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  • 1
    ISSN: 1612-1112
    Keywords: High-performance liquid chromatography ; Expert systems ; System selection and optimisation ; Application to pharmaceutical analysis ; Selectivity optimisation ; Method validation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary This paper gives an overview of the structure and processes going on in the project Expert Systems for Chemical Analysis (ESCA), an international project within the ESPRIT research programme of the European Community. The application area, HPLC method development, is explained and broken down into four domains. The methods used for representing knowledge in the project are discussed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-5001
    Keywords: Distance geometry ; Genetic algorithms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary To improve the convergence properties of ‘embedding’ distance geometry, a new approach was developed by combining the distance-geometry methodology with a genetic algorithm. This new approach is called DG-OMEGA (DGΩ, optimised metric matrix embedding by genetic algorithms). The genetic algorithm was used to combine well-defined parts of individual structures generated by the distance-geometry program, and to identify new lower and upper distance bounds within the original experimental restraints in order to restrict the sampling of the metrisation algorithm to promising regions of the conformational space. The algorithm was tested on cyclosporin A, which is notorious for its intrinsic difficult sampling properties. A set of 58 distance restraints was employed. It was shown that DGΩ resulted in an improvement of convergence behaviour as well as sampling properties with respect to the standard distance-geometry protocol.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this letter we report on the use of selective metalorganic vapor phase epitaxy growth as an original tool to alleviate the problem of random formation of microcracks in thick GaAs-on-Si heteroepitaxial layers. Through the use of a special mask design including the definition of sharp wedges in the SiO2 mask material, the thermally induced stress in the GaAs-on-Si layers preferentially relaxes at precisely located sites on the substrate. The influence of mask configuration, wedge shape, wedge orientation, and layer thickness on microcrack formation has been investigated. Results obtained show that small-area SiO2 wedges are useful for the definition of microcrack location, and thus eventually for optoelectronic device processing.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 168-170 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel shadow masking technique is proposed for the local variation of growth velocity in GaAs/AlGaAs structures grown on GaAs by metalorganic vapor phase epitaxy. This mask makes use of epitaxially grown spacer and mask layers and windows in the mask are lithographically defined. This results in a highly accurate and reproducible shadow mask which can be removed by lift-off. Growth velocity variations up to 50% have been observed. The application of this technique to the coupling of passive and active waveguide structures is proposed.
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  • 5
    ISSN: 0192-8651
    Keywords: nucleic acid ; multivariate analysis ; Ramachandran plots ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A method to construct the equivalent of multidimensional Ramachandran plots for nucleic acids on the basis of singular value decomposition (SVD) is presented. For this purpose, a data matrix containing 244 DNA dinucleoside monophosphate steps, represented by nine torsion angles, was decomposed into a score and loading matrix. It is shown that biplots, containing both score points and loading vectors, provide a simple tool to interpret the principles of DNA class separation. Scores separate the data matrix into one A-DNA class, two different B-DNA classes, and one so-called crankshaft class. Loading vectors correlate torsion angles. The projections of scores on loading vectors indicate which torsion angles play a dominant role in DNA class separation. The results of the biplots are supported by (simple) physical interpretations. From a three-dimensional score space the nine original torsion angles can be reconstructed. Hence, the potential to create the multidimensional equivalent of a Ramachandran plot is available; that is, forbidden and accessible regions in the reduced space reflect these same regions in the nine-dimensional original space.   © 1998 John Wiley & Sons, Inc.   J Comput Chem 19: 695-715, 1998
    Additional Material: 12 Ill.
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  • 6
    ISSN: 1572-817X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract Strained layer superlattices have been used as the active region in asymmetric Fabry-Perot cavity optical modulators. The active layer of the Fabry-Perot modulator consisted of a 50 period In0.15 Ga0.85As/Al0.30Ga0.70As (10nm/10nm) superlattice. These quantum wells operate at typical wavelength of around 960 nm. By varying the length of the Fabry-Perot cavity in the modulator by including AlGaAs space layers of different thicknesses in the cavity, it is shown experimentally that both normally on and normally off devices can be obtained using the same stack of quantum wells. For the first type of device operation, a maximum contrast ratio of 8.3 dB could be measured for a reverse voltage of 7 V at 969 nm, while for the second type, a maximum of 8.9 dB at 957 nm was obtained for a 20 V reverse voltage. Using the same structure with an extra Bragg reflector on top of the quantum well layers to increase the surface reflection, a device with a higher finesse of the cavity was obtained. A maximum contrast ratio of 11.5 dB was measured for a reverse bias voltage of 30 V at 978 nm, with an insertion loss of −4.2dB.
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  • 7
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract This poster illustrates the lecture on Pattern Recognition and gives recently published and unpublished examples, mainly from the laboratory from the first author. The applications concern: - the determination of metabolic pathways of branched chain fatty acids (by clustering), - the development of a genetic classification of meteorites (by clustering), - the classification of cholinergic agents according to their interaction with different receptors (by clustering), - the structure of a data set consisting of gaschromatographic profiles in samples collected in pollution monitoring stations (by factor analysis and pattern recognition), - factors determining GLC behaviour of solutes (by factor analysis and multiple regression), - the classification of olive oils according to geographic origin (by principal components and pattern recognition), - the diagnosis of thyroid status (by pattern recognition).
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  • 9
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An infrared camera with focal plane InSb array detector has been applied to the characterization of macroscopic samples of household waste over distances up to two meters. Per waste sample (singelized), a sequence of images was taken at six optical wavelength ranges in the near infrared region (1100 nm – 2500 nm). The obtained three-dimensional data stack served as individual fingerprint per sample. An abstract factor rotation of this stack of six images into a spectroscopical meaningful intermediate six-element vector by Multivariate Image Rank Analysis (MIRA) finally provided a decision limit for the discrimination of plastics and nonplastics. A correct classification of better than 80% has been reached. The experimental NIRIS set-up has been automated so far to allow an on-line identification of a real world waste sample within a few seconds.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0886-9383
    Keywords: Standard errors ; Eigenvalues ; PCA ; MLR ; GRAM ; Rank estimation ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New expressions are derived for the standard errors in the eigenvalues of a cross-product matrix by the method of error propagation. Cross-product matrices frequently arise in multivariate data analysis, especially in principal component analysis (PCA). The derived standard errors account for the variability in the data as a result of measurement noise and are therefore essentially different from the standard errors developed in multivariate statistics. Those standard errors were derived in order to account for the finite number of observations on a fixed number of variables, the so-called sampling error. They can be used for making inferences about the population eigenvalues. Making inferences about the population eigenvalues is often not the purposes of PCA in physical sciences. This is particularly true if the measurements are performed on an analytical instrument that produces two-dimensional arrays for one chemical sample: the rows and columns of such a data matrix cannot be identified with observations on variables at all. However, PCA can still be used as a general data reduction technique, but now the effect of measurement noise on the standard errors in the eigenvalues has to be considered. The consequences for significance testing of the eigenvalues as well as the usefulness for error estimates for scores and loadings of PCA, multiple linear regression (MLR) and the generalized rank annihilation method (GRAM) are discussed. The adequacy of the derived expressions is tested by Monte Carlo simulations.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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