Retention expressiveness

RETENTION EXPRESSED IN TERMS OF VOLOHE weight of liquid phase in the colvnm... 

Figure 3.8 shows a diagrammatic example of the stress retention, expressed as a percentage, with a fast drop at the beginning of the test followed by a gentle slope. 

Equations (105) and (106) provide an important linkage between the three essential parameters that dictate the overall quality of the chromatographic resolution, namely, the relative retention, expressed in terms of the capacity factor k the relative selectivity a, and the extent of peak dispersion Nt or he i. Higher system performances and thus larger values of Rs, per unit time... 

Retention, expressed by the capacity factor k of nonionic analytes, is a function of the partition coefficient and the volume of pseudostationary phase (micelles), the volume of the mobile phase, the retention time of the analyte, the dead time (corresponding to the migration velocity of the EOF), and the retention time of the micelles. If the micelles were immobilized in the capillary (i.e., if the pseudostationary phase were stationary ), the capacity factor would be similar to the standard equation of retention in chromatography. 

HPLC retention expressed either in absolute values (T ) or in relative terms (retention factor) is a complex function of a multitude of parameters. Type of bonded phase is only one of them. Surface area of base material, pore size, bonding density, end-capping, and even the column history all can significantly affect analyte retention. 

A mapping of the dependence of analyte retention (expressed as the natural logarithm of the retention factor, k) on the mobile phase composition (expressed as the volume fraction of solvent in the mobile phase, (p) in isocratic elution (or as k versus ip in gradient elution) with a minimum of two initial experiments can be used to define the useful range of mobile phase conditions, and can indicate the mobile phase composition at which the band spacing is optimal (see Figure 10). 

Finally, the real retention expression obtained for a binary system (/ = 1, 2) is ... 

Retention Behavior. On a chromatogram the distance on the time axis from the point of sample injection to the peak of an eluted component is called the uncorrected retention time The corresponding retention volume is the product of retention time and flow rate, expressed as volume of mobile phase per unit time ... 

To accomplish any separation of two cations (or two anions), one of these ions must be taken up by the resin in distinct preference to the other. This preference is expressed by the separation factor (or relative retention), using K+ and Na+ as the example ... 

The amino group is readily dia2oti2ed in aqueous solution, and this reaction forms a basis for the assay of sulfas. Aldehydes also react to form anils, and the yellow product formed with 4-(dimethylamino)hen2a1dehyde can be used for detection in thiu-layer and paper chromatography. Chromatographic retention values have been deterrnined in a number of thiu layer systems, and have been used as an expression of the lipophilic character of sulfonamides (23). These values have corresponded well with Hansch lipophilic parameters determined in an isobutyl alcohol—water system. 

A versatile method for the synthesis of a variety of five-membered heterocycles and their ring-fused analogs involves the reaction of a neutral 47r-electron-3-atom system with a 27T-electron system, the dipolarophile, which is usually electron deficient in nature. Available evidence, e.g. retention of dipolarophile stereochemistry in the product and solvent polarity exerting only a moderate influence on the reaction, indicates that the cycloaddition proceeds via a concerted mechanism 63AG(E)565, 63AG(E)633, 68JOC2291) and may be represented in general terms by the expression in Scheme 8. 

For the usual case when R = (total retention of the solute), L petm = 0 3.nd combining these equations gives a general expression for flux in a turbulent-flow membrane system. For any given solute concentration ... 

Once the elution-curve equation is derived, and the nature of f(v) identified, then by differentiating f(v) and equating to zero, the position of the peak maximum can be determined and an expression for the retention volume (Vr) obtained. The expression for (Vr) will disclose those factors that control solute retention. 

The retention volume of a solute is that volume of mobile phase that passes through the column between the injection point and the peak maximum. Consequently, by differentiating equation (10), equating to zero and solving for (v), an expression for the retention volume (Vr) can be obtained. 

Equation (34) is generally quite correct and useful. However, if highly accurate retention measurements are important, then second order effects must be taken into account and equation (33) indicates that, for accurate data, equation (34) is grossly over simplified. From equation (33), a more accurate expression for solute retention would be... 

In general, (Q) and ( ) will be equal, but the general case is assumed, where they are not. Equation (37) gives an explicit and accurate expression for the retention volume of a solute. The importance of each function in the expression will depend on the physical properties of the chromatographic system. At one extreme, using an open tubular column in GC, then... 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

Practically a more convenient way of expressing solute retention in terms of solvent concentration for a binary solvent mixture as the mobile phase is to use the inverse of equation (11), i.e.. 

The data from the separation of the enantiomers of 4-phenyl-2-oxazolidinone [14] gave an expression for the retention volume of the two enantiomers, which are reiterated as follows. 

Thus, if (tr) is the elution time (in seconds) of a solute having a retention volume (Vr), (ocq) is expressed as the change in flow rate per second, (At) is taken as 1 second and (nt) the number of seconds. 

In TLC, plate efficiency is measured in a similar manner to that in GC and LC employing a similar expression. However, the retention distance from the sampling point to the position of the spot center is substituted for the retention volume, time or distance and the spot width as an alternative to the peak base width, viz.. 

MCRT Mean Cell Retention Time - days. An expression of the average time that a microorganism will spend in an activated sludge process. 

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