Rr value

If an ionic surfactant is present, the potentials should vary as shown in Fig. XIV-5c, or similarly to the case with nonsurfactant electrolytes. In addition, however, surfactant adsorption decreases the interfacial tension and thus contributes to the stability of the emulsion. As discussed in connection with charged monolayers (see Section XV-6), the mutual repulsion of the charged polar groups tends to make such films expanded and hence of relatively low rr value. Added electrolyte reduces such repulsion by increasing the counterion concentration the film becomes more condensed and its film pressure increases. It thus is possible to explain qualitatively the role of added electrolyte in reducing the interfacial tension and thereby stabilizing emulsions. 

The effect of temperature on distribution ratios has already been mentioned on page 91. Although the separation proceeds more quickly at elevated temperatures, resolution suffers because of increased rates of diffusion. However, in adsorption TLC only small increases in Rt values are observed even with a 20°C rise. Strict temperature control is not necessary if samples and standards are run at the same time, although large fluctuations should be avoided. The quality of the thin-layer materials, and in particular the presence of impurities in them, determine the extent to which partition, adsorption, ion-exchange and exclusion participate in the sorption process. These factors affect Rr values in an unpredictable manner. Thin layers should be of uniform thickness, between 0.2 and 0.3 mm with thinner layers, local variations in thickness can result in appreciable variations in Rf values. 

Components are identified by comparison of their R( values with those of standards run under identical conditions, or by removing the materials from the chromatogram and subjecting them to further qualitative tests, e.g. spot-tests, mass spectrometry, infrared spectrometry. Factors affecting Rr values were discussed on p. 155. Chromatographic materials and conditions are usually so variable that it is advisable to run standards with samples to ensure that comparisons are valid. 

Interpreting Data Calculate the Rr values for each of the pigments in each chromatogram and record them in the data table. 

The rr values were plotted as a funetion of the square root of the spin-loeking field angular frequeney and the lateral diffusion eoeffieient (Z))... 

Table V. RR-Values From Case-Control Studies (j , 56, j>7) ...

Fig. 3.2 Relations between the pKa values of non-ortho-substituted phenols and the Hammett rr-values of the substituents [8]. Substituents 1, none 2, 4-chloro 3, 4-bromo 4, 3-chloro ...

N,A-dimethylvinylamine (13) is another important model compound in this context. Despite the fact that its IPV has not been determined experimentally, it can be estimated from relations between the IPV of some RNMe2 derivatives and the ca, compounds with R = H, Me, Et, Ph, c-Hex, CF3, CH2CF3, NH2 and NMe2 one obtains equation 12,... 

This technique requires that the chromatographic sorbent be repeatedly developed in the same or different solvent. The sorbent is dried between each development. In other words, one 20-cm development has a longer development time than two 10-cm developments. This technique is most] useful for resolution of components with Rr values below 0.5. ... 

The most commonly described USP procedure for quantification is the scrap and elution approach. Low analyte recoveries can occur but can be minimized by using polar organic solvents such as methanol, ethanol, or acetone. Generally, analytes with high-Rr values can be desorbed with high recoveries by using the mobile phase. One example of this procedure is the USP assay procedure of the steroid methyl prednisolone acetate in cream formulation. This steroid is separated from its excipients by TLC, extracted from the sorbent, derivatized, and measured spectrophotometrically. 

In paper chromatography, the assumption is generally made that there is a partition between a stationary water-phase (held on the paper) and a moving, organic phase. The Rr value of a substance... 

Many methylated sugars are known only in an amorphous state, but even those that are crystalline are frequently difficult to crystallize unless material is available for seeding. From the Rr value of an ether, a tentative estimate of the number of methoxyl groups can be made and, in some... 

Calculation Methods. Partition coefficients are additive-constitutive, free energy-related properties. Log P represents the overall hydrophobicity of a molecule, which includes the sum of the hydrophobic contributions of the "parent" molecule and its substituent. Thus, the rr value for a substituent may be defined as... 

Rr VALUES OF SOME MORPHINE DERIVATIVES USING 200 x 2(K) MM TLC PLATES PRECX3ATED WITH SILICA GEL 60 REPRODUCED WITH PERMISSION FROM [1351... 

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