Simulation of flourine migration in NiF2

The point defect and transport properties of nickel fluoride have been predicted using atomistic simulation. Results suggest that the dominant intrinsic defect reaction is anion Frenkel, however, formation energies are sufficiently high to suggest that intrinsic defect concentrations are low. Migration activation energies are consistent with flourine, as the most highly mobile species, being transported in the ab plane via the formation of a split interstitial configuration. Transport of both flourine and nickel will be considerably anisotropic.     

Missing Gamow-Teller strength in mass 42

The reaction ⁴²Ca(p,n)⁴²Sc at E_p = 160 MeV is used to measure the Gamow-Teller (GT) strength function. Normalization of the dominant peak in the (p,n) spectrum to B(GT) determined from the analogous transition in the beta decay of ⁴²Ti renders the strength function absolute, and the total measured stregth is about half of the minimum value required for a T = 1 nucleus. Shell model calculations are presented which reproduce the shape of the strength function, but overpredict the absolute measured strength by about a factor of two. Evidently the missing strength has been moved out of the region of nucleon particle-hole excitations, and quenching, due possibly to Δ₃₃ coupling, is indicated. Symmetry implications of an observed strong suppression of the T_> component of the GT strength are discussed.     

d.c. Magnetic fields of the human leg as a function of position and relaxation

Summary Investigations of the magnetic field around the human leg using a SQUID magnetometer yield consistent, reproducible results which can be modelled by 2 oppositely oriented line currents parallel to the leg axis. The fields are affected by muscle activity, decaying with the time of relaxation of the leg. Paper presented at the “IV International Workshop on Biomagnetism”, held in Rome, September 14–16, 1982.     

Strontium isotope ratios (87Sr/86Sr) of tooth enamel: a comparison of solution and laser ablation multicollector inductively coupled plasma mass spectrometry methods

Strontium isotope ratios (87Sr/86Sr) in tooth enamel provide a means to investigate migration and landscape use in humans and other animals. Established methods for measuring (87)Sr/(86)Sr in teeth use bulk sampling (5-20 mg) and labor-intensive elemental purification procedures before analysis by either thermal ionization mass spectrometry (TIMS) or multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Another method for measuring 87Sr/86Sr is laser ablation MC-ICP-MS, but concerns have been expressed about its accuracy for measuring tooth enamel. In this study we test the precision and accuracy of the technique by analyzing 30 modern rodent teeth from the Sterkfontein Valley, South Africa by laser ablation MC-ICP-MS and solution MC-ICP-MS. The results show a mean difference in 87Sr/86Sr measured by laser ablation and by solution of 0.0003 +/- 0.0002. This degree of precision is well within the margin necessary for investigating the potential geographic origins of humans or animals in many areas of the world. Because laser ablation is faster, less expensive, and less destructive than bulk sampling solution methods, it opens the possibility for conducting 87Sr/86Sr analyses of intra-tooth samples and small and/or rare specimens such as micromammal and fossil teeth.     

A wireless magnetoelastic biosensor for the direct detection of organophosphorus pesticides

An organophosphorus (OP) pesticide sensor was fabricated by applying a pH-sensitive polymer coating and organophosphorus hydrolase (OPH) enzyme onto the surface of a magnetoelastic sensor, the magnetic analogue of the better-known surface acoustic wave sensor. Organophosphorus hydrolase catalyses the hydrolysis of a wide range of organophosphorus compounds, which changes the pH in the hydrogel. This article describes the application of the magnetoelastic sensor for the detection of OP pesticides by measuring the changes in viscoelasticity caused by the swelling/shrinking of the pH-responsive polymer when exposed to the pesticides. The sensor was successfully used to detect paraoxon and parathion down to a concentration of 1 x 10(-7) and 8.5 x 10(-7) M respectively.     

The effect of the pore structure and zeta potential of porous polymer monoliths on separation performance in ion-exchange mode

Most often, in bioseparations involving charged macromolecules, the chromatographic systems have low Reynolds and high Peclet numbers. For such systems, an expression is developed and presented in this work for evaluating the throughput in polymeric monoliths where ion-exchange adsorption occurs, as a function of (i) the pressure drop along the length of the monolith, (ii) the functional form and width of the throughpore-size distribution of the monolith, and (iii) the magnitude of the zeta potential on the surface of the throughpores of the monolith. Gaussian and log-normal throughpore-size distributions whose mean throughpore-size and standard deviation values are based on experimentally measured throughpore-size distribution data by mercury porosimetry employed on polymeric monoliths are used in this work, and their effect on the throughput relative to that obtained from a polymeric monolith having a uniform throughpore-size distribution is studied for different values of the ratio of the standard deviation to the mean throughpore-size. The results indicate that relatively modest increases in the throughput, when compared with the throughput that could be achieved in a polymeric monolith having a uniform throughpore-size distribution, could be obtained in polymeric monoliths having disperse throughpore-size distributions, and the magnitude of the increase becomes larger when the disperse distribution is skewed to larger throughpore sizes. Furthermore, the results of this work indicate that, under certain conditions, relatively modest increases in the throughput of a charged analyte could also be achieved by altering the value of the zeta potential on the surface of the throughpores of the monolith. Due to the difficulties inherent in controlling the functional form and width of the throughpore-size distribution during the synthesis of polymeric monoliths, it would appear to be more practical to increase the value of the throughput of a charged analyte by altering the value of the zeta potential through prudent selection of the ion-exchange surface functional groups and fine-tuned with the pH of the mobile phase. Thus, for ion-exchange chromatography systems, the zeta potential could be considered an important parameter for column designers and operators to manipulate, since its alteration could increase the through-put of a charged analyte in polymeric monoliths or in columns packed with charged particles.     

