Peter W. Alexander

A reagentless amperometric biosensor for hydrogen peroxide determination based on asparagus tissue and ferrocene mediation

Abstract A new reagentless biosensor for the amperometric detection of hydrogen peroxide was developed by co-immobilisation of ground asparagus tissue ( Asparagus officinalis ) and ferrocene in a carbon paste matrix. The asparagus tissue acts as a source of peroxidase. Ferrocene is utilised as a mediator to facilitate efficient electron transfers between the electrode surface and hydrogen peroxide. The detection of hydrogen peroxide was based on the measurement of amperometric response due to the reduction at the electrode of ferricinium ion generated from the enzymatic reaction at 0.00 mV vs. SCE. The response characteristics and optimisation of the bioelectrode design are evaluated. The bioelectrode exhibits linear response up to a hydrogen peroxide concentration of 6 × 10 −5 M with a detection limit of 4.0 × 10 −7 M. Response times ( t 90 ) as low as 2 s and a relative standard deviation of replicate measurements of 1 × 10 −5 M hydrogen peroxide of 1.95% ( n = 15) were achieved. The bioelectrode sensitivity decreased to 50% of the original value within 30 days of continuous use.

Indirect potentiometric determination of metal ions by flow-injection analysis with a copper electrode

Abstract A copper-wire electrode is shown to give reproducible response when metal ion solutions in the concentration range 6.0×10−5–1.0×10−3M are injected into a phosphate buffer stream containing EDTA. Injections of Mg, Ca, Pb, Co, Cu, Ni, Mn, Ba, and Zn give peak heights dependent on both the metal ion and EDTA concentrations, allowing determination of either single metals or possibly multi-element determination after HPLC separations.

Application of a copper tubular electrode as a potentiometric detector in the determination of amino acids by high-performance liquid chromatography

A copper tubular electrode (CTE) has been used as a potentiometric detector in the determination of amino acids by reversed-phase high-performance liquid chromatography. The reference electrode was an electrically grounded tubular platinum electrode inserted in the flow stream. Comparison with a variable-wavelength UV absorbance detector showed that the CTE gave comparable response time and sensitivity, but was much more selective, showing response only to copper-binding molecules. The CTE has been applied to the analysis of urine and an intravenous amino acid preparation; no sample pre-treatment was required, other than filtration. Microgram quantities of amino acids were readily detected by using the CTE.

Flow-injection Potentiometric Detection of Phosphates Using a Metallic Cobalt Wire Ion-selective Electrode

Flow-injection potentiometric detection of phosphate has been undertaken using a metallic cobalt wire ion-selective electrode. The electrode response to phosphate was characterised by potential readings of high stability versus a Ag/AgCl reference electrode, and results have shown that the electrode displays high sensitivity and selectivity towards the phosphate ion. The metallic cobalt wire electrode was used for the flow-injection potentiometric detection of phosphate using a 0.04 mol l - 1 potassium hydrogen phthalate carrier at pH 5.0. A linear response with a slope of -38.00 ± 0.5 mV decade - 1 change in phosphate activity was obtained in the range 1.0 × 10 - 5 –5.0 × 10 - 3 mol l - 1 , and the detection limit was 1 × 10 - 6 mol l - 1 . The phosphate electrode described in this paper is robust, simple to use, highly selective, inexpensive and highly stable.

Separation of metal complexes of ethylenediaminetetraacetic acid in environmental water samples by ion chromatography with UV and potentiometric detection

An ion chromatographic method is described for the analysis of several metal complexes of ethylenediaminetetraacetic acid (EDTA) in water samples. Separations are achieved on C18 bonded silica, typically using a mobile phase comprising 1% (w/v) cetrimide-1.2 mM phosphate buffer (pH 7)-25% (v/v) acetonitrile-15% (v/v) methanol, as well as on a polymer-based anion exchanger using 2 mM phosphate buffer (pH 7) as eluent. Direct UV detection at 250 nm is employed for EDTA complexes of Fe(III), Cu(II), Pb(II) and Ni(II), whilst UV detection at 250 nm after post-column reaction with copper ions is utilized for EDTA complexes of Zn(II), Cd(II), Co(II), Pb(II), Ni(II) and Cu(II). Detection limits are in the range 1.5–4.0 ng for direct UV detection and 30–50 ng for post-column reaction detection. Indirect potentiometric detection after post-column reaction with copper ions is utilized with metallic copper as the indicator electrode, giving detection limits in the range 1.0–1.5 μg. These separations are applied to the determination of metal-EDTA complexes in river water at the ppbb level and to remobilization studies of metal ions in sediment.

