the required peripherals associated with larger laboratory computers. In this institution, four laboratories having the inexpensive microprocessor systems are linked to the larger microcomputer which is housed in an air-conditioned room better suited for computers. A new approach to data collection and evaluation using different time domains in differential-pulse stripping voltammetry is presented as an example of the use of such instrumentation.     

Review of adsorptive stripping voltammetric methods for cobalt determination in the presence of a zinc matrix

Different polarographic and voltammetric methods for the determination of Co in the presence of a large excess of zinc are reviewed. Adsorptive stripping voltammetry with in-situ matrix exchange and catalytic adsorptive stripping voltammetric methods are recommended for monitoring Co traces in zinc plant electrolyte. The principles of the catalytic adsorptive stripping voltammetric methods for Co determination are presented. The correct selection of the investigated supporting electrolytes, enabling the monitoring of Co traces in a zinc plant electrolyte by means of catalytic adsorptive methods is also discussed. The catalytic adsorptive voltammetric procedures offer the possibility of the determination of Co or Co and Ni traces in metallic zinc and zinc salts.     

Electrochemical real-time polymerase chain reaction

In this work, we report the first electrochemistry-based real-time polymerase chain reaction technique for sequence-specific nucleic acid detection. This new technique builds upon the advantages of the well-established fluorescence-based counterpart, such as short assay time (simultaneous target DNA amplification and detection). In addition, this electrochemical approach could employ simple and miniaturizable instrumentation compared to the bulky and expensive optics required in the fluorescence-based schemes. We have demonstrated a proof-of-concept experiment showing that the utilization of solid-phase extension of the electrode surface-immobilized capture probe with Fc-dUTP during PCR resulted in the accumulation of the redox marker on the transducer surface. This new technique can be applied to a microfabricated PCR electrochemical device for point-of-care diagnostics as well as on-site environmental monitoring and biowarfare agent detection.     

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.     

Metal film electrodes prepared with a reversibly deposited mediator in voltammetric analysis of metal ions

Electrochemical stripping analysis is recognized as a powerful technique for trace metals owing its remarkable sensitivity, relatively inexpensive instrumentation, ability for multielement determination and capable of determining elements accurately at trace and ultra-trace levels. The success of voltammetric sensing procedure depends mainly on the proper choice and preparation of the working electrode. The article reviews the development and application of metal film electrodes (bismuth, lead, and antimony film electrodes) prepared with a reversibly deposited mediator for stripping voltammetric determination of metal ions (Ni(II), Sn(IV), Hg(II), U(VI), Cd(II), Pb(II), and Cr(VI)). In this electrochemical, defect-mediated, thin-film growth method, the mediator is periodically deposited and the stripped from the surface, and this serves to significantly increase the density of islands of atoms of interest metal, what in consequence improves the electrochemical properties of these electrodes, because of the increase in the active surface area of electrode.     

Membrane-introduction mass spectrometry (MIMS)

Membrane-introduction mass spectrometry (MIMS) for chemical analysis involves directly sampling analytes in gaseous, liquid and solid samples through a semi-permeable membrane coupled to a mass spectrometer, yielding selective and sensitive quantitation. Because MIMS is an on-line technique, in which samples can be continuously flowed over a membrane interface, it can yield analytical results in real time without the need for sample clean-up and chromatographic separation. This review highlights trends and developments in MIMS over the past decade and describes recent studies that pertain to its use for on-site, in-situ and in-vivo chemical analysis. We report on advancements in instrumentation, including membrane materials, interface configurations and ionization techniques that have extended the range of analytes amenable to MIMS.We summarize the progress made in the miniaturization of mass spectrometers that have resulted in field-portable systems and review recent applications of continuous mobile monitoring and on-site environmental monitoring to yield both temporally and spatially resolved quantitative and semi-quantitative data. Finally, we describe recent work involving the use of MIMS for in-vivo chemical analysis.     

用微机电化学系统研究方波伏安法和循环方波伏安法——Ⅰ.可逆过程的理论和验证

本文应用微计算机电化学系统研究可逆过程的方波伏安法,特别是作者提出的循环方波伏安法,建立了循环方波伏安法的理论电流函数表示式。理论和实验验证都表明,循环方波伏安法正向扫描的峰电流和峰电势分别等于逆向扫描的峰电流和峰电势,这是氧化态和还原态都溶于电解质溶液的可逆电极过程体系的重要特征和判据。同时研究了参数如方波振幅(E_sw),方波周期(τ),阶梯电势增量(ΔE)的影响,理论预期与实验结果一致。     

Contributions to automated trace analysis

Summary A reliable and robust automated device for simultaneous voltammetric analysis (by differential pulse anodic stripping voltammetry) of several of the most toxic trace metals (Cu, Pb, Cd, Zn) in drinking and ground water is described. It enables in water works the required round the clock on line-monitoring of drinking water over extended periods (one week or longer) without supervision by staff. The device consists of adapted commercial polarographic instrumentation, a punch card program controller and automatic sampling and standard addition facilities. Extended tests under realistic routine working conditions in water works over several months have established the suitability and the potentialities of the concept and the satisfactory performance of the device.Ref. [4], [10] and [11] referring to automated flameless AAS methods for the study of biological materials constitute part I–III of this series     

