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  • 1
    ISSN: 1612-1112
    Keywords: Supercritical fluid chromatography ; Mixed mobile phases ; Capillary and packed columns ; Antibiotics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The separation of polar thermally labile solutes is one of the potentially most rewarding fields of SFC application. A presupposition for such applications is, however, mobile phases having relatively high solvent strengths. A promising approach to achieve this is the use of mobile phases consisting of carbon dioxide with a polar additive. In this work, the chromatographic effects of different concentrations of an additive, isopropanol, in carbon dioxide have been studied on capillary and packed columns. A series of antibiotics was used as test substances. Best results were obtained with carbon dioxide/8% isopropanol as mobile phase on a capillary column coated with a cyanopropyl-substituted polysiloxane stationary phase.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1612-1112
    Keywords: Ligand exchange chromatography ; Supercritical fluid chromatography ; Selective stationary phases ; Argentation open tubular columns ; Poly(methylpropylthiol)siloxanes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Poly(methyl-3-propylthiol)siloxane has been evaluated for use as stationary phase in open tubular columns for gas and supercritical fluid chromatography. Immobilization of the stationary phase was achieved by crosslinking with 2,4,6-trichloro-1,3,5-triazine. The poly(methyl-3-propylthiol)siloxane stationary phase film was in situ oxidized to the disulphide and sulphonic acid forms and both have been evaluated for use in supercritical fluid chromatography. Good selectivity towards polar polycyclic aromatic compounds has been shown. All the modified forms of the stationary phase were loaded with silver ions and were evaluated with regard to ligand exchange chromatography using supercritical fluid carbon dioxide as a mobile phase. The utility of the stationary phase, having sulphonic acid groups loaded with silver ions, has been demonstrated by separation of fatty acid methyl esters according to their unsaturation.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0935-6304
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0935-6304
    Keywords: Gas chromatography-mass spectrometry ; Capillary columns ; Stationary phases ; Silicones ; Thermal degradation ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The applicability of capillary columns for gas chromatography is often limited by stationary phase degradation at elevated temperatures. In order to achieve a better understanding of column thermostability, column bleed products have been analyzed qualitatively by gas chromatography-mass spectrometry (GC-MS). Three silicone stationary phases were studied: SE-52, SE-54, and OV-1701. Each of these was immobilized in the columns. The proportion of cyclics containing diphenyl in column bleed from SE-52 reflected to a large extent the composition of the polymer, while for SE-54 the proportion of such cyclics was unexpectedly high. The polar moiety of OV-1701, the cyanopropyl(phenyl)siloxy unit, was found to exert a highly destabilizing effect on the polymer, and the thermal degradation products consisted mainly of cyclosiloxanes containing this unit.
    Additional Material: 6 Ill.
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  • 5
    ISSN: 0935-6304
    Keywords: Enantiomer separation ; mixed chiral selectors ; modified cyclodextrins ; gas chromatography ; PCB ; o,p´-DDD ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Columns containing different types of cyclodextrin derivatives have been evaluated for chiral gas chromatographic separation of atropisomeric PCBs, o,p´-DDT and o,p´-DDD. Separation was attempted on columns containing mixed chiral selectors, and the performance of two closely related selectors was also examined. The cyclodextrins were: permethylated-β-CD (PM-β-CD), heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-β-CD (2,3-M-6-TBDMS-β-CD), heptakis(2,3-di-O-methyl-6-O-tert-hexyldimethylsilyl)-β-CD (2,3-M-6-THDMS-β-CD), and heptakis(2,3-di-O-ethyl-6-O-tert-hexyldimethylsilyl)-β-cyclodextrin (2,3-E-6-THDMS-β-CD). The cyclodextrins were dissolved in OV-1701 or in a dimethylsiloxane/silarylene copolymer containing 5% phenyl in the backbone. The application of mixed chiral selectors led to improved separations, however; at most eleven PCB congeners were separated on a single column. Chiral resolution of o,p´-DDD was achieved. The use of a dimethylsiloxane/silarylene copolymer as a matrix for the cyclodextrins is a promising approach. With such a matrix, blocking of the CD cavities by silicone substituent groups can be avoided, and a reasonable CD solubility can be provided. The selectivity of heptakis(2,3-di-O-ethyl-6-O-tert-hexyldimethylsilyl)-β-CD and heptakis(2,3-di-O-methyl-6-O-tert-hexyldimethylsilyl)-β-CD was quite different, the former selector could separate four congeners, while the latter separated ten congeners.
