The analysis of conductive solid samples by r.f. capacitively coupled plasma at atmospheric pressure

A radiofrequency capacitively coupled plasma (rf CCP) with tip-ring electrode geometry has been used for the analysis of Al, Co, Cr, Cu, Mn, Mo, Ni, and V in low and medium alloyed steel. The sample is used as one of the electrodes of the plasma torch. The influence of plasma power, argon flow rate and distance between the electrodes on the analytical signals has been studied. The limits of detection are in the range of 0.001 to 0.048%. The dynamic range is three orders of magnitude.     

Analytical characterisation of a capacitively coupled plasma torch with a central tube electrode

A new type of radiofrequency capacitively coupled plasma torch is presented. The torch electrode geometry is coaxial with a tubular central electrode and one or two outer ring electrodes. The argon plasma is generated at 275 W radiofrequency power and 27.12 MHz and it has a very good stability and a low gas consumption of 0.4 l min(-1). The nebulized sample is introduced through the tubular electrode into the core of the annular shaped plasma thus achieving a better atomisation and a lower background. The limits of detection for 20 elements are in the range of ng ml(-1) and the dynamic range between 2.5 and 3.5. The best results are obtained with the torch with two outer ring electrodes.     

Microwave and RF surface wave sustained discharges as plasma sources for plasma chemistry and plasma processing

The surface wave produced plasma belongs to a class of RF and microwave induced plasmas. It results from the propagation of an electromagnetic wave which uses the plasma column it sustains and the plasma tube as its sole propagating media. This type of plasma offers several advantages compared to the positive column plasma of dc discharges or to other RF and microwave produced plasmas. Surface wave plasmas require no internal electrodes, and they can be applied over an extremely broad range of wave frequencies (27 MHz to 10 GHz demonstrated) and gas pressures (about 10^–4 Torr to a few times the atmospheric pressures). Using the surface wave plasma technique, a large variety of plasma column diameters have been created (0.5–150 mm demonstrated) and no limitation on plasma column length (column up to 6 m long demonstrated) has been found. The surface wave produced plasma is used in elemental analysis and to sustain emission in lasing media. This article is intended as a guide for potential users of surface wave plasmas in the field of plasma processing and plasma chemistry.     

新型多尖端旋转电极甲烷偶联等离子体反应器

研制了一种新型的带有一个多尖端旋转电极对一个同心圆筒型固定电极的甲烷放电制备碳二烃的等离子体反应器.以氢气共存条件下甲烷偶联研究对此反应器作出了评价.在此反应器中，反应物流垂直穿过两电极之间的环行等离子体反应区.在大约40 V供电电压、20 kHz脉冲放电等离子体条件下，在长时间连续反应后没有产生大量积炭.在同样的条件下，此多尖端旋转电极工艺比固定的尖端 平板电极工艺具有较高的甲烷转化率、碳二烃单程收率以及较高的能量效率.     

Atmospheric Pressure Plasma Discharge for Polysiloxane Thin Films Deposition and Comparison with Low Pressure Process

An atmospheric pressure dielectric barrier plasma discharge has been used to study a thin film deposition process. The DBD device is enclosed in a vacuum chamber and one of the electrodes is a rotating cylinder. Thus, this device is able to simulate continuous processing in arbitrary deposition condition of pressure and atmosphere composition. A deposition process of thin organosilicon films has been studied reproducing a nitrogen atmosphere with small admixtures of hexamethyldisiloxane (HMDSO) vapours. The plasma discharge has been characterized with optical emission spectroscopy and voltage-current measurements. Thin films chemical composition and morphology have been characterized with FTIR spectroscopy, atomic force microscopy (AFM) and contact angle measurements. A strong dependency of deposit character from the HMDSO concentration has been found and then compared with the same dependency of a typical low pressure plasma enhanced chemical vapour deposition process.     

Ion-selective electrodes and enzyme electrodes in environmental and clinical studies.