Voltammetric determination of platinum in inorganic complexes and in water, geological and biological matrices using laboratory- and field-based instrumentation

The extremely sensitive catalytic hydrogen ion reduction wave observed after the formation of a platinum-formazone complex at a mercury electrode in a hydrazine-formaldehyde-H2SO4 medium has been utilised to determine platinum voltammetrically in well characterised platinum inorganic complexes (oxidation states O, II and IV) and in biological, geological and water samples. Experimental conditions have been optimised and sample-treatment procedures for various matrices have been critically evaluated for the quantification of platinum by the standard additions method. The determination of platinum in geological samples by this method has been compared with an inductively coupled plasma mass spectrometric method. Both conventional and portable field-based instrumentation have been used in the studies, and the possibility of developing a field-based method for the determination of platinum has been investigated. Despite the inherent sensitivity of the method, which enables concentrations down to 0.01 p.p.b. to be detected in simple matrices, natural levels in water and biological materials, where matrix effects suppress the voltammetric response, are often below the detection limit.     

In vivo Electrochemical Monitoring of Signaling Transduction of Plant Defense Against Stress in Leaves of Aloe vera L.

In situ recording of the voltammetric response of Aloe vera L. leaves using graphite and platinum microelectrodes inserted in the subcuticular region is described. The plant response against the mechanical stress results in changes of the voltammetric signals associated to signal transduction compounds salicylic and jasmonic acids and hydrogen peroxide, which were separately monitored. A kinetic model is proposed to describe the time variation of the respective voltammetric signatures, denoting the possibility of in vivo electrochemical monitoring of biochemical processes in plants. The obtained results suggest that two defense pathways with different participation of ROS are operative.     

Metalloporphyrin chemically modified glassy carbon electrodes as catalytic voltammetric sensors

Metalloporphyrin-coated glassy carbon electrodes are used as electrocatalytic voltammetric sensors for numerous clinically important solutes. For such compounds, heterogeneous charge- transfer rates are often very slow at carbon electrodes, leading to poorly defined voltammetric responses. The metalloporphyrin-modified electrodes are shown to decrease by several hundred millivolts the potential required for the oxidation of ascorbic acid, penicillamine, acetaminophen, dihydronicotinamide adenine dinucleotide, hydralazine, epinephrine, cysteine and oxalic acid. The faster rates of electron transfer result in a well defined voltammetric response and increased sensitivity. The differential pulse peaks for caffeic acid, ascorbic acid, acetaminophen and dopamine are enhanced by 18, 10.5, 9.4 and 8.4, respectively. When used for amperometric monitoring of flowing streams, the coated electrode permitted detection at lower potentials than at the naked surface and greatly facilitated assays of urine samples.     

Battery-operated microcomputer-based electrochemical instrumentation

There is a need for microprocessor-controlled, battery-operated, analytical instrumentation in environmental and industrial monitoring programmes. The electrochemical techniques of ion-selective electrode potentiometry and voltammetry are ideally suited for this task because of their inherent simplicity and compatability with digital computer-based instrumentation. The task for the future is to build computerized intelligent stand-alone battery-operated devices with comparable performance to their mains-powered laboratory-based cousins.     

Nanodiamond Decorated with Silver Nanoparticles as a Sensitive Film Modifier in a Jeweled Electrochemical Sensor: Application to Voltammetric Determination of Thioridazine

Nanodiamond? graphite (NDG) decorated with Ag nanoparticles (AgNPs‐NDG) was prepared and used to construct a novel sensitive sensor for the voltammetric determination of thioridazine (TR). The results indicate a remarkable increase in the oxidation peak currents together with a negative shift in the oxidation peak potentials, in comparison to the bare pyrolytic graphite electrode. Remarkable enhancement in microscopic area of the electrode along with strong adsorption of TR on the surface of the modified electrode resulted in a considerable increase in the peak current of TR. The surface morphology and the nature of the composite film deposited on PGE were characterized by scanning electron microscopy, atomic force microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Experimental variables, such as the deposited amount of the modifier suspension, pH of the supporting electrolyte, the accumulation potential and time are optimized by monitoring the CV responses of TR. Under the optimal conditions, the modified electrode showed a wide linear response to the concentration of TR in the range of 0.08–100 µM with a detection limit of 0.01 µM. The prepared modified electrode showed several advantages: simple preparation method, high stability and uniformity in the composite film, high sensitivity, long‐term stability and remarkable voltammetric reproducibility in response to TR. The modified electrode can be successfully applied for accurate determination of trace amounts of TR in pharmaceutical and clinical preparations.     

A battery-powered,computerized instrument for the field-based determination of fluoride by the ion-selective electrode technique

Battery-powered complementary metal-oxide semiconductor (CMOS) electronic devices are used to develop computerized instrumentation based on an ion-selective electrode for environmental monitoring in the field. The development of CMOS-based instrumentation for the determination of fluoride in drinking water, river water, lake water and sea water is described. The instrument is portable and completely field-programmable. Under software control, the equilibrium potential and fluoride concentration, based on a double standard-addition method, are calculated. The associated temperature, date and time form part of the data-logging record. Data obtained in the field are shown to compare satisfactorily with those obtained on samples which were stored and returned to the laboratory for examination by conventional mains-powered instrumentation. Accuracy is good and precision is only slightly poorer than that of mains-powered laboratory instrumentation.