    Additional Material: 5 Ill.
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  • 6
    ISSN: 1040-7685
    Keywords: light scattering detection ; supercritical fluid chromatography ; packed microcolumns ; argentation chromatography ; triacylglycerols ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Packed capillary columns provide excellent separation of neutral lipids by supercritical fluid chromatography. For these separations, it is necessary to add a polar modifier to the mobile phase; hitherto, a detector suitable for quantitative analysis has been lacking. For this purpose, a miniaturized evaporative light scattering detector for combination with packed microcolumns in supercritical fluid chromatography has been developed. Since the detector response is dependent on the mobile phase flow rate, a drop in response was found at flow rates above 16 mL min-1 of expanded mobile phase. However, below this flow rate, there was a region of relatively constant response. In this region, quantitative analysis could also be successfully performed when the pressure and/or the temperature of the separation were programmed. The detection limit was lower than 6 ng. With pressure programming, the relative standard deviation in peak area was less then 4%. Minor peaks gave higher RSD values.Further, it is demonstrated that argentation SFC of triacylglycerols provides separation according to chain length as well as to the degree of unsaturation.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A description is given of theFreja double-probe electric field instrument. Its capability to perform high-resolution measurements of the aurora and its fine-structure as well as collect information on sub-auroral and low-latitude phenomena is illustrated by selected results from the first six months of operation. The instrument is highly flexible and possible to operate in a number of different modes. It is also equipped with a 4-Megabyte burst memory for high data sampling rate and temporary storage of data. It has been fully operational since October 1992, and delivers data from ≈22 hr day−1 including about 5–6 auroral crossings of the northern and southern auroral ionosphere. New and important information on the auroral fine structure and electrodynamics is obtained by means of burst resolution data (6144 samples s−1) and normal resolution data (768 samples s−1). Common burst data collection triggered by the electric field event detector has turned out to be very useful for the selection of scientifically interesting events. This is illustrated by high-resolution data of a pair of extremely intense and narrow electric field structures (1 V m−1) which are associated with a total absence of precipitating particles, depletions of the thermal plasma and with an intense wave activity. The low inclination of theFreja orbit provides a new perspective for studying largescale phenomena associated with east-west gradients as is exemplified by electric field data from a satellite crossing over north-south oriented auroral structures presumably resulting from rotational distortions of east-west aligned auroral arcs. The different plasma regimes encountered byFreja are continuously monitored by means of current sweeps applied to the probes and by the satellite potential. In addition, overview data (8 samples s−1) are collected from full orbits and stored in the on-board memory and have proved to be extremely valuable, providing new information on global electric field phenomena at subauroral and lower latitudes, such as the intense poleward electric fields and Pc-1 observations that have been made near the plasmapause during substorm activity.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A new approach to the study of ionospheric plasma characteristics is presented using data from the Freja double probe electric field instrument. Plasma characteristics are derived from continuous measurements of the satellite potential and from intermittent Langmuir sweeps. These provide information on both relative variations in the plasma density and absolute density and temperature, useful for comparisons with other plasma measurements on Freja, and essential for the interpretation of the electric field measurements. The on-board memory makes it possible to obtain full-orbit coverage of this type of information, which is a new feature of the Freja measurements. The memory is also used for high time resolution Langmuir sweeps which allow for the first time detailed studies of the time behavior of the probe response and computation of the probe-plasma capacitance. Comparisons are also made with similar measurements on earlier missions.
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  • 9
    ISSN: 0935-6304
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0935-6304
    Keywords: Thin-layer chromatography, HPTLC ; Reversed phase mode ; Modification of commercially available plates ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The condition for in situ chemical modification of commercially available thin-layer plates have been varied and the influence thereof upon reversed phase plate properties has been studied. The reaction variables studied were: average silica pore size, silica water content, use of tri- or monochlorosilanes as modifiers, reaction temperature, and addition of base. Coverage and activity of residual silanol groups was studied on modified plates.Desired plate properties are high speed of development in aqueous solvents and low content of active residual silanol groups. Under the conditions tested, the lowest amount of residual silanols was achieved at the expence of the speed of development in aqueous media and vice versa.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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