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JPC – Journal of Planar Chromatography – Modern TLC - This paper reports the efficiency of the C30 phase for HPLC and TLC separation of a mixture of tocopherol homologs and... 相似文献
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Rapp E Jakob A Schefer AB Bayer E Albert K 《Analytical and bioanalytical chemistry》2003,376(7):1053-1061
A mixture of unsaturated fatty acid methyl esters was separated with a new splitless capillary set-up. With the employed apparatus configuration different capillary separation techniques such as capillary high-performance liquid chromatography (cHPLC), capillary electrochromatography (CEC) and pressurized capillary electrochromatography (pCEC) could be applied. The detection and identification of the sample compounds were accomplished by hyphenating these capillary separation techniques with nuclear magnetic resonance (NMR) spectroscopy using a novel configuration of the detection capillary set-up. Using modified electrokinetically driven separation techniques, the electric field was applied solely across the separation column. With this improved interface for capillary liquid chromatography-NMR on-line coupling, the stereochemical assignment of the cis and trans configuration of unsaturated fatty acids could be easily accomplished. Finally, the results of cHPLC-NMR, CEC-NMR and pCEC-NMR coupling experiments were compared.Dedicated to Professor Günter Häfelinger on the occasion of his 65th birthday 相似文献
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J. Schefer T. Woike M. Imlau B. Delley 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):349-352
The metastable states of sodiumnitroprusside are extremely stable at temperatures below 200 K. It is possible to allocate structural changes measured by neutron diffraction
to measured spectroscopic parameters, but the amount of the structural change is relatively small for a reaction co-ordinate
as the metastable states have an extremely long lifetime. New hypotheses for related systems try to explain such a phenomena
in two ways: The first way is a bending of the NO-bond in the metastable state, the second one an exchange of the oxygen and nitrogen atoms in the NO-bond
(which can be regarded as an bending). As such changes would be possible also from our density functional calculations, we re-investigated our neutron
diffraction data using the new models. However, our results are not compatible with one of these models. On the contrary,
the neutron diffraction data show partially opposite tendencies. We compare both models with EXAFS measurements, with vibrational
spectroscopic results and the data found by M?ssbauer spectroscopy. We propose a potential scheme for all three states (GS,
and ) extracted from absorption and thermodynamic data to explain the electronic and energetic rearrangement, and the population
dynamics.
Received: 23 June 1997 / Accepted: 13 October 1997 相似文献
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Rayleigh scattering has become an accepted technique for the determination of total number density during the combustion process. The interpretation of the ratio of total Rayleigh scattering signal as a ratio of densities or temperatures is hampered by the changing composition through a flame, since the average Rayleigh scattering cross-section depends on the gas composition. Typical correction factors as a function of degree of reaction, fuel and equivalence ratio were calculated. The fuels considered were H2, CH4, C2H4, C2H6 and C3H8. Factors as low as 0.7 and 0.56 were found for the heaviest hydrocarbon fuel at large equivalence ratio for interpreting the Rayleigh scattering intensity as gas density and inverse temperature, respectively. This is primarily due to the presence of CO and H2 as intermediates. As CO and H2 are subsequently oxidized to CO2 and H2O, these factors approach 1.0. Conversely, the worst case, when using H2 as a fuel, occurs in the post flame zone. However, the correction factors for H2 are near 1.0 and the errors involved will, in general, remain within the expected experimental accuracy of a typical Rayleigh scattering system. Linear correlations of correction factors with equivalene ratio and with the product of equivalence ratio and fuel molecular weight were found and presented. The interpretation of Rayleigh scattering as temperature was found to have larger errors than the interpretation as density. Corrections for changes in gas composition were applied to Rayleigh scattering temperature measurements in the post flame region of CH4 and C3H8 flames with equivalence ratios of 0.75 and 1.0. The corrected temperatures were in excellent agreement with thermocouple measurements.List of symbols
A
1, A
2
correlation coefficients
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B
1, B
2
correlation coefficients
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C
1, C
2
correlation coefficients
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D
1, D
2
correlation coefficients
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C
calibration constant for Rayleigh scattering optics
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H
total enthalpy
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Î
I
R
/I
RO
-
I
i
incident laser intensity
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I
R
Rayleigh scattering intensity
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I
R0
Rayleigh scattering intensity at reference condition
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N
total number density of gas
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N
0
total number density of gas at reference condition
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n
i
index of refraction of species i
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–
T/T
O
-
T
temperature
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T
a
adiabatic flame temperature
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T
0
reference temperature
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t
time
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W/W
0
-
W
mean molecular weight
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W
0
mean molecular weight at reference condition
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W
ij
rate of production of species i by reaction j
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X
i
mole fraction of species i
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degree of reaction (T – T
0)/(T
a
– T
0)
-
laser wavelength
- 0
Loschmidt number
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/0
-
density
- 0
density at reference condition
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dimensionless mean Rayleigh scattering cross-section
- Ri
Rayleigh scattering cross-section of species i
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scattering angle measured from the electromagnetic field vector
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equivalence ratio 相似文献
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Schefer AB Braumann U Tseng LH Spraul M Soares MG Fernandes JB da Silva MF Vieira PC Ferreira AG 《Journal of chromatography. A》2006,1128(1-2):152-163
Separation and characterization of limonoids from Switenia macrophylla (Meliaceae) by HPLC-NMR technique has been described. Analyses were carried out using reversed-phase gradient HPLC elution coupled to NMR (600 MHz) spectrometer in stopped-flow mode. Separated peaks were collected into an interface unit prior to NMR measurements, which were performed with suppression of solvent signals by shaped pulses sequences. Structure elucidation of the limonoids was attained by data obtained from 1H NMR, TOCSY, gHSQC and gHMBC spectra without conventional isolation that is usually applied in natural products studies. 相似文献
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A series of three para-benzoquinone mono-oximes and four oxy-derivatives were prepared and tested for their antidepressant properties. The (4E) oxime of 2-isopropyl-5-methyl-para-benzoquinone (4) and the corresponding 2-diethylamino-ethyl derivative (10) present antidepressant activities and were slightly more potent than the reference standard. 相似文献