Dispersion sample volume influence

As noted earlier, the sample volume influences the height of the analyte peak and hence the dispersion coefficient of a flow system. Figure 1.37 illustrates this influence in FIA, allowing several conclusions... 

Another parameter influencing axial sample dispersion is the number of sample plugs for a given sample volume, which is defined by the number of bubbles along the sample zone or, in other words, the... 

The injected sample volume is by far the most important parameter affecting dispersion in flow injection analysis, as it has a pronounced influence on the height, width and area of the recorded peak (Fig. 5.11). 

In the frequency response method, first applied to the study of zeolitic diffusion by Yasuda [29] and further developed by Rees and coworkers [2,30-33], the volume of a system containing a widely dispersed sample of adsorbent, under a known pressure of sorbate, is subjected to a periodic (usually sinusoidal) perturbation. If there is no mass transfer or if mass transfer is infinitely rapid so that gas-solid mass-transfer equilibrium is always maintained, the pressure in the system should follow the volume perturbation with no phase difference. The effect of a finite resistance to mass transfer is to cause a phase shift so that the pressure response lags behind the volume perturbation. Measuring the in-phase and out-of-phase responses over a range of frequencies yields the characteristic frequency response spectrum, which may be matched to the spectrum derived from the theoretical model in order to determine the time constant of the mass-transfer process. As with other methods the response may be influenced by heat-transfer resistance, so to obtain reliable results, it is essential to carry out sufficient experimental checks to eliminate such effects or to allow for them in the theoretical model. The form of the frequency response spectrum depends on the nature of the dominant mass-transfer resistance and can therefore be helpful in distinguishing between diffusion-controlled and surface-resistance-controlled processes. 

V shonld be similar for the calibration standards and for the sample. Apparently, this advice is convenient to follow experimentally, however it may comphcate treatment of wide and flat chromatograms of broad molar mass dispersity polymers. The effect of v on the peak apex position of narrow cahbration standard differs from the influence of v. on the retention volumes of fractions comprised in broad molar mass dispersity sample. Qnantitative evaluation of the above difference is a complex matter... 

In principle, then, through equation (45), light-scattering studies of concentrated colloidal dispersions yield structural information in the form of h(r). In practice, however, the usefulness of equation (45) is severely limited by the fact that it is valid only in the absence of multiple scattering. With concentrated aqueous dispersions of latex particles, this means studying very small sample volumes in which structure could be influenced by the surface properties of the container. Even so, no measurements of this sort have yet been reported due to the experimental difficulties involved. 

Thus, the dispersion coefficient is a function of variables including the injected sample volume, tubing length and flow-rates, among others. Optimizing a flow system for a specific application requires a prior knowledge of the way the dispersion coefficient is influenced by these variables. This... 

The standards should match the samples in order to accurately determine the latter. Viscosity differences result in injected volume differences. If a compound is determined in the presence of high concentrations of other compounds in the sample, these can influence the migration time and peak shape by electromigration dispersion (EMD), and should therefore be present in the standards as well (Figure 1). ... 

Another example of synthesis of TiC-based compact cermets is reported by Xing et al. [90]. These authors describe the synthesis of dense NiAl-20 vol-% TiC composites through a reaction of a mixture of Ni, Al, Ti, and C powders in a hot press. The reaction was performed using the volume combustion mode heating the green mixture until a reaction is observed in the entire sample. The samples were heated up to 1500°C under the influence of an applied pressure. The products were 98.9% dense but the TiC particles (0.2-1 pm) were not uniformly dispersed in the matrix. The combustion process had similar ignition characteristics to the... 

To suppress the effect of certain molecular characteristic on sample retention volume so that the resulting chromatogram reflects mainly or even exclnsively other molecular characteristic(s) of sample. In practice, it is usually attempted to partially or fully suppress the influence of polymer molar mass. In this instance, the coupling of LC retention mechanisms may allow assessment of chemical structure or physical architecture of a complex polymer irrespective of its molar mass average and dispersity. Under favorable conditions, also the constituents of a complex polymer system with similar molar masses can be discriminated and molar mass of one constituent determined. For example, in the case of a two-component polymer system, the molar mass effect can be suppressed selectively for one constituent so that it elutes in a completely different retention volume compared with the retention volume pertaining to SEC. In some cases, the... 

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