Electrodes have been developed for the assay of glucose, urea, amino acids, uric acid, phosphate, nitrate and perchlorate. The electrodes for the organic compounds are enzyme electrodes which are prepared by chemically immobilizing an enzyme over the outside of a conventional ion-selective electrode. These electrodes will be discussed in depth. The progress and the development of the electrodes that show sensitivity and selectivity for phosphate, nitrate and perchlorate will be outlined. The basis of these sensors is a complex of a transition metal of either an analog of thiourea or an organic chelator, such as 1,10-phenanthraline. Such electrodes respond linearly to phosphate, nitrate or perchlorate, and show selectivity over sulphate, halides and acetate. The linear range of all these electrodes is approx. 10(-1)-10(-5) M with a near Nernstian slope and a reproducibility of 1%. The electrodes are stable and can be used continuously.     

Microband electrodes fabricated by screen printing processes: Applications in electroanalysis

A novel method for the simple and cheap manufacture of microband and multiple microband electrodes is described. The construction technique is based entirely on the screen printing of the surface of alumina tiles with conducting and insulating ink materials. The range of potentials over which the electrodes may be used has been determined by basic electrochemical studies. These electrode systems have been used successfully for the measurement of vitamin C, vitamin B(1) and paracetamol, which illustrates, their potential as useful tools in the field of electroanalysis.     

Electrochemical Determination of Uric Acid,Dopamine and Tryptophan at Zinc Hexacyanoferrate Clay Modified Electrode

A Cameroonian smectite clay has been transformed into Zn²⁺ homoionic form and then used to prepare film modified glassy carbon electrodes and carbon paste electrodes. These electrodes containing Zn²⁺ were exploited to prepare a mixed valence zinc hexacyanoferrate (ZnHCF). Cyclic voltammetry has been employed to monitor the in situ growth of ZnHCF on clay modified electrodes. Although interesting electrocatalytic properties toward UA were observed with these modified electrodes, the modified carbon paste electrodes were the most suitable for dopamine, uric acid and tryptophan detection and exhibited for these analytes extended linear range, high sensitivities, selectivity and low limit.     

Electrochemical performance of multi-walled carbon nanotube composite electrodes is enhanced with larger diameters and reduced specific surface area

Multi-walled carbon nanotubes (MWCNT) have been widely used to fabricate composite electrodes due to their electrochemical properties. MWCNTs can be fabricated by various approaches and a range of MWCNT types and sizes has been developed. This study focuses on understanding the influence MWCNT diameter and specific surface area has on the electrochemical properties of a composite electrode. MWCNTs with fixed length range and diameters ranging from 10–20 to 50–80 nm were examined in this study. The amount of MWCNT utilised to fabricate the electrodes was identical and above the percolation threshold. MWCNT electrodes fabricated with larger diameters showed enhanced thermodynamic and kinetic properties towards common redox species which covered surface-insensitive, surface-sensitive and adsorption-based processes. Overall, these findings indicate that the number of strands of MWCNT alone is not essential for enhanced conductivity in composite materials but other geometric parameters play important roles.     

Immunoassay of C-reactive protein by hot electron induced electrochemiluminescence using integrated electrodes with hydrophobic sample confinement

C-reactive protein (CRP) was determined in the concentration range 0.01–10 mg L⁻¹ using hot electron induced electrochemiluminescence (HECL) with devices combining both working and counter electrodes and sample confinement on a single chip. The sample area on the electrodes was defined by a hydrophobic ring, which enabled dispensing the reagents and the analyte directly on the electrode. Immunoassay of CRP by HECL using integrated electrodes is a good candidate for a high-sensitivity point-of-care CRP-test, because the concentration range is suitable, miniaturisation of the measurement system has been demonstrated and the assay method with integrated electrodes is easy to use. High-sensitivity CRP tests can be used to monitor the current state of cardiovascular disease and also to predict future cardiovascular problems in apparently healthy people.     

