Retardation factor definition

With a growing implementation of the thin-layer chromatographic laboratories with densitometric scanners and particularly in the case of PLC, it seems quite important to reconsider definition of the Rp coefficient and the ways of its determination. Further, it seems strongly advisable to recommend the retardation factor in its i P(max) form as a more practical one for the preparative layer chromatographic usage. 

Finally, an officially updated definition of the retardation factor, R, issued by lUPAC is important to the whole field of planar chromatography (the linear and the nonlinear TLC mode included). The importance of such a definition has two reasons. First, it is promoted by the growing access of planar chromatography users for densitometric evaluation of their chromatograms and second, by the vagueness of the present definition in the case of skewed concentration profiles with the samples developed under mass overload conditions. 

In gas chromatography the analyte partitioning between mobile gas phase and stationary liquid phase is a real retention mechanism also, phase parameters, such as volume, thickness, internal diameter, and so on, are well known and easily determined. In liquid chromatography, however, the correct definition of the mobile-phase volume has been a subject of continuous debate in the last 30 years [13-16]. The assumption that the retardation factor, i /, which is a quantitative ratio, could be considered as the fraction of time that components spend in the mobile phase is not obvious either. 

If precipitation-dissolution reactions dominate (this is sorption by Sposito s definition), there are simply no differentiable isotherms (defined as a functional relationship between S and C) relating solid and aqueous concentrations (Bryant et al., 1987). The above-mentioned retardation factor formulations (equation (10.3)) are not applicable. A more complete form of the ADR equation may be (Fetter, 1993, p.l 15),... 

Combining equations 10,13, and 14 yields the computational definition of the retardation factor ... 

DAL influences practically all stages of the elementary flotation act. Buoyancy velocity of bubbles of definite size with retarded and non-retarded surfaces can differ from each another by a factor of about 2 (see Fig. 8.2). According to the theory of quasi-elastic (Section 10.1) and inelastic (Section 10.2) collisions, a smaller film thickness h corresponding to the beginning... 

In conclusion, although it is yet to be definitively shown, much evidence supports a link between oxidation of lipoproteins and arteriosclerosis in human beings. If this relationship can be conclusively demonstrated, it is likely there will be a need to measure lipoprotein oxidation products in the clinical laboratory for risk assessment. Establishing a definitive link between oxidation and arteriosclerosis is not a simple task because not only will it be necessary to demonstrate that oxidation is a risk factor for arteriosclerosis in large prospective studies, but it will be important to show it is a marker independent of lipoprotein and inflammatory markers. Furthermore, it would be desirable to identify treatments by which modification of oxidation markers leads to reduction in disease (SI). To date, the failure of randomized trials with antioxidant nutrients to retard arteriosclerosis (Al, Y3) has made it clear that demonstrating a definite relationship in humans may be more difficult that expected. In the meantime, there is a need to measure these... 

Note that, unlike in the definition of effectiveness factor for catalytic reactions (Chapter 7) where the normalizing rate was the rate of reaction, here the normalizing rate is the rate of mass transfer. Thus the reaction is considered the intruder (albeit benevolent or enhancing), whereas for catalytic reactions, diffusion was the intruder (often, but not always, retarding). For a pseudo-mth-order reaction, one can write... 

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