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.     

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.

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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.

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

A new graphic characterization of non singular quadrics of PG(r,q)

LetK be ak-set of class [0, 1,m,n]₁ of anr-dimensional projective Galois space PG(r, q) of orderq. We prove that: Ifr = 2s (s 2),k = _2s–1 and if through each point ofK there are exactlyq ^2(s–1) tangent lines and at most _2s–3 n-secant lines, thenK is a non singular quadric of PG(2s,q). Ifr = 2s–1 (s2),k=_2(s–1) +q ^s–1 and if at each point ofK there are exactlyq ^2s–3 –q ^s–2 tangents and at most _2(s–2)+q ^s–2 n-secant lines, thenK is a hyperbolic quadric of PG(2s–1,q).     

Theoretical and spectroscopic study of the effect of ring substitution on the adsorption of anisole on platinum

The adsorption of anisole, 3,5-dimethylanisole, and 3,5-bis-(trifluoromethyl)-anisole on Pt(111) was studied theoretically and compared to the adsorption of benzene using relativistically corrected density functional theory. A cluster of 31 platinum atoms was used to simulate the surface. The three anisoles were found to be less strongly adsorbed than the parent molecule benzene, 3,5-bis-(trifluoromethyl)-anisole showing weakest adsorption, with an adsorption energy of only one-third that of benzene. The theoretical study was complemented by in situ ATR-IR spectroscopy of the adsorption of the anisole derivatives on a polycrystalline Pt film. The spectroscopic study indicated that the adsorption strength of the anisoles follows the same order as predicted by the calculations. In addition, catalytic hydrogenation tests showed that the propensity to aromatic ring hydrogenation can also be correlated to the mode and strength of adsorption of the anisoles. The degree of saturation followed the same order as the adsorption strength found by the calculations and indicated by spectroscopy. Although 3,5-dimethyl substitution on anisole resulted in only a partial loss of adsorption energy and reactivity toward ring hydrogenation as compared to anisole, the substitution by CF(3) groups led to a large loss of adsorption energy and complete loss of reactivity toward aromatic ring saturation. Along with the study of the substituent effect on the adsorption of aromatic molecules, the correlation between adsorption and propensity to saturation of aromatic substrates could be corroborated.     

Enzyme-catalyzed synthesis of furanosyl nucleotides

A bacterial alpha-d-glucopyranosyl-1-phosphate thymidylyltransferase was found to couple four hexofuranosyl-1-phosphates, as well as a pentofuranosyl-1-phosphate, with deoxythymidine 5'-triphosphate, providing access to furanosyl nucleotides. The enzymatic reaction mixtures were analyzed by electrospray ionization mass spectrometry and NMR spectroscopy to determine the anomeric stereochemistry of furanosyl nucleotide products. This is the first demonstration of a nucleotidylyltransferase discriminating between diastereomeric mixtures of sugar-1-phosphates to produce stereopure, biologically relevant furanosyl nucleotides.     

Discrimination of active palladium sites in catalytic liquid-phase oxidation of benzyl alcohol

Knowledge of the structure of active sites is a prerequisite for the rational design of solid catalysts. Using site-selective blocking by CO and isotope labeling combined with in situ attenuated total reflection infrared (ATR-IR) spectroscopy, we were able to discriminate the different sites involved in the liquid-phase oxidation of benzyl alcohol on Pd/Al(2)O(3). The main reaction, that is, the oxidative dehydrogenation of the alcohol to the corresponding aldehyde, showed only little dependence on structure and occurred on all exposed Pd faces, whereas the undesired product decarbonylation occurred preferentially on hollow sites on (111) Pd faces. This explains why specific blocking of the latter sites, as realized in the industrially used Pd-Bi/Al(2)O(3) catalysts, leads to improved catalytic performance.     

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.     

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.