Analytical selenoamino acid studies by chromatography with interfaced atomic mass spectrometry and atomic emission spectral detection

Consumption of selenium enriched plants or yeast-based nutritional supplements has been reported to provide anticarcinogenic benefits which are selenium compound dependent. Separation and identification of these selenium compounds is critical to understand the activity. Plants and yeast convert inorganic selenium in the soil or growth media into organoselenium compounds, probably following a route similar to the sulfur assimilatory pathway. Non-volatile selenium compounds produced include selenoamino acids, some of which have shown anticarcinogenic activity. Volatile compounds produced by chemical reaction of involatile precursors have also been found. An ion pair chromatographic method with ICP-MS detection for the separation of selenoamino acid standards potentially present in real samples is given. The method allows separation of selenoamino acids including such analytes as the cis-trans isomers of Se-1-propenyl-dl-selenocysteine. The method also provides the capability of determining the presence of selenoxides and possibly selenones, and tracking of other functionalities and reactions by selective derivatization. Alternatively, selenoamino acids are treated with ethylchloroformate to produce stable volatile derivatives which are amenable to GC separation with element specific atomic emission detection (GC-AED). Results of total selenium determination and speciation of selenium enriched yeast-based nutritional supplements, selenium enriched allium vegetables and bioremediation samples are presented. Received: 16 February 1998 / Revised: 4 June 1998 / Accepted: 9 June 1998     

Surface potential of hematite in aqueous electrolyte solution: hysteresis and equilibration at the interface

Electrostatic potential at the inner plane of the hematite aqueous interface, i.e., surface potential, was measured by means of a single-crystal hematite electrode. Acidic solutions were titrated with base and then back-titrated with acid. Surface potentials were evaluated from electrode potentials by setting the zero value at the isoelectric point. In the case of fast titrations the equilibration time was approximately 10 min, and significant hysteresis was obtained, more pronounced at higher electrolyte concentrations. Hysteresis disappeared in slow titration runs when the equilibration time was extended up to 120 min, and also when ultrasound was applied. Hysteresis was observed in the pH region close to neutrality, where the concentrations of potential-determining H+ and OH- ions are low. Equilibration was fast in acidic and basic regions. These results are explained on the basis of the kinetics of surface reactions, supported by the following rate of single-crystal electrode equilibration. It is concluded that the equilibration rate at the interface is specific for a given system and is not a general phenomenon. As several systems may undergo fast equilibration, such data may be regarded as equilibrium data and interpreted by the surface complexation model. In other cases, one should perform kinetic tests and apply extended equilibration times.     

W3 theory: robust computational thermochemistry in the kJ/mol accuracy range

We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T3 (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T3 contribution overall. They almost universally decrease molecular binding energies. Their inclusion in isolation yields less accurate results than CCSD(T) nearly across the board: It is only when T4 (connected quadruple excitations) effects are included that superior performance is achieved. T4 effects systematically increase molecular binding energies. Their basis set convergence is quite rapid, and even CCSDTQ/cc-pVDZ scaled by an empirical factor of 1.2532 will yield a quite passable quadruples contribution. The effect of still higher-order excitations was gauged for a subset of molecules (notably the eight-valence electron systems): T5 (connected quintuple excitations) contributions reach 0.3 kcal/mol for the pathologically multireference X 1Sigmag+ state of C2 but are quite small for other systems. A variety of avenues for achieving accuracy beyond that of W3 theory were explored, to no significant avail. W3 thus appears to represent a good compromise between accuracy and computational cost for those seeking a robust method for computational thermochemistry in the kJ/mol accuracy range on small systems.     

An Optical Carrier Generation Approach for Cellular Millimeter-Wave Radio Systems

An optical carrier generation approach is presented for cellular millimeter-wave radio systems. The well-known methods distribute the millimeter wave signal via fibers. They have drawbacks because they need very high-speed photonic compo nents that are expensive. This problem is overcome by the new approach utilizing an optically transmitted low-frequency reference signal to generate the millimeter wave carrier. The new approach applies inexpensive photonic components and is less sensitive to the fiber dispersion.     