Physicochemical Characteristics of Polypyrrole/(Glucose oxidase)/(Prussian Blue)‐based Biosensor Modified with Ni‐ and Co‐Hexacyanoferrates

In this work, three types of electrodes suitable for amperometric glucose biosensors were designed. One type of electrode was based on bio‐selective layer of polypyrrole/(glucose oxidase)/(Prussian Blue) (Ppy/GOx/PB) and it was used as a control electrode regarding to which electrochemical properties of two other types of electrodes were compared. During the formation of Prussian blue layers graphite electrodes were additionally modified by Ni‐hexacyanoferrate (NiHCF) and by Co‐hexacyanoferrate (CoHCF) in order to design Ppy/GOx/PB‐NiHCF and Ppy/GOx/PB‐CoHCF electrodes, respectively. Some physicochemical characteristics of all three types of electrodes were evaluated and compared. The Ppy/GOx/PB‐NiHCF electrode showed wider linear range of the calibration curve than Ppy/GOx/PB and Ppy/GOx/PB‐CoHCF electrodes. The effect of temperature on analytical performance of the Ppy/GOx/PB‐NiHCF based biosensor has been evaluated and activation energy of enzyme catalysed reaction has been calculated within the temperature range of 15 °C to 30 °C.     

Separation of erythrocytes and latex beads by dielectrophoretic levitation and hyperlayer field-flow fractionation.

A model system consisting of a mixture of latex beads and erythrocytes has been investigated to demonstrate the practical feasibility of particle separation by means of the combined application of negative dielectrophoresis and hyperlayer field-flow fractionation. The dielectrophoretic levitation of latex beads is demonstrated by energizing interdigitated electrodes, of widths and separation ranging from 5 to 40 μm, with AC signals of 0–10 V (rms) in the frequency range 1 kHz–10 MHz. Maximum levitation was attained at 1 MHz, at which frequency levitation is relatively independent of the suspending medium conductivity. Levitation was also independent of particle size, but dependent on particle density and dielectric properties. At 1 MHz the erythrocytes were attracted to the electrodes by positive dielectrophoresis, and so could be separated from the latex beads by fluid flow. The electric field and field gradient above the electrodes were also computer modelled, and this information was used to design the electrode and chamber geometries for optimum DEP-field-flow fractionation.     

Nanocarbon structures formed in a universal plasma reactor

The results of an instrumental study of a deposit formed on the electrodes of an arc plasma torch with propane-butane mixture feeding into the interelectrode gap are presented. The optical, electron, and Raman microscopy techniques have been used in the study. According to Raman spectra, the cathode deposit contains various forms of nanocarbon. It has been found that maleic anhydride is synthesized and covalently grafted to nanographite in the absence of a specialty catalyst during plasma torch operation. Having a large specific surface area, the nanocarbon itself acts as a heterogeneous catalyst in this case. It has been shown that an arc plasma torch of this design with a hydrocarbon feed gas can be considered a mini-reactor for synthesis of different forms of nanocarbon, its surface modification, and alteration of physicochemical properties.     

Time-resolved Optical Emission Spectroscopy in Water Electrical Discharges

The characteristics of the plasma initiated in ultrapure water between pairs of tungsten and tantalum electrodes were investigated by time-resolved optical emission spectroscopy. The deexcitation processes of the reactive species formed in the water plasma depended on the electrode material, but had been independent on the polarity of the applied voltage pulses. All the reactive species presented the same evolution with time and have been identified with high concentration in the emission spectra between the pulses. The current–voltage characteristics showed the features of a spark discharge for the both types of electrodes used in the process. When tantalum electrodes were used to generate the discharge, a broad emission continuum (350–940 nm) dominated the spectrum due to a transition to arc discharge.     

Parameters important in fabricating enzyme electrodes using self-assembled monolayers of alkanethiols.

The fabrication of enzyme electrodes using self-assembled monolayers (SAMs) has attracted considerable interest because of the spatial control over the enzyme immobilization. A model system of glucose oxidase covalently bound to a gold electrode modified with a SAM of 3-mercaptopropionic acid was investigated with regard to the effect of fabrication variables such as the surface topography of the underlying gold electrode, the conditions during covalent attachment of the enzyme and the buffer used. The resultant monolayer enzyme electrodes have excellent sensitivity and dynamic range which can easily be adjusted by controlling the amount of enzyme immobilized. The major drawback of such electrodes is the response which is limited by the kinetics of the enzyme rather than mass transport of substrates. Approaches to bringing such enzyme electrodes into the mass transport limiting regime by exploiting direct electron transfer between the enzyme and the electrode are outlined.     

