Recurrence plots of experimental data: To embed or not to embed?

A recurrence plot is a visualization tool for analyzing experimental data. These plots often reveal correlations in the data that are not easily detected in the original time series. Existing recurrence plot analysis techniques, which are primarily application oriented and completely quantitative, require that the time-series data first be embedded in a high-dimensional space, where the embedding dimension d(E) is dictated by the dimension d of the data set, with d(E)>/=2d+1. One such set of recurrence plot analysis tools, recurrence quantification analysis, is particularly useful in finding locations in the data where the underlying dynamics change. We have found that for certain low-dimensional systems the same results can be obtained with no embedding. (c) 1998 American Institute of Physics.     

Line-dependent saturation in CO2 lasers

Line dependent saturation has been studied in a CO₂ waveguide laser, operating cw in 100 lines of the 9 m and 10 m bands. In both bands, for constant discharge conditions, the saturation intensity is found to be strictly inversely proportional to the small signal gain for lines in the range J=2 to J=48 in both the P and the R branches. For lasers operating well above threshold this implies an output power which is essentially independent of the line number. The effect is verified experimentally, and discussed theoretically.Supported by the Danish Science Research Council under grant no. 11-7777     

Selective solid-liquid extraction of lithium halide salts using a ditopic macrobicyclic receptor

A ditopic salt receptor that is known to bind and extract solid NaCl, KCl, NaBr, and KBr into organic solution as their contact ion pairs is now shown by NMR and X-ray crystallography to bind and extract solid LiCl and LiBr as water-separated ion pairs. The receptor can transport these salts from an aqueous phase through a liquid organic membrane with a cation selectivity of K+ > Na+ > Li+. However, the selectivity order is strongly reversed when the receptor extracts solid alkali metal chlorides and bromides into organic solution. For a three-component mixture of solid LiCl, NaCl, and KCl, the ratio of salts extracted and complexed to the receptor in CDCl3 was 94:4:2, respectively. The same strong lithium selectivity was also observed in the case of a three-component mixture of solid LiBr, NaBr, and KBr where the ratio of extracted salts was 92:5:3. This observation is attributed to the unusually high solubility of lithium salts in organic solvents. The study suggests that ditopic receptors with an ability to extract solid salts as associated ion pairs may have application in separation processes.     

Near-edge elastic photon scattering from dilute aqueous ions

For ions in aqueous solutions, measurements of elastic photon scattering cross-sections have been made in the vicinity of core atomic orbital energies. It is proposed that the aqueous system provides a suitable situation for testing predictions obtained through use of the independent particle approximation (IPA); using intermediate atomic number elements, and at ionic concentrations ranging from 0.01 to 0.1 M/l, resonance structures accord with predictions of elastic scattering cross-sections calculated within IPA. Conversely, at higher ionic concentrations (>0.1 M/l) the resonance structure is distorted. The amplitudes calculated using modified form-factors and anomalous scatter factors computed from a Slater exchange potential, were convolved with a Lorentzian of several eV. This results in cancellation of the increasingly rapid fluctuations, producing a smoothly continuous cross-section across the ionisation threshold.     

Determination of calcium, iron and magnesium in rabbit arteries by inductively coupled plasma atomic emission spectrometry

Regional variations in arterial concentrations of Ca, Mg and Fe may influence susceptibility to atherosclerosis. However, investigation of such hypotheses requires the availability of a sensitive, reliable method for the determination of elements in small arterial samples. These biologically important elements are determined in rabbit arteries by inductively coupled plasma atomic emission spectrometry (ICP-AES). Arterial samples (aorta and iliac arteries) are collected from 4- and 6- to 7-month-old rabbits fed rabbit chow. Closed-vessel microwave acid digestion is used to prepare the samples. The accuracy of the method is tested with a NIST bovine liver (1577b) standard reference material, and the amount of each metal found is within the reported uncertainty in the certified concentration. Also, the recovery from artery samples spiked with 0.5 μg of each metal is nearly 100% (96-105% Ca, 93-105% Fe, and 92-104% Mg). The simultaneous multielement detection of Ca, Fe and Mg at levels more than 1000-fold higher than the detection limit, in arterial samples weighing as little as 5 mg, suggests that this method may be applicable to very small clinical samples or arterial samples from very small animals.     

