Methodological reflections on a three-step-design combining observation,stimulated recall and interview

In this paper a tripartite qualitative design combining abservation, stimulated recall and interview is presented and discussed. This three-step-design makes it possible to get insight into the interaction of internal and external processes when solving mathematical tasks. The data analysis depends on the research question and the methodological approach. In the light of two research projects in mathematics education two different methods of data analysis are presented and methodologically reflected.     

A new technique for fluid velocimetry based on near field scattering

We show that the time evolution of near-field scattering speckles, originated by a fluid suspension of particles, provides information about the velocity field in the fluid. This information can be extracted from a statistical analysis of speckle fields taken at different times, either by measuring their cross-correlation function or by recovering the power spectrum corresponding to the difference between the two speckle fields. Experimental data are in accordance to the expected behaviors. The results are independent of the scatterer's size, allowing one to exploit the technique also with sub-wavelength tracking particles.     

An electrochemical multienzymatic biosensor for determination of cholesterol

This paper describes an electrochemical biosensor for free cholesterol monitoring. The sensor is a multienzymatic electrodic system in which horseradish peroxidase and cholesterol oxidase are simultaneously immobilized within a polymeric film, on the surface of a pyrolitic graphite electrode. From voltammetric and amperometric (flow-injection) data obtained, the efficiency, reproducibility and stability of the system are discussed. Results obtained, of interest for basic and applied biochemistry, represent a first step for construction of a mediator-free biosensor with potentialities for a successful application in the biosensor area.     

Selenium speciation in foods: Preliminary results on potatoes

The present paper deals with the speciation of selenium in potatoes (enriched or not in selenium). The study was carried out by using differential pulse cathodic stripping voltammetry (DPCSV) for quantifying selenium. Results obtained provide evidence that the selenium content in the protein fraction is rather independent from the selenium added to the plants during their growth. On the contrary, the amount of Se in the non-protein fraction (water and starch) in Se-enriched sample is significantly higher than in non-enriched one, suggesting that it is the main selenium-storing site. In this fraction the Se(VI)/Se(IV) ratio seems independent from selenium application but it may be related to the redox conditions. The accumulation of selenium in the non-protein fraction is tentatively ascribed to the “Se–starch interaction” that should be able to modulate both the Se absorption into proteins and, possibly, its toxic effect for the plant itself.     

Radio TLC in quality control and development of radiopharmaceuticals

Radio TLC has been used for determining the radiochemical purity and composition of two^99mTc-radiopharmaceuticals, available as kits and commonly used for diagnostic imaging. Moreover, the same technique has been applied to detect impurities which decrease the specific activity of¹³¹I-derivatized dermatan sulfate, a new potential radiopharmaceutical, and for establishing the best labeling conditions.     

Glutamate receptor incorporated in a mixed hybrid bilayer lipid membrane array, as a sensing element of a biosensor working under flowing conditions

The realization of a reliable receptor biosensor requires stable, long-lasting, reconstituted biomembranes able to supply a suitable biomimetic environment where the receptor can properly work after incorporation. To this end, we developed a new method for preparing stable biological membranes that couple the biomimetic properties of BLMs (bilayer lipid membranes) with the high stability of HBMs (hybrid bilayer membranes); this gives rise to an innovative assembly, named MHBLM (mixed hybrid bilayer lipid membrane). The present work deals with the characterization of biosensors achieved by embedding an ionotropic glutamate receptor (GluR) on MHBLM. Thanks to signal (transmembrane current) amplification, which is typical of natural receptors, the biosensor here produced detects glutamate at a level of nmol L(-1). The transmembrane current changes linearly vs glutamate up to 100 nmol L(-1), while the limit of detection is 1 nmol L(-1). In addition, the biosensor response can be modulated both by receptor agonists (glycine) and antagonists (Mg(2+)) as well, and by exploiting the biosensor response, the distribution of different kinds of ionotropic GluR present in the purified sample, and embedded in MHBLM, was also evaluated. Finally, one of the most important aspects of this investigation is represented by the high stability of the biomimetic system, which allows the use of biosensor under flowing conditions, where the solutions flow on both biomembrane faces.     

Plasma antioxidant capacity determination: comparative evaluation of chemiluminescent and spectrophotometric assays

All aerobic organisms have developed different mechanisms for neutralising the free radicals, mostly produced by the monoelectronic reduction of O(2), and preventing the severe damages these can provoke. The efficiency of these mechanisms can be assessed, in different matrices, by a simple and direct chemiluminescent assay (CL) based on luminol oxidation catalysed by horseradish peroxidase. Light emission is mediated by the production of free radicals and it is inhibited after a sample addition in a way that is directly proportional to the sample total content of molecules displaying antioxidant activity. The performances of this chemiluminescent assay were compared with those of two spectrophotometric methods already applied in clinical practice. First spectrophotometric method measures, like CL assay, the total antioxidant capacity, whereas the second one determines free thiol groups content. The chemiluminescent assay has a linearity interval between 0.60 and 9.46 mumol l(-1) of Trolox (y=34.91x+3.10; r=0.999; n=5) with an imprecision, expressed as CV, of 3.8% and an inaccuracy, expressed as percentage recovery, of 109%. The first spectrophotometric method, based on the same reference standard, the Trolox molecule, has a linearity interval between 0.2 and 2.5 mmol l(-1) of Trolox (y=-0.01x+4.54; r=0.95; n=5); the thiol groups assay has a linearity interval between 0.1 and 1 mmol l(-1) of l-cysteine (y=1.68x-47.09; r=0.998; n=5). Different clinical samples of plasma from healthy individuals, obese subjects and patients with liver diseases were tested. Interesting correlations were obtained among the three methods, but no significant correlations emerged between antioxidant capacity and clinical parameters. Significant differences were there only between men and women among obese subjects and between drinkers and non-drinkers among liver disease patients.     