A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane

The authors recently developed a high-frequency pulsed plasma process for methane conversion to acetylene and hydrogen using a co-axial cylindrical (CAC) type of reactor. The energy efficiency represented by methane conversion rate per unit input energy has been improved so that such a pulsed plasma has potential for commercial acetylene production. A pulsed plasma consists of a pulsed corona discharge and a pulsed spark discharge. Most of energy is injected over the duration of the pulsed spark discharge. Methane conversion using this kind of pulsed plasma is a kind of pyrolysis enhanced by the pulsed spark discharge. In this study, a point-to-point (PTP) type of reactor that can produce a discharge channel over the duration of a pulse discharge was used for the pulsed plasma conversion of methane. The energy efficiency and carbon formation on electrodes have been improved. The influences of pulse frequency and pulse voltage on methane conversion rate and product selectivity were investigated. The features of methane conversion using PTP and CAC reactors were discussed.     

等离子体聚合法修饰半导体光电极的研究——聚丙烯腈/n-GaAs,n-GaP电极

Polyacrylonitrile (PAN) film attached to n-GaAs, n-GaP photoelectrode surface by plasma polymerization has been studied. Both electrochemical behaviors and stability of the modified electrodes have been investigated. The results showed that the semiconductor electrodes with PAN film decreased photocorrosion in a certain degree, and their electrochemical characteristics and stability were improved remarkably after plasma doping. Cyclic voltammetric analysis demonstrated that the redox reaction occurred in I-PAN films could be carried out rapidly on electrode surfaces, in favor of the capture and transfer of photoinduced holes by the films to redox couples in solution.     

Extending the dynamic range of electrochemical sensors using multiple modified electrodes

Multiple electrodes, combined with a chemometric strategy to calibrate the measurement response, have been used for the determination of an analyte across a broader dynamic range than is possible with a single electrode. The model system used for the detection of copper comprised electrodes modified with a self-assembled monolayer. The electrodes were modified with the copper-complexing species (3-mercaptopropionic acid, thioctic acid, and the peptides cysteine and Gly-Gly-His) and copper was determined over concentrations ranging from nanomolar to millimolar using voltammetric analysis. We have demonstrated that by combining the calibration functions from the four electrodes a better estimate (i.e. with smaller variance) of the concentration of the analyte is obtained. Measurement uncertainty is expressed for independently prepared electrodes, which allows the possibility of commercial production and factory calibration. The principles of using multiple electrodes modified with recognition elements with different affinities for the target analyte to extend the dynamic range of sensors is a general one that could be applied to other analytes.     

New, high-sensitivity high-performance liquid chromatographic method for the determination of acyclovir in human plasma, using fluorometric detection.

A high-performance liquid chromatographic method for the determination of acyclovir in human plasma has been developed. It is the first published chromatographic method capable of determining acyclovir in plasma with sufficient sensitivity and for sufficiently long periods of time following oral administration of a standard dose of acyclovir during pharmacokinetic investigations. Following precipitations of the proteins with perchloric acid, the sample is chromatographed with a strongly acidic mobile phase on a reversed-phase column, and is then subjected to fluorometric detection (excitation 260 nm, emission 375 nm). The determination limit is 6-10 ng/ml human plasma. The calibration is linear in the range 10-12,400 ng/ml plasma, with the coefficients of variation less than 8%. The absolute recovery rate is between 102 and 113%. This method has already been used to analyse several thousand plasma samples.     

Arc plasma in air with calcium and fluorine—I. Calculation of the plasma composition

The equilibrium composition of the gaseous system containing N, O, C, F and Ca was calculated in the temperature range between 298 and 7000 K. This gas mixture corresponds to the plasma of a d.c. low current arc freely burning in the air between graphite electrodes in the presence of fluorine and calcium.