On the chemical nature of graphene edges: origin of stability and potential for magnetism in carbon materials

Heretofore disconnected experimental observations are combined with a theoretical study to develop a model of the chemical composition of the edges of graphene sheets in both flat and curved sp(2)-hybridized carbon materials. It is proposed that under ambient conditions a significant fraction of the oxygen-free edge sites are neither H-terminated nor unadulterated sigma free radicals, as universally assumed. The zigzag sites are carbene-like, with the triplet ground state being most common. The armchair sites are carbyne-like, with the singlet ground state being most common. This proposal is not only consistent with the key electronic properties and surface (re)activity behavior of carbons, but it can also explain the recently documented and heretofore puzzling ferromagnetic properties of some impurity-free carbon materials.     

Development of carbohydrate-based scaffolds for restricted presentation of recognition groups. Extension to divalent ligands and implications for the structure of dimerized receptors

The solution structure of glycosyl amides has been studied by using NMR. A strong preference is displayed by tertiary aromatic glycosyl amides for E-anti structures in contrast with secondary aromatic glycosyl amides where Z-anti structures predominate. The structural diversity displayed by these classes of molecules would seem to be important as the directional properties of the aromatic ring, or groups attached to the aromatic ring, would be determined by choosing to have either a secondary or tertiary amide at the anomeric center and could be considered when designing bioactive molecules with carbohydrate scaffolds. The structural analysis was also carried out for related divalent secondary and tertiary glycosyl amides and these compounds display preferences similar to that of the monovalent compounds. The constrained divalent compounds have potential for promoting formation of clusters that will have restricted structure and thus have potential for novel studies of mechanisms of action of multivalent ligands. Possible applications of such compounds would be as scaffolds for the design and synthesis of ligands that will facilitate protein-protein or other receptor-receptor interactions. The affinity of restricted divalent (or higher order) ligands, designed to bind to proteins that recognize carbohydrates which would facilitate clustering and concomitantly promote protein-protein interactions, may be significantly higher than monovalent counterparts or multivalent ligands without these properties. This may be useful as a new approach in the development of therapeutics based on carbohydrates.     

Thin-film polymer light emitting diodes as integrated excitation sources for microscale capillary electrophoresis

We report the use of a thin-film polymer light emitting diode as an integrated excitation source for microfabricated capillary electrophoresis. The polyfluorene-based diode has a peak emission wavelength of 488 nm, an active area of 40 microm x 1000 microm and a thickness of similar 2 mm. The simple layer-by-layer deposition procedures used to fabricate the polymer component allow facile integration with planar chip-based systems. To demonstrate the efficacy of the approach, the polyfluorene diode is used as an excitation source for the detection of fluorescent dyes separated on-chip by electrophoresis. Using a conventional confocal detection system the integrated pLED is successfully used to detect fluorescein and 5-carboxyfluorescein at concentrations as low as 10(-6) M with a mass detection limit of 50 femtomoles. The drive voltages required to generate sufficient emission from the polymer diode device are as low as 3.7 V.     

Two-dimensional optical spectroscopy: two-color photon echoes of electronically coupled phthalocyanine dimers

Two-color photon echo peak shift spectroscopy was used to study electronic coupling in a phthalocyanine homodimer. Two optical parametric amplifiers were used to produce pulses to excite the split lower states of LuPc2-. The existence of a two-color peak shift indicates the existence of correlation between these two dipole-allowed states. The nature of this correlation is discussed based on theoretical predictions of the interactions between exciton and charge resonance states.