Retention factor

The second term,. represents the zone spreading that each sample component exhibits due to diffusion along the column axis. Diffusion coefficients in aqueous solution are generally low, so the contribution of this term is relatively small unless the retention time is quite long due to a very slow flow rate or a high retention factor. 

Resistance to mass transfer, is by far the major contributor to sample zone spreading within the column. This term is minimized by a column packing that attains equilibrium of the analyte between the mobile and stationary phases as quickly as possible. A moderate linear flow rate, u, should be employed because cr mt increases with flow rate. 

The last term, u ec, points out that substantial peak broadening may occur outside the column. To avoid this, the transfer lines from sample injection to column and column should be as short as possible. Stagnant areas in the system must be avoided. These can occur, for example, if the connection between two pieces of tubing bows out. The detector cell should have a low dead volume. 

Perhaps the single most important term in any kind of chromatography is the retention factor (or capacity factor), k. Conditions must be adjusted so that there is a sufficient difference in the k values of the various analytes to give a good separation. It is also necessary to select conditions so that the range of k values is such that a separation may be completed within a reasonable time. A k range of 2 tolO has often been specified as desirable. 

A satisfactory chromatographic separation depends on having a column with a sufficient plate number, N, as well as an adequate difference in k values. Resolution in terms of separation factor (a = k2lki), the average retention factor, kav = ( i + 2)/2, and the plate number, N, is given by  

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The retention factor is defined as a dimensionless peak locator as ... 

In reversed-pViase chromatography (RPC), the mobile phase modulator is typically a water-miscible organic solvent, and the stationary phase is a hydrophobic adsorbent. In this case, the logarithm of solute retention factor is commonly found to be linearly related to the volume fraction of the organic solvent. 

Other cases, involving an arbitrary relationship between the solute retention factor and the modulator concentration can be handled analytically using the approaches of Frey [Biotechnol. Bioeng., 35, 1055 (1990)] and Carta and Striugfield []. Chromatogr, 605, 151 (1992)]. 

Solute equilibrium parameters (X5,S for RPC and (Xz,Z for lEC Solute retention factor for initial mobile phase A ... 

Retention factor of mobile phase modulator A h Plate number obtained for k = IZr NpR... 

The stationary phase is selected to provide the maximum selectivity. Where possible, the retention factor is adjusted (by varying the mobile phase composition, temperature, or pressure) to an optimum value that generally falls between 2 and 10. Resolution is adversely affected when k 2, while product dilution and separation time... 

The known models for describing retention factor in whole variable space ar e based on three-phase model and containing from three to six par ameters and variety combinations of two independent factors (micelle concentration, volume fraction of organic modifier). When the retention models are comparing or the accuracy of fitting establishing, the closeness of correlation coefficient to 1 and the sum of the squared residuals or the sum of absolute deviations and their relative values is taken into account. A number of problems ar e appear in this case ... 

Different tests for estimation the accuracy of fit and prediction capability of the retention models were investigated in this work. Distribution of the residuals with taking into account their statistical weights chai acterizes the goodness of fit. For the application of statistical weights the scedastic functions of retention factor were constmcted. Was established that random errors of the retention factor k ai e distributed normally that permits to use the statistical criteria for prediction capability and goodness of fit correctly. 

The main equation of the model describes the dependence of retention factor, k, from surfactant concentration, c and modifier concentration, c ... 

Mobile phases with some solvating potential, such as CO2 or ammonia, are necessary in SGC. Even though this technique is performed with ambient outlet pressure, solutes can be separated at lower temperatures than in GC because the average pressure on the column is high enough that solvation occurs. Obviously, solute retention is not constant in the column, and the local values of retention factors increase for all solutes as they near the column outlet. 

According to Equation 3, the resolution of two peaks in column separation is controlled by three major variables retention defined in terms of the retention factor k column efficiency expressed as the number of theoretical plates N and selectivity characterized by the selectivity factor a [48] ... 

In order to illustrate the critical process parameters of SMB process validation, we will consider the separation of the racemic drug as described in Process design. The study represents the effect of the influence of feed concentration, number of plates and retention factor on the second eluting enantiomer. The simulation of the process for different values of feed concentration is performed and the variations of the extract and raffinate purities are shown in Fig. 10.10. 

Figure 10.12. Influence of retention factor of the more retained compound on extract and raffinate purity. Solid line without adjustment of the operating flow rates dotted lines with adjustment of the operating flow rates.

The effect on purity and the influence of retention factor by adjusting operating flowrates is illustrated in Fig. 10.12. 

The graph in Fig. 10.12 shows that the purity decreases very quickly below acceptable levels as retention factor of the more retained enantiomer decreases. Flowever, with minor adjustment of the SMB internal flow rates, a variation of more than 10 % of the retention factor of the more retained enantiomer still meets required purity, productivity, and eluent consumption. Control of critical parameters such as retention factors can be made without modification of the feed and eluent flowrates. 

The 300-ton inventory unit in Example 3-2 is changing catalyst type and planning to add 3.5 tons per day of new catalyst. Determine the percent of changeover after 60 days of operation. Assume a retention factor of 0.7. 

This method can also be used to calculate the catalyst retention factor. The above equations assume steady-state operation, constant unit inventory, and constant addition and loss rate. 

Plutonium is transported by the groundwater in fractures in the rock (usually <1 mm wide). A typical groundwater velocity (vw) at >100 m depth in Swedish bedrock is 0.1 tn/y. The fractures are filled with crushed, weathered, clayish minerals, which have a high capacity to sorb the plutonium. Assuming instantaneous and reversible reactions, the sorption will cause the plutonium to move considerably slower (with velocity vn) than the groundwater. The ratio between these two velocities is referred to as the retention factor (RF), defined by... 

Introducing the values into the equation, using a minimum Kd-value of >300, gives a retention factor of >750. If this value is combined with a representative water transport time from repository to recipient (>1000 years for a distance >100 m), the transport equation indicates that it will take the plutonium >750,000 years to reach the recipient which is the water man may use. This estimate is supported by findings at the ancient natural reactor site at Oklo in Gabon (67). 

Another relatively new lipophilicity scale proposed for use in ADME studies is based on MEKC [106]. A further variant is called BMC and uses mobile phases of Brij35 [polyoxyethylene(23)lauryl ether] [129]. Similarly, the retention factors of 16 P-blockers obtained with micellar chromatography with sodium dodecyl sulfate as micelle-forming agent correlates well with permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments [130]. 

Based on the partitioning of the solute between a polar mobile and an apolar sta-honary phase (RPLC), the chromatographic data were expressed as retention factors (logfe) given by ... 

Retention Factors Used as RPLC Lipophilicity Indices... 

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