Retention ratio

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Note that from Eqs. (7.17) and (7.18), R EmCw(0/EpCp(O) for t > Tlag- The substitution of the apparent partition coefficient with the retention ratio allows us to state the mole balance at time t (provided t > Tlag) in a much simplified form ... 

Separation Parameters Retention Ratio, Plate Height, Resolution,... 

FFF process can be theoretically described because (a) the flow regime inside the channel of well-defined geometry can be mathematically represented (see Equation 12.1), and (b) the tractability of the various force fields employed in the different techniques allows one to describe the analyte concentration profile (see Equation 12.3). The retention ratio expresses the retardation of an analyte zone caused by its interaction with the field, and it is given by the ratio of the average velocity of the analyte zone and the average velocity of the carrier liquid (n) ... 

Equation 12.11 is obtained by substituting Equations 12.1 and 12.3 into Equation 12.9 and by performing the integration in order to calculate (o) [3]. Well-retained compounds have A,<0.1 and, under these conditions, the retention ratio can be approximated as a linear function of X ... 

This cryptand effect which results in increases of retention ratio (5) and of the rate, also can be explained by a higher level of anion HOMO (Scheme 10). As a consequence, the frontier-orbital interaction is increased. Therefore, retention is kinetically favored. 

If there are many peaks in the sample, or if the peaks are eluted in close proximity to one another, it may be necessary to determine peak identities on the basis of the selectivity, a, as defined in Chapter 1 (Section 1.3.2), where t refers to the retention time of the internal standard.3 The advantage of the selectivity approach is that the relative retention ratio provides more reproducible numbers.4... 

At the end of the stop-flow period in which the particle clouds reach equilibrium, flow is resumed and the particles are displaced downstream. The displacement velocity is characterized by the dimensionless retention ratio R, which is described by the equation... 

The downstream motion of component bands in F(+) methods is characterized by the retention ratio R, a dimensionless parameter defined as... 

Figure 9.11. Variation of retention ratio R with A according to Eqs. 9.29 (solid line) and 9.30 (broken line).

With this A substituted into Eq. 9.29 or 9.30, we get retention ratio R. Clearly, R varies strongly with molecular weight M because A depends directly on M. In the simple case in which A is small and Eq. 9.30 can be used for R, we obtain... 

Equation 10.1 shows that R is a measure of the retardation or slowing of the zone or peak with respect to mobile phase velocity. A peak that experiences no retardation because its solute does not partition into the stationary phase (R = 1) is termed a nonretained peak or void peak such a peak travels at mobile phase velocity v. Solute retained to some extent by the stationary phase migrates as a retained peak, for which R< 1. (The smaller R, the greater the retention.) Because of its key role in specifying retention, R is termed the retention ratio. 

The above equations can be rearranged to yield the retention ratio R explicitly. The last equation gives... 

Write Eq. 10.38 in terms of capacity factor k instead of retention ratio R. 

Calculate the plate height contributed by sorption-desorption mass transfer (nonequilibrium) through a uniform liquid layer (configuration factor q = 2/3) of thickness 1.0 x 10 3 cm coated on the inside of an open tubular (capillary) column. The gas velocity v is 10 cm/s. The solute retention ratio is 0.10 and its diffusion coefficient Ds through the stationary liquid is 1.0 x 10 5 cm2/s. 

R retention ratio zone velocity /mean fluid velocity R equilibrium fraction in mobile phase Re Reynold s number... 

When these relationships are compared with the analogous ones for liquid-liquid extractions, it is noted that retention ratio in chromatography is similar in concept to the fraction extracted in extractions. 

Injection ports for packed columns are aligned so that the sample can be deposited on a heated surface just before the column or directly on the end of the column. In the first instance, the injection port is heated above the boiling point of the sample in order to get rapid volatilization, but for on-column use, the injection port is kept at column temperature. On-column injection is usually preferred because there is less chance of decomposition and the sample is not exposed to a high injection port temperature. Remember also that a typical GC analyte will have a retention ratio of 0.25 or much less. This means that 75% or more of it is sorbed in the stationary phase, and that is where the on-column technique puts it—in the stationary phase. It is probably for this reason that on-column injection is so efficient. The use of large volumes of solvents will wash... 

Reduced parameters, 66-69 Refractive index (RI) detector, 206-207 Regular solution, 49 Relative retention, 20-21, 22, 77 Repeatability, see Precision Reproducibility, see Precision Resolution, 17-19, 55 Response factors (detector), 104, 125 Response time, 94 Retardation factor, Rf, 71 Retention index of Kovats, 78 Retention ratio, 11, 12, 71 Retention time, 6, 9 Retention volume, 9, 75 adjusted, 10, 75 corrected, 62-63, 75 net, 63, 75 specific, 110 Reverse phase LC, 158 Rohrschneider/McReynolds constants, 137-140... 

Another chromatographic parameter is the retention ratio Rr. It is the relative average speed v of an analyte through a chromatographic system compared to the average mobile phase speed or velocity u. 

There is another difference between column techniques and planar techniques in LC. It is the way that the retention ratio is expressed. In Chapter 1, the retention ratio Rr was defined for column techniques as... 

Figure 5.7. Comparison of equations for calculating retention ratio Rr and retardation factor Rf. Reprinted courtesy of Gow-Mac Instrument Co.

Also, there are some operational differences that cause differences between the retention ratio and the retardation factor. Even if the exact same stationary phase is coated on a TLC plate and packed in a column, the TLC material usually contains an additional binder to hold the stationary phase on the plate. This binder will most likely alter slightly the characteristics of the stationary phase and result in differences between... 

It is likely that a relative retention volume that is ratioed to a standard would be more reproducible. Such has been found to be true a is a relative retention ratio it was defined in Chapter 1, where it was called the column selectivity or a separation factor ... 

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