Retardation factor in TLC

Rf value retardation factor in TLC and PC as a measure of the retention of a component on the stationary phase as it is carried along in the mobile phase ... 

Retardation factor in TLC, PC Corrected retardation factor Retention index, Kovat s RI Resolution between adjacent peaks Signal output from detector Separation number Stationary phase Time... 

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. 

The fundamental parimeter used to characterize the position of a saaple zone in a TLC chromatograa is the retardation factor, or Rf value. It represents the ratio of the distance migrated by the saaple compared to that traveled by the solvent front. With respect to Figure 7.1, the Rf value for linear development is given by equation (7.1)... 

In TLC each component is defined by its Rf value (retardation factor) that corresponds to its relative migration compared to the solvent ... 

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... 

TLC Analysis. Small sample requirements, minimal sample preparation, high sensitivity, and low cost make TLC an attractive method for organic archaeometry. Its suitability for the detection of resin acids in complex mixtures was tested by subjecting the Carthaginian samples to a two-dimensional technique. Ether solutions of the organic material were spotted onto the plate and first freed from nonpolar components by elution in one direction with heptane. The residual carboxylic acids were then developed, with reference standards in adjacent tracks, in the second direction with heptane-toluene-ether (1 1 1). Under these conditions, 7-ketodehydro-abietic acid remains at or very near the origin (maximum retardation factor [Rf] = 0.04), but abietic acid and dehydroabietic acid are readily identified. 

The basic parameter used in TLC for solute identification is the Retardation Factor (Rp), which is given by... 

Similar to the retention factor in HPLC the retardation factor Rf describes retention behavior during TLC experiments. It represents the ratio of the distance migrated by the sample to the distance traveled by the solvent front. Boundaries are 0 < R < 1. For Rf = 0, the product does not migrate from the origin and for R = 1, the product is not retained. Rf values are calculated to two decimal places, while some authors prefer to tabulate values as whole numbers, as hR values (equivalent to 100Rf) (Poole, 2003). 

In gas chromatography (GC) the mobile phase is a gas (e.g. nitrogen, N ) and moves rapidly compared to the movement of the separated substances. Thus, R values (retardation factor, i.e. the distance travelled by an analyte divided by the distance travelled by the mobile phase (these values are routinely used in TLC)) would be very small and hard to determine. Retention times are therefore employed in preference to Revalues. The retention tune of a compound (also used in liquid chromatography) may be described simply as the length of time it takes a compound to be detected following injection. This quantity has units of time, that is, seconds or minutes. 

In TLC, Equation 6.22 is expressed in terms of the retardation factor, R A plot of Rj/(l -R vs. Qnjij, should give a straight line and the partition coefficient can be found from the slope/intercept ratio in the same way as for HPLC. 

As in TLC studies the accepted method of evaluating paper chromatograms is by determination of retardation factors. These can be obtained with equal facility for vertical and horizontal development techniques and the underlying principles and concepts are as for TLC. However, coincidence of R( values should never be taken as absolute identification and for complete and unambiguous structural characterisation the components should be eluted from the sorbent layer, isolated and further spectroanalytical studies carried out. 

The retardation factor just defined for column chromatography is similar to the i F factor in thin-layer chromatography, permitting liquid chromatog-raphers to use these two parameters to compare TLC and HPLC data. And finally, it may be helpful in understanding the meaning of retention factor to note that the concept is similar in principle to fraction extracted concept in liquid-liquid extraction. 

In TLC, the movement of a compound is described using the retardation factor R(. The i f of a compound is described as the distance the compound is eluted (icompound). divided by the distance from the application line to the mobile phase front (Imobiie phase) (Figure 4.5). 

For planar chromatography (PC and TLC), retardation factors, Rf values, for standards and unknowns are compared by chromatographing them simultaneously so as to eliminate variations in laboratory materials and conditions. For column separations, retention times, f, or volumes, Vg, are compared by chromatographing standards and unknowns sequentially under stable conditions with as little time between runs as possible. 

Figure 4.23 Calculation of retardation (retention) factor Rf for TLC. In diffuse spots, the distance is measured from the line of origin to the center of the spot. 

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