Géza Nagy

Injection techniques in dynamic flow-through analysis with electroanalytical sensors

Abstract The theoretical and practical aspects of injection techniques used in flow-through systems with ion-selective electrodes and voltammetric detectors are discussed. Mathematical descriptions of the measuring systems based on the use of mixing chambers in the analytical channel are briefly outlined. The effects of different parameters on the analytical signals are described theoretically and experimentally. The methods developed for the evaluation of the analytical signals are presented in detail.

Novel polypyrrole based all-solid-state potassium-selective microelectrodes

Potassium-selective potentiometric microelectrodes with a polypyrrole solid internal contact were fabricated by the application of a potassium-selective bis-crown ether ligand based, plasticized poly(vinyl chloride) (PVC) membrane to the surface of conducting polymer modified Pt, Au or C micro disk electrodes. The selectivity and sensitivity of the new type of potentiometric potassium microelectrode were found to be comparable with those of the conventional macro ion-selective electrodes and of the micropipet type microelectrodes based on the same ionophore. The ease of preparation and robustness are the main advantages of this new electrode design, which can replace the classical micropipet type microelectrodes in many applications. The microelectrodes showed good dynamic characteristics and were used successfully in a wall-jet cell incorporated flow injection analysis system and as a measuring tip in scanning electrochemical microscopy. The polypyrrole modified substrate electrodes can be made selective for different ions just by changing the ionophore in the plasticized PVC membrane, which can extend their use to a wide range of applications.

Amperometric microcells for alkaline phosphatase assay

To develop simple electrochemical immunoassays, a screen printed amperometric microcell with graphite working and Ag/AgCl reference electrodes was tested for the determination of alkaline phosphatase enzyme (ALP) and anti-humanIgG conjugated ALP (alpha-hIgG-ALP) activity in 5-10 microl samples. To ensure reproducible, steady state conditions, the working electrode surface was coated with mass-transport controlling hydrogel layer. The kinetic response curves of the hydrogel coated electrodes were linear. In addition, the hydrogel layer reduced the nonspecific adsorption of the alpha-hIgG-ALP conjugate on the working electrode surface. The measurements were made in the range of 2 divided by 4000 mU ml(-1) enzyme activities using ascorbic acid 2-phosphate (AAP) as the enzyme substrate. AAP is commercially available, non-toxic and has excellent stability. The sensitivity of the determinations was about 71% of the sensitivity which could be achieved using p-aminophenylphosphate (PAPP), a not easily accessible and unstable enzyme substrate. The experimentally determined kinetic parameters of the ALP enzyme catalyzed reactions were the same with the bare and hydrogel layer coated electrodes.

Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps

Electron microprobe studies on the age, mineral chemistry and alteration on accessory LREE-phosphate minerals have been carried out in medium-grade metamorphic rocks of the Sopron Hills belonging to the Lower Austroalpine tectonic unit. Monazite (and xenotime) is relatively common, whereas rhabdophane and florencite are restricted to certain rock types. A first generation of monazite was formed in mica schists during the pre-Alpine, Hercynian metamorphism at 575–700 jC and 1.8– 3.8 kbar as evidenced by P–T data from the literature, their mineral paragenetic and textural characteristics and supported by Th–U–total Pb ages of ca. 300 Ma. In orthogneisses, monazite is rare and of igneous origin. Kyanite quartzites and leucophyllites that were formed by Mg metasomatism contain inherited monazite from the precursor rocks. A new generation of monazite was also formed during the Alpine metamorphism at V550 jC, 13 kbar according to the literature data, giving ages around 75 Ma. Pronounced negative Eu anomalies were found in the igneous monazites (Eu/Eu* 0.4). Small differences have been observed in Yand HREE contents, whereas the LREE sections of the rare-earth element (REE) patterns nearly coincide. Th and Ca enter the monazite structure at the expense of REE, nearly according to the brabantitic replacement 2REE 3+ XTh 4+ +Ca 2+ . In some mica schists, monazite is altered to rhabdophane. Rhabdophane, distinguished from monazite by quantitative electron microprobe analysis by low-oxide total, is found in many mica schists and orthogneisses. It forms fine-grained aggregates, often attached to apatite or monazite. It usually has higher Yand Ca contents and a less pronounced negative Eu anomaly than that of coexisting monazite. It may have been formed either by crystallization from REE-containing hydrous solutions or from monazite reacting with Y–Ca-containing solutions. Florencite appears only in some leuchtenbergite-bearing leucophyllites, kyanite quartzites and REE-rich clasts. It is often idioblastic and may be grown on apatite or monazite. It is chemically close to its ideal composition, but Ca, Sr and Th may replace REE in minor amounts. In some grains, ThO2 may reach 10 wt.%. The data indicate that the charge balance is maintained by different mechanisms in low- and high-thorian florencite. No Yor HREE (above Gd) could be measured in florencite. No fractionation was observed between coexisting monazite and florencite; however, monazite inclusions in florencite are depleted in La–Ce and enriched in HREE. D 2002 Elsevier Science B.V. All rights reserved.

