Negligible Memory Effects Research Articles

An optimized sampling system for highly reproducible isotope ratio measurements (δ13C and δ18O) of pure CO2 gas by infrared spectroscopy

The characteristics of an optimized sampling system for measurements of isotope ratios in pure CO2 gas with Isotope Ratio Infrared Spectroscopy measurement systems that has achieved reproducible measurement of δ13 C and δ18O values with 0.02‰ reproducibility (1 σ) is described. The key elements of the sampling system revolve around almost identical treatment of sample and reference gases allowing two-point calibration of up to 14 samples, and appropriate flushing protocols to remove any biases from memory effects of previously sampled gases. Measurements are performed by the Isotope Ratio Infrared Spectroscopy system at a mole fraction of nominally 700 μmol mol−1 CO2 in air, by dilution of pure CO2 gas controlled by individual low-flow mass flow controllers (0.07 ml min−1), and with a feedback loop to control mole fractions to ensure that differences between references and sample gas mole fraction stay below 2 μmol mol−1. This level of control is necessary to prevent biases in measured isotope ratios, the magnitude of which has also been studied with a sensitivity study. The system has been validated using pure CO2 samples which range in δ13 C delta values of −1‰ and −45‰ vs VPDB-CO2, and in all cases measurement reproducibility over several days of testing of 0.02‰ or better (1 σ) was achieved for both δ13 C and δ18O, with negligible memory effects. The amount of sample gas used for each measurement was less than 5 ml of CO2 at (RTP), making the system easily deployable for isotope ratio value assignment of bulk CO2 gas, and adaptable to atmospheric mole fractions of CO2 in air, and for value assignments of standards. Using the sampling system described the measurement reproducibility of current Isotope Ratio Infrared Spectroscopy systems approaches measurement reproducibility that can be achieved with some IRMS systems.

Cobalt Chalcogenides/Cobalt Phosphides/Cobaltates with Hierarchical Nanostructures for Anode Materials of Lithium-Ion Batteries: Improving the Lithiation Environment.

Lithium-ion batteries (LIBs) are widely used in electric vehicles and portable electronic devices due to their high energy density, long cycle life, environmental friendliness, and negligible memory effect, though they also suffer from low power density, safety issues, and an aging effect. Cobalt chalcogenides/phosphides as promising anode materials have attracted intensive interests due to their high theoretical capacity based on the conversion mechanism. Cobaltates (XCo2 O4 , X = the other metal) have attracted attention because the X element can partially replace the high cost and toxic cobalt element. The serious volume variation during the cycling process has an impact, however, on the lithiation environment of above materials. Hierarchical construction can provide more active sites and shorten the diffusion pathways of Li ions as well as accommodating the volume expansion during lithiation processes. Herein, the research progress on the synthesis methods, structural characteristics, and electrochemical performances of cobalt chalcogenides/cobalt phosphides/cobaltates with hierarchical nanostructures for LIBs is presented. The concluding remarks highlight the research challenges and possible development directions of cobalt chalcogenides/cobalt phosphides/cobaltates with tailored hierarchical nanostructures for LIBs.

Reference materials selection for the stable carbon isotope analysis of dissolved carbon using a wet oxidation system.

Wet chemical oxidation combined with isotope ratio mass spectrometry has become a routine technique for analyzing the stable carbon isotope composition of dissolved organic (DOC) and inorganic (DIC) carbon. However, methodological inconsistencies between laboratories in using different reference materials lead to a discrepancy in results. We experimentally tested the precision and accuracy of the analysis of commonly available international reference materials and other chemicals potentially suitable for laboratory standards. The solid international reference materials and other simple chemicals were used to prepare water solutions. A range of carbon concentrations was chosen to optimize tests for (1) precision and accuracy, (2) linearity, (3) detection limits, (4) memory effects, and (5) efficiency of DIC removal from a DOC/DIC mixtures. Samples were analyzed using an LC-IsoLink coupled with a Delta V Plus isotope ratio mass spectrometer (Thermo Fisher Scientific). The analytical setup had a negligible memory effect, good reproducibility (<0.21‰) and accuracy (maximum difference from the true values <0.35‰) for the analyzed organic compounds if approximately ≥9 × 10-09 moles of dissolved carbon was injected into the system (~11 mg C L-1 if a 10-μL loop was used). Analyses of sodium bicarbonate or calcium carbonate solutions had a two-fold lower accuracy despite maintaining a high precision. Aqueous solutions of international reference materials such as L-glutamic acids (USGS40, USGS41), benzoic acid (IAEA-601) and sucrose (IAEA-CH-6) can be successfully used for direct normalization of results to the VPDB scale. By contrast, analyses of caffeine and urea returned very reproducible but highly inaccurate results and these materials are not recommended for standards.