A coated-metal enzyme electrode for urea determinations

A potentiometric enzyme electrode is reported in which an enzyme immobilized in polyvinyl chloride is used to coat an antimony metal electrode to detect changes in pH when the electrode is immersed in a solution of the enzyme substrat. As an example, urea is determined in solution by using immobilized urease on an antimony electrode, giving a linear concentration range of 5.0 × 10-4–1.0 × 10-2 M urea with a slope of 44 mV per decade change in urea concentration. The response slope is stable for about 1 week, with response times in the range 1–2 min, but with absolute potential changes occurring from day to day.

Flow injection potentiometry for enzymatic assay of cholesterol with a tungsten electrode sensor

Flow injection potentiometry (FIP) for the enzymatic determination of cholesterol is reported. The assay utilises a combination of three enzymes: cholesterol esterase (CE), cholesterol oxidase (COD) and peroxidase (POD). The method is developed by the use of a tungsten wire electrode as a sensor vs. Ag/AgCl in conjunction with a redox mediator ferrocyanide. CE converts esterified cholesterol to free cholesterol, which is then oxidised by COD with hydrogen peroxide as product. Ferrocyanide is converted to ferricyanide by hydrogen peroxide, catalysed by POD, and the tungsten electrode responds to the ratio of ferricyanide to ferrocyanide. Flow injection potentiometry gave well-defined peaks for cholesterol samples with a fast response (30 s). Linear calibration was obtained from 0.05 to 3.0 mM cholesterol, with a slope of 60.2 mV/decade change in cholesterol concentration, and detection limit 0.01 mM cholesterol (S/N=3). Repeatability was 3% (CV). Interferences from commonly found species were shown to be negligible. The sensor cell is simple to construct, and it was free from surface contamination problems over long periods of use. The application of the sensor for the determination of serum cholesterol was demonstrated.

Rapid flow analysis with inductively coupled plasma atomic-emission spectroscopy using a micro-injection technique

An introduction system for liquid micro-samples in inductively coupled plasma atomic-emission spectroscopy is described that allows the injection of 5–500-µl volumes into a rapidly flowing carrier reagent stream leading to the nebuliser. The effect on analyte signal was studied as a function of flow-rate, injection volume and sample concentration. It is shown that the carrier flow-rate determines the response time, sensitivity, precision and sample carry-over in the nebuliser. By the use of relatively rapid flow-rates of up to 7.5 ml min–1, fast injection of 10-µl samples is achieved at an injection rate of 240 h–1 with a relative standard deviation of 1.5% for a single-element analogue readout. Digital readout is used for multi-element determinations with similar or better precision. Detection limits of the order of 0.1 mg l–1 are obtained for 10-µl injections, limited by the volume injected, with a proportionate decrease in detection limit for increasing volumes.

Ion chromatography of inorganic anions with potentiometric detection using a metallic copper electrode

The determination of inorganic anions by ion chromatography with potentiometric detection using a metallic copper indicator electrode is described. The electrode response to inorganic anions can result from consumption of cuprous and cupric ions in the diffusion layer at the electrode surface, from production of copper ions due to oxidation of metallic copper, or as a result of displacement of a copper-complexing ligand from the eluent by an eluted non-complexing inorganic anion. The first possibility is exemplified by the determination of cyanide, chloride, bromide, iodide and thiocyanate, whereas the second possibility is illustrated by the determination of iodate, bromate and chlorate. An example of the indirect detection method is the determination of nitrite, nitrate and sulphate, using sodium tartrate as eluent. Calibration data for all of the above detection methods are provided and are interpreted in terms of theoretical response equations. Detection limits are also presented and are shown to be strongly dependent on the chromatographic conditions used and on the electrode response mechanism applicable to each anion.

High-performance liquid chromatography of organic acids with potentiometric detection using a metallic copper electrode

Abstract The use of a metallic copper wire electrode for potentiometric detection of organic acid anions in ion-exchange chromatography is described. These anions were detected by changes in electrode potential resulting from complexation of copper(I) or copper(II) ions at the electrode surface. The direction of this potential change, and hence the direction of the peak produced, was found to depend on the relative strengths of copper complexation between the injected ligand and the eluent ligand. Calibration relationships between the electrode potential and the amount of injected solute were studied and were observed to depend on the type of solute and the amounts injected. Several separations obtained using the copper wire electrode detector are presented as examples. Included are separations of glycinate, glutamate, and oxalate, and of acetate, lactate, formate, succinate, and benzoate.