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  • 1
    Electronic Resource
    Electronic Resource
    On-line chromatographic pretreatment of samples with varying salt concentrations for capillary electrophoresis (1997)
    Springer
    Chromatographia 44 (1997), S. 581-588 
    Details
    ISSN: 1612-1112
    Keywords: Capillary electrophoresis ; On-line LC-CE ; Sample pretreatment ; Matrix effects ; Benzoic acids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Various methods are currently under investigation to improve concentration detection limits in capillary electrophoresis (CE). Stacking and isotachophoresis coupled in-line with free-solution electrophoresis are the techniques most frequently used. Samples containing a high and/or varying salt concentration, such as urine and serum, are difficult to handle: usually a significant loss in electrophoretic efficiency is observed compared with samples possessing a low electric conductivity. In the present paper an alternative approach is developed, i.e. a liquid chromatographic (LC)-type of sample pretreatment is coupled on-line with CE. To demonstrate the feasibility of this approach, the separation of three model compounds (benzoates) in water containing up to 400 mM of sodium chloride is studied using a 50 mM borate CE buffer of pH 9.5. The direct injection of samples with high salt concentrations in CE results in peak splitting and/or serious band broadening. These problems are not encountered when using the present LC-CE system. In addition, the detection limits are hardly influenced by the salt concentration of the sample; this underlines the robustness of the system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02466659
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  • 2
    Electronic Resource
    Electronic Resource
    Low-level interferences in peroxyoxalate chemiluminescence (1990)
    New York, NY : Wiley-Blackwell
    Biomedical Chromatography 4 (1990), S. 92-95 
    Details
    ISSN: 0269-3879
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The role of interferences at concentrations lower than 10-3 M in peroxyoxalate chemiluminescence is examined based on experimental results available in the literature. Implications for fluorophore and for hydrogen peroxide determinations are discussed. An interpretation in terms of the reaction mechanism is proposed.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bmc.1130040303
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  • 3
    Electronic Resource
    Electronic Resource
    The NMR spectra of the carbanions of xanthene and thioxanthene. Evidence for paratropism (1974)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 6 (1974), S. 245-249 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The proton NMR spectra of the carbanions of xanthene [XH]- and thioxanthene [TxH]- have been recorded and interpreted. Paratropism in the central rings of [XH]- and [TxH]- is inferred from a comparison of the chemical shifts with those of the carbanion of 9,10-dihydroanthracene [AH]-. The contributions to the chemical shifts arising from n-electron excess charges, local dipoles and magnetic anisotropies are discussed. Numerical values for the various ring currents have been estimated by a least squares analysis of the observed chemical shifts after applying corrections for the excess charge effect. The results point to a strongly increasing paramagnetic ring current in the central ring in the order [AH]-, [TxH]-, [XH]-.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270060413
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  • 4
    Electronic Resource
    Electronic Resource
    The proton exchange reaction between indene and the indenyl anion (1974)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 6 (1974), S. 574-576 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The proton exchange reaction between the indenyl carbanion and its parent compound indene has been studied by NMR as a function of temperature. The rate of this bimolecular reaction is very low and has been found to be strongly dependent on the polarity of the solvent. In solvents like dimethoxyethane (∊ = 7·2) and diglyme the reaction becomes manifest in the NMR spectrum only at elevated temperatures (T 〉 150°C). In hexamethylphosphortriamide (∊ = 30) the rate is much greater and line broadening may be observable at room temperature. The reaction in this solvent is characterised by a frequency factor f = 7 × 107 1 mol-1 s-1, an activation enthalpy ΔH ≠ = 9·5 kcal mol-1 and an entropy of activation ΔS≠ = -23 e.u. The low reaction rate and its solvent dependence are briefly discussed.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270061104
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  • 5
    Electronic Resource
    Electronic Resource
    The alkali cation effect on the 1H NMR chemical shifts of the indenyl carbanion (1979)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 12 (1979), S. 684-686 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complex character of the temperature dependence of the 1H NMR chemical shifts of indenyl-lithium and -sodium in dimethoxyethane is explained. It is shown that the cation causes a polarisation of the C—H bonds and thus influences the proton shifts of the anion, both directly by its electric field along the bonds (the direct effect) and indirectly via its effect on the π-electron distribution (the indirect effect). The indirect effect is inferred from 13C NMR chemical shift data. By subtracting the temperature dependent contribution of the indirect effect from the experimental 1H data, the direct effect is visualised. It appears that information on ion paris obtained by 1H and 13C NMR on the one hand and optical spectroscopy on the other hand is complementary. Apparently, aggregation of ion pairs does not seriously affect the chemical shift data.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270121207
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  • 6
    Electronic Resource
    Electronic Resource
    Interpretation of the asymmetrical ESR hyperfine spectrum of sodium pyrazine [Na P] in DME in the low temperature region. Evidence for the triple ion [Na P Na]+ (1974)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 6 (1974), S. 448-451 
    Details
    ISSN: 0030-4921
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ESR spectrum of Na pyrazine in DME, at temperatures below -50°C, has been studied in detail. It is shown that two paramagnetic species with slightly different g values are present in the solution, i.e. the sodium pyrazine ion pair [Na P] and the iriple ion [Na P Na]+. The interpretation is based on the concentration dependence of the spectrum.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1270060811
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