Virial-bias Monte Carlo methods

A new sampling technique is proposed for the isothermal-isobaric (T, P, N) ensemble Monte Carlo computer simulation. The new method selects the volume perturbations by using the partial derivative of the volume with respect to the internal energy. Test calculations on the Lennard-Jones fluid show significant improvements over the conventionally used method.     

Electrical interfacial layer at TiO2/poly(4-styrene sulfonate) aqueous interface

The interfacial properties of the system titanium(IV) oxide/poly(4-styrenesulfonate) (PSS) over a broad pH region in the presence of different alkali metal chlorides of different concentrations were investigated by means of electrokinetic, adsorption and surface potential measurements. Adsorption and electrokinetic data were obtained with colloid TiO₂ particles, while surface potential data were obtained using a single crystal rutile electrode with the 001 plane exposed to the liquid medium. The electrokinetic and surface potentials of TiO₂ were measured in the absence and presence of PSS. Since the presence of PSS did not significantly affect surface potentials, it was concluded that negative PSS molecules adsorbed at the surface by forming an outer-sphere surface complex rather than inner-sphere complex. The adsorption decreases significantly with pH, while the electrokinetic potential in the presence of PSS is negative in the whole investigated pH region. Amount of adsorbed PSS molecules is limited by the electrostatic repulsion which suppresses further adsorption, i.e. above critical potential of ?50 millivolts. In the acidic region, where the surface is originally positively charged the amount of adsorbed PSS molecules is high since negative PSS molecules should at first compensate original positive charge and in the second step reverse the charge to reach the critical potential. In the basic region the surface charge is already negative so that small amount of adsorbed PSS molecules creates critical potential that prevents further adsorption.     

Popoviciu's inequality for functions of several variables

T. Popoviciu (1965) [13] has proved an interesting characterization of the convex functions of one real variable, based on an inequality relating the values at any three points x₁,x₂,x₃, with the values at their means of different orders: (x₁+x₂)/2, (x₂+x₃)/2, (x₃+x₁)/2 and (x₁+x₂+x₃)/3. The aim of our paper is to develop a higher dimensional analogue of the usual convexity based on his characterization.     

The category of 3-computads is not cartesian closed

We show, using [A. Carboni, P.T. Johnstone, Connected limits, familial representability and Artin glueing, Math. Structures Comput. Sci. 5 (1995) 441-459] and Eckmann-Hilton argument, that the category of 3-computads is not cartesian closed. As a corollary we get that neither the category of all computads nor the category of n-computads, for n>2, do form locally cartesian closed categories, and hence elementary toposes.     

Untersuchungen über die Aufnahme von Nitratstickstoff durch Hafer mit Hilfe von 15N

Die Aufnahme von Nitratstickstoff durch Hafer wurde in Gegenwart von Ammoniumstickstoffüberschuß sowie verschiedener Stickstoffdügermichungen untersucht. Ebenfalls wurde der Einfluß phosphor-, kalium-, und spurenelementhaliger Düngemittel auf den Ausmutzungskoeffizienten des Nitratstickstoffs durch Hafer geprüft. Der Nitratstickstoff des angewandten Ammonium- nitrats wurde mit dent stabilen Stickstoffisotop ¹⁵N markiert. Die Masse der trockenen Planzen, der Gesamtstickstoffgehalt und der Ausnutzungskoeffizient des Nitratstickstoffs durch die Planzen wurden bestimut. Die Versuche haben gezeigt, daß der Ausmutzungskoeffizient des Nitratstickstoffs durch die Pfanzen in Geganwart con Ammoniumstickstoffüberschuß nicht wesentlich beeinflußt wird.     

Precipitation and association of silver laurate in aqueous solutions

The temperature dependency of silver laurate solubility at various concentrations of precipitation components was investigated. Interpretation of the data indicated the formation of silver laurate ion pairs. The equilibrium constants for the precipitation and for ion pairing processes were calculated for the temperature range between 20 and 60 °C. The enthalpy of silver laurate precipitation was obtained by calorimetry. The agreement of the solubility and calorimetric data showed that no change in the structure of silver laurate occurred as the temperature was varied.