Mixed hybrid bilayer lipid membrane incorporating valinomycin: improvements in preparation and functioning

The paper deals with the reconstruction of lipid bilayer membranes on a Au-covered polycarbonate membrane. Such a kind of like-biomembranes (namely mixed hybrid bilayer lipid membrane (MHBLM)) are characterised by appreciable long-term stability. Here we describe changes that have been made in the geometry of the experimental device in order to avoid artefacts and render membrane reproduction easier. Incorporation of valinomycin was performed to check the membrane and its stability: conductance and membrane potential following the changes of ion concentration were recorded. This new approach permits increase of successful trials and renders possible, when it breaks, easily formation of a new MHBLM on the same Au-covered polycarbonate membrane support. Finally, the stability shown by the MHBLM renders this system a promising tool for use under flowing conditions.     

Differential pulse polarographic determination of nitrate and nitrite in nuclear wastes

Summary Solid particles for nuclear fuel fabrication can be produced by gel-supported precipitation. Process by-products are aqueous wastes containing organic materials, ammonium nitrate and ammonia that can be decomposed into nitrogen compounds. A differential pulse polarographic determination of nitrate- and nitrite-nitrogen in such mixtures is described.The proposed method is based on the previous separation of nitrate and nitrite ions from inorganic and organic interferents. After ammonia volatilization, the separation is achieved with a column of Amberlite IRA 410 (Cl^– form) anion exchange resin that is then eluted with 2.5M NaCl. An aliquot of the eluate is made 1M NaCl, 0.012M HCl and 0.0001M U(VI) and polarographic cumulative determination of nitrite and nitrate follows at –0.93 V (vs. Ag/AgCl) potential. Then a solution 6.5·10^–5 M in diphenylamine, 2.5·10^–3 M in sodium thiocyanate and 3·10^–2 M in perchloric acid is added to a second aliquot of the eluate and differential pulse polarographic determination of nitrite follows at –0.52 V (vs. Ag/AgCl).The proposed method permits determinations of nitrite-nitrogen levels as low as 5 ng N ml^–1 and of nitrate-nitrogen levels at 200 ng N ml^–1. Relative standard deviations less than 4% may be routinely achieved. Analysis time is approximately one hour per sample.

---

Bestimmung von Nitrat und Nitrit in nuklearen Abfallprodukten

Zusammenfassung Feste Teilchen für die Herstellung nuklearer Brennstoffe lassen sich durch Fällung bei Anwesenheit von Gel herstellen. Dabei entstehen wäßrige Abfälle, die organisches Material, Ammoniumnitrat und Ammoniak enthalten, woraus durch Zersetzung stickstoffhältige Verbindungen entstehen können. Die Bestimmung von Nitratund Nitrit-Stickstoff in solchen Gemischen durch Differential-Puls-Polarographie wurde beschrieben.Die vorgeschlagene Methode beruht auf der vorhergehenden Abtrennung von Nitrat- und Nitritionen von störenden anorganischen und organischen Substanzen. Nach Verflüchtigung des Ammoniaks wird die Trennung mit Hilfe einer Säule aus Amberlit IRA 410 (Cl^–-Form) durchgeführt, die dann mit 2,5M NaCl ausgewaschen wird. Ein aliquoter Teil des Eluats wird auf 1M NaCl-Konzentration gebracht, 0,012M HCl und 0,0001M U(VI) eingestellt und polarographisch die Gesamtbestimmung von Nitrat und Nitrit bei einem Potential von –0,93 V (gegen Ag/AgCl) durchgeführt. Dann wird eine Lösung, bestehend aus 6,5·10^–5 M Diphenylamin, 2,5·10^–3 M NaCNS und 3·10^–2 M Perchlorsäure, einem zweiten Aliquot des Eluates zugesetzt und die Bestimmung des Nitrits durch Differential-Puls-Polarographie bei –0,52 V (gegen Ag/AgCl) vorgenommen.Dieses Verfahren ermöglicht die Bestimmung von Nitrit-Stickstoff bis auf 5 ng N/ml und von Nitrat-Stickstoff bis auf 200 ng/ml. Relative Standardabweichungen unter 4% lassen sich erzielen. Ungefähr eine Stunde je Probe ist erforderlich.

---

---

Presented at the 9th International Symposium on Microchemical Techniques, Amsterdam, August 1983.