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  • 1
    ISSN: 1432-1106
    Keywords: Long latency reflexes ; Electromyographic activity ; Wrist muscles ; Humans
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Reflex EMG responses to angular displacements of the wrist joint were recorded from 12 normal human volunteers. A mechanical stop was used to suddenly arrest displacements at varying times following the onset of the stimulus. With unrestricted movement of the handle, the EMG response consisted of an early component (M1) with a latency of 30–35 ms and a long-latency component (M2–3) beginning 55–65 ms after the onset of the displacement. When the displacements were arrested prior to a critical time occurring between 40 and 50 ms after the onset (mean of 44 ms), the M2–3 component of the response was not present. Increasing the duration of the displacement beyond this time resulted in a rapid increase in the size of M2–3. Facilitation provided by volitional intent to oppose the perturbation was not sufficient to generate an M2–3 response to either a brief, low velocity displacement produced by the torque motor or to a phasic, high velocity stretch produced by a tendon tap. The timing relationships between the onset latency of M2 and the minimum duration of displacement required to generate an M2–3 response are not easily reconciled with the notion that the segmentation of the EMG responses into components is mediated by repeated activation of the same central reflex pathway by phasic afferent bursts. Two mechanisms that could account for these results are either inhibition in response to the sudden stop of phasically-active “linking” interneurons which are part of the long latency pathway, or the loss of an essential convergent facilitatory input which serves to monitor the continuation of the movement.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Chromatographia 17 (1983), S. 451-453 
    ISSN: 1612-1112
    Keywords: Static coating ; Mixed solvents ; Capillary gas chromatography ; Capillary supercritical fluid chromatography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Mixed solvents in coating solutions were employed to facilitate (lower temperatures and shorter times) the static coating of capillary columns. Different volume ratios of mixed solvents used in the coating solutions do not affect the quality of capillary columns coated with stationary phases of different polarity. Small diameter (〈100 μm i.d.) capillaries of up to 50 m long were successfully coated with a methylphenylpolysiloxane (50% phenyl) for use in supercritical-fluid chromatography. In addition, an 80 m×0.31 mm i.d. capillary was statically coated with Superox 20 M (0.25 μm film thickness) in a 1∶1n-pentane/methylene chloride mixture at 55 °C. The coating time was only 7 hrs, and the resultant column exhibited a total of 170,000 theoretical plates (k′=5.5).
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1612-1112
    Keywords: Surface hydroxyl quantification ; Siliceous capillary columns ; Capillary gas chromatography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The concentration of surface hydroxyl groups on three types of capillary columns was determined by exchanging the hydroxyl protons with tritium. The tritium was quantified by combustion to tritiated water followed by scintillation counting. The number of hydroxyl groups on a leached and presumably hydroxylated Pyrex glass capillary column was found to be about 2.8 groups per square nanometer. This value was slightly less than the 4.6 groups per square nanometer that is generally accepted for a fully hydroxylated porous silica. Dehydroxylation of the same glass by heating at 600°C left only 0.4 groups per square nanometer while an untreated fused silica had only 0.2 groups per square nanometer.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1612-1112
    Keywords: Deactivation of columns ; Capillary gas chromatography ; Capillary supercritical fluid chromatography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary In order to prevent plugging during deactivation of small diameter (50 μm i.d.) capillary columns for gas and supercritical fluid chromatography, various high temperature deactivation methods were employed. Pure hexamethyldisilazane and hexamethyldisiloxane (a substitute for D4) were dynamically coated on the column, while a film (0.05 μm) of OV-101 was statically coated, before high temperature (450°C) treatment. Excellent deactivated columns were obtained, and no significant difference in column activity was observed using any of these three methods.
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  • 5
    ISSN: 1612-1112
    Keywords: Capillary gas chromatography ; Fused silica capillary columns ; Crosslinked phenyl polysiloxanes ; Non-extractable stationary phases
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The synthesis of methylphenylpolysiloxane polymers and their use in the preparation of crosslinked, non-extractable stationary phases for fused-silica capillary columns are described. By preparing more viscous phenyl-containing polymers than are commercially available, stationary phase films of these polymers could be efficiently coated on fused-silica capillary columns and stabilized by a free radical crosslinking mechanism using peroxides. Four methylphenylpolysiloxane polymers containing different phenyl concentrations were prepared. These included three polymers containing 50% phenyl and one polymer containing 70% phenyl. Two of the 50% phenyl polymers had one phenyl and one methyl group attached to each silicon atom. One of these also had 1% vinyl incorporated. The third 50% phenyl polymer was synthesized in such a way that one half of the silicon atoms had two phenyl groups attached while the rest contained dimethyl groups. The 70% phenyl polymer also had 4% vinyl incorporated. Due to the intrinsic thermal stability of these phenyl phases and the enhanced film stability achieved by crosslinking, the 70% phenyl phase could be utilized up to 400 °C. Using the methods described in this paper, highly efficient and thermally stable fused silica capillary columns coated with crosslinked methylphenylpolysiloxane stationary phases can be successfully prepared.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 0935-6304
    Keywords: Capillary gas chromatography ; Stationary phase stability ; Stationary phase viscosity ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With recent advances in column technology it is now possible to prepare highly efficient, very inert, and thermally stable capillary columns coated with nonpolar polysiloxane stationary phases. Unfortunately, the same degree of success has not been achieved for some of the more polar polysiloxane phases. A parameter that has been studied little in the past in relation to stationary phase film stability is the viscosity of the stationary phase. In this paper the efficiency and stability of coated columns are correlated to the viscosity of the phase. Due to their structure, the viscosity of the phenyl-containing polysiloxanes change rapidly with temperature and hence, thin-film coatings are not stable at elevated temperatures. By using high viscosity phenyl-containing methylphenylpolysiloxanes which were recently synthesized, efficient and stable columns have been prepared.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 0935-6304
    Keywords: Free radical crosslinking ; Capillary gas chromatography ; Azo compounds ; Nonextractable Polysiloxane stationary phases ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: There has been much effort spent in recent years developing the technology for free radical crosslinking of polysiloxane polymers to prepare capillary columns coated with thermally stable and nonextractable stationary phases. Organic peroxides have been used extensively as the free radical initiators for the in situ polymerization of the stationary phases. However, these peroxides adversely affect the phase polarity and column activity. Seven azo compounds were studied for crosslinking efficiency. Azo compounds can be used as free radical initiators to prepare nonextractable stationary phases without the adverse effects caused by peroxides.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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