Retention factor definition

H is the plate height (cm) u is linear velocity (cm/s) dp is particle diameter, and >ni is the diffusion coefficient of analyte (cm /s). By combining the relationships between retention time, U, and retention factor, k tt = to(l + k), the definition of dead time, to, to = L u where L is the length of the column, and H = LIN where N is chromatographic efficiency with Equations 9.2 and 9.3, a relationship (Equation 9.4) for retention time, tt, in terms of diffusion coefficient, efficiency, particle size, and reduced variables (h and v) and retention factor results. Equation 9.4 illustrates that mobile phases with large diffusion coefficients are preferred if short retention times are desired. 

More complex is the separation of charged analytes in CEC, which is the result of the interplay of chromatographic and electrophoretic processes that is considered in the definition of the electrochromatographic retention factor, or overall retention factor, k introduced by Rathore and Horvath [140] ... 

Figure 1.7—Separation factor (or selectivity factor) between two adjacent components, a alone does not determine if the separation is, in fact, possible. Here, the separation factor is in the order of 1.3. For two non-adjacent peaks, the relative retention factor can be obtained and it is designated by r. By definition, neither a nor r can be less than one.

We have shown recently that the different separation mechanisms of HPLC and CZE preclude the definition of any retention or velocity factor for CEC that would have comparable significance to that of k in HPLC [4], However, as shown in the following section, the CEC system was defined by a combination of a retention factor that measures the retention in CEC and a velocity factor that characterizes the electrophoretic migration. Further, expressions were proposed for possible peak locators that, just like k and k, in HPLC and CZE, respectively, can serve as useful indicators of peak position on the electrochromatogram and facilitate estimation of various separation parameters such as selectivity and resolution. 

In modem chromatography it is more convenient to use the capacity factor (k or k ) instead of the distribution coefficient. The name retention factor has been suggested to replace the name capacity factor. The definition for capacity factor or retention factor is the same ... 

Figure 13.41 Definition and calculation of retention factor ( f) and Plate Number (AO from a developed TLC chromatogram.

A dimensional analysis of the perfonnance index shows that it has the dimension of an inverse viscosity. With a little algebra, using the equations for permeability [Eq. (2.51)3, die definitions of the flow-resistance parameter [Eq. (154)3, linear velocity [l s. (110 and (111)3. retention factor [Eq. (161)3 and the reduced plate height [Eq. (113)3, we can lit the performance index into its components ... 

This is the basic definition of the retention factor. With this equation, we can now make the link between the retention time of a compound and the chemistry and the thermodynamics of the separation. 

Definition of the Retention Factor Retention Factor and Chemical Equilibrium Relative Retention a Relationships to the Free Energy... 

Most of the specifics of the chemistry of chromatography will be covered in the chapters dedicated to each separation mechanism. Nevertheless, a few topics, including some definitions, are sufficiently generic to be treated separately. In the following chapter, we look at the definition of the retention factor and the relative retention, and then look at the relationship of retention with the free energy of the interaction with the stationary phase. 

By definition, the retention factor is the ratio of the number of molecules in the stationary phase to the number of molecules in the mobile phase ... 

Retention is the basis of chromatographic separation, as it refers to the fact that the different compounds are retained by the column to a varied degree. The phenomenon is quantified by the definition of the retention factor k, which... 

The definition of polymer thermal stabiUty is not simple owing to the number of measurement techniques, desired properties, and factors that affect each (time, heating rate, atmosphere, etc). The easiest evaluation of thermal stabiUty is by the temperature at which a certain weight loss occurs as observed by thermogravimetric analysis (tga). Early work assigned a 7% loss as the point of stabiUty more recentiy a 10% value or the extrapolated break in the tga curve has been used. A more reaUstic view is to compare weight loss vs time at constant temperature, and better yet is to evaluate property retention time at temperature one set of criteria has been 177°C for 30,000 h, or 240°C for 1000 h, or 538°C for 1 h, or 816°C for 5 min (1). 

A general approach to the problem of identification, should more definitive detectors not be available, is to change the chromatographic system , which in the case of HPLC is usually the mobile phase, and redetermine the retention parameter. The change obtained is often more characteristic of a single analyte than is the capacity factor with either of the mobile phases. 

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