Optical biosensor for urea with improved response time

An optical biosensor for urea measurements was developed. The operation of the sensor is based on the well-known urease enzyme-catalyzed hydrolysis of urea. The ammonium ions liberated in the reaction are detected with an ion selective optode membrane containing nonactin as ion selective ionophore and ETH 5294 chromoionophore in a thin (1 microm) plasticized poly(vinylchloride) film. The basic sensing element was home made of a microscope glass slide, a HeNe laser light source, photodiode light detector and light in coupling, de-coupling elements. The transducer membrane and the enzyme containing reaction layer were sandwich-cast with spin coating onto the surface of the sensing slide. The attenuation of the laser light propagating inside the glass wave-guide was used as signal for urea measurements. With this arrangement membranes provided good sensitivity (0.05 absorption unit when going from 0.1 to 1 mM urea) and short (16-20 s) response time. Taking advantage on the improved response time, flow injection urea measurements were made in the 0.01-2 mM concentration range. Thirty sample/hour analysis-rate, good peak-to-peak reproducibility (RSD=0.02) and recovery (95-104%) was achieved with buffer diluted urea solutions. Applications for the analysis of real samples are planned to do in the future.

Copper electrode based amperometric detector cell for sugar and organic acid measurements

Abstract Electrochemical behavior of sugars and organic acids at catalytic copper electrode was investigated in strongly alkaline media. Linear relationship was found for different analytes between concentration and voltammetric current wave or amperometric current. Electrode fouling was not observed neither in voltammetric nor in amperometric measurements. The number of electrons per molecule transferred during electrochemical oxidation was measured with controlled potential electrolysis and using the Koutecky–Levich function. The high number of electrons taking part in the electrode process and the stable operation of the electrode makes the catalytic copper electrode a well applicable sensor in FIA analysis and in chromatographic detection of different sugars and some organic acids. In the work a thin layer type detector cell was prepared and its function was studied.

The application of electroanalytical detectors in continuous flow analysis

Abstract A survey is given of the different fields of application of flow-through techniques employing electroanalytical detectors. Emphasis is placed on recently developed titration techniques.

Enrichment of Amadori products derived from the nonenzymatic glycation of proteins using microscale boronate affinity chromatography

Amadori peptides were enriched using boronate affinity tips and measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS). As demonstrated by electrochemical measurements, the tips show the highest binding efficiency for glucose at pH 8.2 employing ammonium chloride/ammonia buffer with ionic strength of 150 mM, exceeding taurine buffer at the same concentration. The bound constituents were released by sorbitol and formic acid. It was also demonstrated that elution with sorbitol at 1.2 M is superior to acidic media. Comparison of results was based on the numbers of detected peptides and their glycated sites. Using sorbitol for elution requires desalting prior to analysis. Therefore, three different sorbents were tested: fullerene-derivatized silica, ZipTip (C18), and C18 silica. Fullerene-derivatized silica and ZipTip showed the same performance regarding the numbers of glycated peptides, and sites were better than C18 silica. The elaborated off-line method was compared with liquid chromatography-tandem mass spectrometry (LC-MS/MS) measurements, by which considerable less modified peptides were detected. Affinity tips used under optimized conditions were tested for the analysis of human serum albumin (HSA) from sera of healthy and diabetic individuals. A peptide with a mass of 1783.9 Da could be detected only in samples of diabetic patients and, therefore, could be a very interesting biomarker candidate.

The influence of some organic complexing agents on the potential of copper(II) - selective electrodes. application of the silicone rubber-based electrode to the determination of citrate ion and 8-hydroxyquinoline

Abstract The behaviour of the silicone rubber-based copper(II)-selective electrode in the presence of complexing agents is discussed. The electrode shows well defined responses to several complexing agents in solutions practically free of copper(II) ions. Titrimetric determinations of citrates and 8-hydroxyquinoline with potentiometric end-point detection are described.

The electrochemical determination of phenolic derivates using multiple pulsed amperometry with graphite based electrodes.

The electrochemical determinations of 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) by chronoamperometry (CA) and multiple pulsed amperometry (MPA) using expanded graphite-epoxy composite (EG-Epoxy) and rotating spectral graphite disc (SG) electrodes are reported. The electrochemical behaviours of both electrodes in the presence of organics informed about oxidation peak potential and the electrode fouling with organics concentration increasing. Setting up the oxidation peak potential as detection potential, only SG gave good electroanalytical performance using CA. However, by MPA applying both electrodes exhibited the capability to assess electrochemically and quantitatively the pollutants from aqueous solutions. UV spectrometric method detecting 4-CP and 4-NP at lambda=280nm and lambda=398nm wavelength, respectively was used for validation and parallel determinations.