System peaks peak profiles

The total polysaccharide preparations from wine, apple and tomato juices were analysed in our HPSEC system. The obtained profiles (Figure 2) were all characterized by the presence of a main sharp peak eluted at the same elution volume (18.2 to 18.6 min) as a previously purified wine RG-II [20]. 

For systems, which cannot be marked easily, the displacement method is an alternative, in particular for low concentration ranges (36). This is a modification of isotachophoresis, because current flow is not constant with time and field strength is a function of the position along the capillary. Instead of analyte peaks, profiles are obtained (Fig. 6a) as injection and separation are carried out in one step. Although this method is not suitable for all micellar systems, one outstanding advantage is the higher UV sensitivity (which is important for most surface-active compounds). Because of the reliance of the method on displacement, the micellar phase is not diluted. 

Profiling radiometers measure and display the peak power and total density ion of a UV curing system and also profile the temperature and irradiance as a function of time. The information is transferred into a computer. They are capable of comparing characteristics of multiple lamps, comparing UV systems over time, or comparing different systems to each other. They track and store archival data. An example of profiling radiometers is in Figure 9.6. 

Up to this point it may have appeared that the terms hand, peak, and zone have been used synonymously, but let us take a closer look. As commonly used, all three terms describe the distribution of analyte molecules in space (a concentration profile), but band represents this distribution while the analyte is still in (or on) the system, while peak refers to a distribution of analyte that has eluted from the system. The term zone is more general and includes both bands and peaks it is used in those cases when we do not wish to be more specific. In the context of our current discussion, this means that the zones in TLC and PC are called bands and those in column LC and in GC are called peaks. 

Figure 9.85 Normal phase HPLC profiles of the reaction product of the cholesterol side chain cleavage system. Peaks were identified on the basis of their retention times. (i4) Without cholesterol oxidase treatment. Cholesterol (100 nmol) was incubated with cytochrome P450scc (70 pmol) in the presence of adrenodoxin, adrenodoxin reductase, and an NADPH-generating system. Monitoring was at 214 nm. Peaks 1, cholesterol 2, pregnenolone 3, deoxycorticosterone acetate (internal standard) (B) The reaction mixture of (A) was further incubated with cholesterol oxidase at 37°C for 10 minutes. Monitoring was at 240 nm. Peaks 1, cholestenone 2, progesterone 3, deoxycorticosterone acetate (internal standard). (From Sugano et al., 1989.)...

Chapters 10 to 13 review the solutions of the equilibrium-dispersive model for a single component (Chapter 10), and multicomponent mixtures in elution (Chapter 11) and in displacement (Chapter 12) chromatography and discuss the problems of system peaks (Chapter 13). These solutions are of great practical importance because they provide realistic models of band profiles in practically all the applications of preparative chromatography. Mass transfer across the packing materials currently available (which are made of very fine particles) is fast. The contribution of mass transfer resistance to band broadening and smoothing is small compared to the effect of thermodynamics and can be properly accounted for by the use of an apparent dispersion coefficient independent of concentration (Chapter 10). 

When a small sample is injected, the problem can be considered as a mere perturbation of the phase equilibrium, and simple solutions are easily derived. When large samples are injected, the elution profiles are more complex, sometimes surprisingly so. Thus, a separate discussion of these problems in linear and nonlinear chromatography is in order. Note that system peaks arise only when chromatography is carried out under conditions that, although they may be linear for the analytes, are not linear for the additive(s). 

From a theoretical point of view, the discussion of the profiles of the component and the additive bands at high concentrations is complicated because the perturbation due to the injection of the sample cannot be considered small and cannot be treated by assxuning the system to behave linearly aroxmd the steady equilibrium point, as is done in the study of system peaks in analytical chromatography. Band profiles are accessible only through numerical calculations. The experimental results are still difficult to account for because of the scarcity of studies and data on the phenomenon, and because of the strange and unexpected shape of the profiles obtained imder some sets of experimental conditions. 

The mathematical approach used to account for system peaks at high concentration of the sample components is the same as that described earlier in the linear case, when the analyte concentrations are low, and the terms Yli could be neglected in the isotherms. These terms can no longer be ignored at high concentrations, and this has a corisiderable influence on the shape of the solute and the system peak profiles. 

In Figure 13.13b, the compound has a bimodal profile the signal obtained with a selective detector suggests strongly that there are two partially resolved isomers. The signal of a nonselective detector exhibits a trough behind a major peak, features that are difficult to explain for an analyst who is not well aware of the riddles that system peaks may present occasionally. 

Experimental Study of System Peak Profiles. Influence of Sample Size... 

In this discussion, we assume that the mobile phase contains a single additive or modifier dissolved in a weak solvent, that the sample is a binary mixture, that the competitive isotherms of the additive and the two components are described by the Langmuir model, and that the sample is less retained than the additive, so that ao > 1, as stated above. The presence of an adsorbed modifier in the mobile phase (1) decreases the retention of the two components, (2) causes the elution of an additive system peak and two component system peaks, and (3) may result in considerable departure of one or both component band profiles from their expected Langmuirian behavior. 

If the mobile phase additive is much less retained than the sample components, the chromatogram (Figure 13.20) exhibits a positive additive system peak which appears shortly after the colurtm dead volrune (the additive is weakly retained). The profile of this peak is nearly Gaussian, because the additive being weakly... 

Figure 13.3 PD (100) (line), (110) (dash) and (111) (dot) peak profiles for a system made of cubic crystallites (edge Z) = 10nm). Normalized profiles in reciprocal space.

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