Fire-Retardant Pyrrolinium-Based Ionic Liquids As Electrolytes for Lithium Ion Batteries

Lithium ion batteries (LIBs) have some advantages such as a high energy density, negligible memory effect and a less self-discharge. Consequently, they have been applied to not only small electronic devices but also large energy storage systems. Although LIBs have been considered as one the most promising energy conversion/storage systems, the safety issue of LIBs must be overcome to realize large, high-power, and high-voltage LIBs. To get over the limitation of safety, many kinds of approach have been attempted to enhance the safety of the LIBs and one of the efficient ways is partial or total replacement of combustible organic electrolytes with a new class viscous materials, acting as poorly flammable and/or flame retardant components. In this respect, ionic liquids (ILs) have been considered as attractive electrolyte candidates because they are non-flammable owing to its unique chemical nature. The ILs are salts that remain as liquid at room temperature even if they are composed of ionic species, cations and anions. Due to the ionic nature in bonding, ILs show favorable physicochemical properties such as high ionic conductivity, non-volatility, and non-flammability as well as a wide liquid range and wide electrochemical stability window. Previously reported imidazolium-based ILs with low viscosity and high ionic conductivity have some defects such as decomposition of an acidic proton at the C-2 position[1]. To overcome the decomposition imidazolium ring, we have studied saturated cyclic ammonium-based ILs based on pyrrolidinium[2] or piperidinium[2], which showed more stable electrochemical behavior compared to the imidazolium-based ILs, but less favored physicochemical properties such as high viscosity and lower ionic conductivity. In this work, some novel pyrrolinium-based ionic liquids have been prepared and studied as electrolytes for LIBs to address those problems. Introduction of a beneficial functional group such as an oxygen atom into the partially saturated pyrrolinium ring was expected to give some positive effects on both the electrochemical stability and the physicochemical properties. Therefore, several target ILs that were synthesized in the present study showed higher thermal stability than the combustible organic solvent such carbonates. We will present the thermal, physicochemical, and electrochemical properties of several pyrrolinium-based ionic liquids as electrolytes, which are compared to those of the conventional carbonate. [1] Bull. Korean Chem. Soc., 2006, 27, 847 [2] Bull. Korean Chem. Soc., 2007, 28, 1567 Figure 1

Prevalence of the adiabatic exchange-correlation potential approximation in time-dependent density functional theory

Time-dependent (TD) density functional theory (TDDFT) promises a numerically tractable account of many-body electron dynamics provided good simple approximations are developed for the exchange-correlation (XC) potential functional (XCPF). The theory is usually applied within the adiabatic XCPF approximation, appropriate for slowly varying TD driving fields. As the frequency and strength of these fields grows, it is widely held that memory effects kick in and the eligibility of the adiabatic XCPF approximation deteriorates irreversibly. We point out, however, that when a finite system of electrons in its ground-state is gradually exposed to a very a high-frequency and eventually ultra-strong homogeneous electric field, the adiabatic XCPF approximation is in fact rigorously applicable. This result shows that adiabatic XCPF has a larger scope of applicability than previously suspected and in this sense is compliant with recent numerical findings by Thiele et al. [M. Thiele, E.K.U. Gross, S. Kümmel, Phys. Rev. Lett. 100 (2008) 153004] of negligible memory effects in strong-field double ionization.

Online CE-MALDI-TOF MS using a rotating ball interface.

We report on the construction and performance of a rotating ball interface for online coupling of capillary electrophoresis (CE) to matrix-assisted laser desorption ionization (MALDI) mass spectrometry with a time-of-flight (TOF) mass analyzer. The interface is based on a rotating stainless steel ball that transports samples from atmospheric pressure to the high vacuum of the mass spectrometer for desorption and ionization. The sample is deposited directly from a 50-microm-i.d. separation capillary onto the 19-mm ball that is rotating at 0.03 to 0.3 rpm. The sample is mixed online with matrix flowing from a separate 50-microm-i.d. capillary. The sample deposit dries before it is rotated past a polymer gasket and into the laser ionization region. Cleaning of the interface is accomplished using solvent-saturated felt, which cleans the ball surface after it rotates out of the ionization chamber. On-line CE-MALDI is demonstrated, and the performance is evaluated with the analysis of a mixture of three peptides: [Lsy8] vasopressin, substance P, and neurotensin. The rotating ball interface to MALDI-TOF MS demonstrated mass detection limit in the high femtomole range. The interface has negligible memory effect and shows no significant electrophoretic peak broadening when operated under optimized conditions.

Electron-beam-heated solid source for carbon doping in GaAs and AlGaAs alloys grown by molecular-beam epitaxy

A wide range of carbon doping in molecular‐beam‐epitaxy‐grown GaAs and AlGaAs alloys was obtained by means of a new electron‐beam heated solid source. Free hole concentrations from low 1015 cm−3 to over 1×1020 cm−3 were reproducibly obtained in GaAs and in direct‐gap AlxGa1−xAs while maintaining good surface morphology. These results extend the doping range in both directions with respect to commercial solid carbon sources. High activation percentage, negligible memory effects and diffusion were demonstrated via secondary ion mass spectrometer and photoluminescence measurements.

