Quenched phosphorescence as alternative detection mode in the chiral separation of methotrexate by electrokinetic chromatography

Quenched phosphorescence was used, for the first time, as detection mode in the chiral separation of methotrexate (MTX) enantiomers by electrokinetic chromatography. The detection is based on dynamic quenching of the strong emission of the phosphorophore 1-bromo-4-naphthalene sulfonic acid (BrNS) by MTX under deoxygenated conditions. The use of a background electrolyte with 3 mg/mL 2-hydroxypropyl-β-cyclodextrin and 20% MeOH in 25 mM phosphate buffer (pH 7.0) and an applied voltage of 30 kV allowed the separation of l-MTX and its enantiomeric impurity d-MTX with sufficient resolution. In the presence of 1 mM BrNS, a detection limit of 3.2 × 10⁻⁷ M was achieved, about an order of magnitude better than published techniques based on UV absorption. The potential of the method was demonstrated with a degradation study and an enantiomeric purity assessment of l-MTX. Furthermore, l-MTX was determined in a cell culture extract as a proof-of-principle experiment to show the applicability of the method to biological samples.     

Erbium incorporation in LiNbO3 by diffusion-doping

 The erbium incorporation into LiNbO₃ by diffusion doping is investigated in detail by means of Secondary Ion Mass Spectrometry, Secondary Neutral Mass Spectrometry, Rutherford Backscattering, Atomic Force Microscopy, X-ray Standing Wave technique and optical site-selective spectroscopy. The diffusion of erbium in LiNbO₃ can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient. The diffusion constants and activation energies for Z-cut (X-cut) LiNbO₃ are 4.8×10^-5 cm²/s (12.0×10^-5 cm²/s) and 2.28 eV (2.44 eV), respectively. A limited solubility of erbium in LiNbO₃ has to be taken into account increasing exponentially with rising temperature. During the first step of diffusion an Er _x Nb _y -oxide layer is formed at the surface of the sample acting as diffusion reservoir. Erbium is incorporated into LiNbO₃ on vacant Li-sites slightly shifted from the original Li-position along the (-c)-direction. Site-selective spectroscopy found four distinguishable energetically different erbium centres at this lattice site resulting from locally different symmetries of the crystal field. Received: 21 March 1996 / Accepted: 12 August 1996     

Isotope selective photoionization of calcium using two-step laser excitation

The isotope specific two-step photoionization of metastable Ca atoms was investigated using a continuous wave dye laser and an Ar⁺ laser. The photoionization was performed via a transition to an autoionizing state. Therefore an ionization probability as high as 10⁻² could be obtained.     

Satelliten; Singuläre Erweiterungen und Derivationen

This paper is divided in two parts. In chapter one we generalize the concept of satellite for abelian categories, substituting Ext by an arbitrary functor E:A ^op×BEns. A very close relation to Kan functor extensions turns out. In chapter two we give a comprehensive formulation of the wellknown cohomology theories for groups and algebras-with the recently used modification in taking the group of derivations as the lowest cohomology group- starting with a categorical definition of the concept of singular extension. There is a zeroth chapter with a construction on functors, needed in both parts, and a third with examples.

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Gekürzte Fassung der Dissertation des Autors zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Hamburg.     

Über Biegeschwingungen von Stäben bei allgemeinem Elastizitätsgesetz

Ohne Zusammenfassung     

Addition von Korrespondenzen in abelschen Kategorien

Ohne ZusammenfassungDer Autor dankt dem Forschungsinstitut für Mathematik der E.T.H. in Zürich und der Deutschen Forschungsgemeinschaft für die Unterstützung seiner Arbeit.     

Silver ion dynamics in the Ag5Te2Cl-polymorphs revealed by solid state NMR lineshape and two- and three-time correlation spectroscopies

The relation between structure and ion dynamics in the three polymorphs of Ag(5)Te(2)Cl has been investigated using (109)Ag, (125)Te, and (35)Cl NMR spectroscopies. Specifically, the influence of the structural phase transitions observed near 240 K (P2(1)/c<-->P2(1)/n) and near 332 K (P2(1)/n<-->I4/mcm) upon silver ion mobilities has been studied by temperature dependent (109)Ag NMR lineshapes and spin-lattice relaxation times. While the superionic high temperature phase alpha-Ag(5)Te(2)Cl is characterized by a molten cationic sublattice, fast ion dynamics in the medium-temperature phase beta-Ag(5)Te(2)Cl occurs in spatially restricted regions comprising all the crystallographically distinct silver sites. Temperature dependent magic-angle-spinning linewidths yield an activation energy of 0.38 eV, consistent with 0.44 eV measured from dc electric conductivities. For the low-temperature gamma-modification, results of two- and three-time (109)Ag correlation spectroscopies provide a detailed insight into the nature of the silver ionic hopping motion. Temperature dependent jump rates measured by two-time correlation functions yield an activation energy E(a)= 0.48 eV. (109)Ag NMR three-time correlation functions indicate that the non-exponential relaxation behavior of the silver ions can be attributed to a broad distribution of jump rates rather than correlated forward-backward jumps. Nevertheless, all the silver ions are mobile down to temperatures of about 185 K.