Optical spectroscopy study of organic-phase lanthanide complexes in the TALSPEAK separations process

Time-resolved fluorescence spectroscopy and Fourier transform IR spectroscopy have been applied to characterize the coordination environment of lipophilic complexes of Eu(3+) with bis(2-ethylhexyl)phosphoric acid (HDEHP) and (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (HEH[EHP]) in 1,4-diisopropylbenzene (DIPB). The primary focus is on understanding the role of lactate (HL) in lanthanide partitioning into DIPB solutions of HDEHP or HEH[EHP] as it is employed in the TALSPEAK solvent extraction process for lanthanide separations from trivalent actinides. The broader purpose of this study is to characterize the changes that can occur in the coordination environment of lanthanide ions as metal-ion concentrations increase in nonpolar media. The optical spectroscopy studies reported here complement an earlier investigation of similar solutions using NMR spectroscopy and electrospray ionization mass spectrometry. Emission spectra of Eu(3+) complexes with HDEHP/HEH[EHP] demonstrate that, as long as the Eu(3+) concentration is maintained well below saturation of the organic extractant solution, the Eu(3+) coordination environment remains constant as both [HL](org) and [H(2)O](org) are increased. If the total organic-phase lanthanide concentration is increased (by extraction of moderate amounts of La(3+)), the (5)D(0) → (7)F(1) transition singlet splits into a doublet with a notable increase in the intensity of both (5)D(0) → (7)F(1) and (5)D(0) → (7)F(2) electronic transitions. The increased multiplicity in the emission spectra indicates that Eu(3+) ions are present in multiple coordination environments. The increased emission intensity of the 614 nm band implies an overall reduction in symmetry of the extracted Eu(3+) complex in the presence of macroscopic La(3+). Although [H(2)O](org) increases to above 1 M at high [HL](tot), this water is not associated with the Eu(3+) metal center. IR spectroscopy results confirm a direct Ln(3+)-lactate interaction at high concentrations of lanthanide and lactate in the extractant phase. At low organic-phase lanthanide concentrations, the predominant complex is almost certainly the well-known Ln(DEHP·HDEHP)(3). As lanthanide concentrations in the organic phase increase, mixed-ligand complexes with the general stoichiometry Ln(L)(n)(DEHP)(3-n) or Ln(L)(n)(DEHP·HDEHP)(3-n) become the dominant species.     

Partial ordering of glass networks adjacent to simulated glass–crystal interfaces

Interfaces between sodium lithium borosilicate glasses and the (100) and (110) surfaces of MgO and CaO crystals were simulated using a melt-quench procedure, employing classical pair potentials and molecular dynamics. The density of network forming species within the glass at these interfaces was considered as a function of distance from the plane of the interface and the positions of network formers were calculated in relation to sites in the crystal surface.For each interface, a strong correlation was found between the position and orientation of borate and silicate coordination polyhedra within the glass and particular positions of ions in the crystal surface, indicating that different partial ordering of the glass had occurred. In addition, examination of oxygen density profiles revealed a sequence of consistently spaced layers of increased density extending into the glass for all interfacial systems.     

Flow-through pore characteristics of monolithic silicas and their impact on column performance in high-performance liquid chromatography

In order to elucidate the role of the flow-through characteristics with regard to the column performance in high-performance liquid chromatography (HPLC) native and n-octadecyl bonded monolithic silica rods and columns, respectively of 100 mm length and 4.6 mm ID with mesopores in the range between 10 and 25 nm and macropores in the range between 0.7 and 6.0 μm were examined by mercury intrusion/extrusion, scanning electron microscopy, image analysis and permeability. The obtained data of the flow-through pore sizes and porosity values as well as surface-to-volume ratio of the stationary phase skeleton enabled to predict their influence to the chromatographic separation efficiency. Our data demonstrate that mercury porosimetry is a reliable technique to obtain all the characteristic parameters of the flow-through pores of silica monoliths. An important result of our examination was that the surface-to-volume ratio of monolithic silica skeletons had more significant impact to the separation process, rather than the average flow-through pore sizes. We could also show the essential differences between the particulate and monolithic stationary phases based on theoretical computation. The results, obtained from other characterization methods also indicated the structural complexity of monolithic silica samples. Permeability of columns is a generally applicable parameter to characterize all chromatographic phases no matter the chemistry or format. The correlation coefficient obtained for mercury intrusion and permeability of water was 0.998, though our investigation revealed that the surface modification is more likely influencing the obtained results. Further, the assumption of the cylindrical morphology of flow-through pores is not relevant to the investigated monolithic silica columns. These results on the morphology of the flow-through pores and of the skeletons were confirmed by the image analysis as well. Our main finding is that the flow-through pore sizes are not relevant for the estimation of the chromatographic separation efficiency of monolithic silica columns.