Carbon p+ doping of molecular-beam epitaxial GaAs films using carbon tetrabromide

Carbon-doped GaAs films have been grown by molecular-beam epitaxy using carbon tetrabromide as the carbon source. The films were doped up to 1.3 × 1020 cm−3 with mobilities which compare favorably to beryllium-doped films. Secondary-ion mass spectrometry measurements made on these films indicate sharp transitions and negligible memory effects as well as near unity doping incorporation. Room temperature photoluminescence intensities were equivalent to comparably beryllium-doped films at 5.0 × 1019 cm−3. Photoluminescence intensities and hole concentrations were found to be dependent on both arsenic to gallium flux ratios and substrate temperature. Annealing studies on a film doped with carbon at 4.6 × 1019 cm−3 indicate good thermal stability of the carbon in the arsenic lattice site.

Combination of chemical modifiers and graphite tube pre-treatment to determine boron by electrothermal atomic absorption spectrometry

Several ways of increasing the atomization efficiency in the determination of B by electrothermal atomic absorption spectrometry were evaluated: the use of a chemical modifier (Ni, Pd and ascorbic acid); pretreatment of the graphite tubes with carbide forming elements (Zr, Ta and V); and addition of NaF as a cleaning solution between measurements. The tubes were pre-treated by soaking in the carbide forming metal solution under vacuum, drying for 4 h at 150 °C and finally subjecting them to slow electrothermal heating to at least 1600 °C in order to form the metal carbide on the inner side of the tube. The best results were obtained using Zr-treated tubes in conjunction with Ni as chemical modifier. This resulted in high sensitivity (80 pg), a negligible memory effect and an increased tube lifetime (200 atomization cycles of 3 s at 2650 °C). The method was successfully applied to determine B in waste and river waters.

H2Se “memory effects” upon doping profiles in GaAs grown by metalorganic chemical vapor deposition (MO-CVD)

A chemical vapor deposition “memory effect” is the tendency of a source material to adsorb onto the internal walls of the system and subsequently desorb after nominal source shutoff. H2Se, the most common Se transport agent in metalorganic chemical vapor deposition (MOCVD), adsorbs onto stainless steel and glass surfaces. Among the consequences are: graded doping profiles at the end of Se-containing epitaxial layers and the depletion of H2Se gas mixtures with time. By contrast, SiH4, the most common Si transport agent, exhibits negligible memory effect and is recommended as an alternate n-dopant source when memory effects must be entirely eliminated. Capacitance-voltage measurements are used to quantify the extent of the t2Se “memory effect” in GaAs to identify methods for its reduction or elimination, and to distinguish the effects of diffusion and memory upon observed doping profiles. Memory effects are also anticipated when Se is a primary constituent of a semiconductor and H2Se the transport agent.

Graphite Furnace Atomic Absorption Spectrometry with a Tantalum Boat for the Determination of Yttrium, Samarium, and Dysprodium in a Mish Metal

Abstract The determination of yttrium, samarium, and dysprodium by means of graphite-furnace atomic absorption spectrometry (AAS) was studied by a tantalum boat inserted into a graphite tube atomizer. These elements could not be determined by the use of a commercial graphite tube. In the atomization from a tantalum boat, better analytical sensitivities and negligible memory effects for these rare earths are obtained. The analytical sensitivities of yttrium, samarium, and dysprodium with the tantalum boat were 0.60 ng, 0.86 ng, and 0.17 ng respectively. This method was applied for the determination of yttrium, samarium, and dysprodium in a mish metal. The measurements were performed with slightly acidified solutions (0.01 mol dm−3 HCl or HNO3). The sensitivities and the precisions for these elements decreased with increasing acid concentration. An enhancement in the sensitivities of yttrium and dysprodium upon the addition of a large excess of lanthanum, neodymium, and praseodymium salts were observed. The yttrium, samarium, and dysprodium in a mish metal were determined with both analytical curves of standard solutions containing an excess of lanthanum, cerium, and neodymium ions and of the standard addition. The precisions for this work were in the 3–9.3% range.

Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry

Atomization of yttrium in tube-type electrothermal atomizers was studied using various atomization surfaces: pyrocoated graphite surface, carbidized graphite surface and tantalum or tungsten metal surfaces. Carbidizing of pyrocoated graphite tubes with other carbide-forming metals (Ta, Zr or La) produces refractory metal-carbide surfaces thereby preventing the carbide-forming yttrium to come in physical contact with the reactive graphite surface. The result is an enhancement in the analytical sensitivity (peak height absorbance) of yttrium. The atomization of Y from a metal surface (Ta or W) gives better analytical sensitivity, lower atomization temperature, and negligible memory effect compared with those from metal-carbidized surfaces.